GB2572090A

GB2572090A - Optical corrosion sensor

Info

Publication number: GB2572090A
Application number: GB1908272.6A
Authority: GB
Inventors: Munn Stephen
Original assignee: Hevasure Ltd
Current assignee: Hevasure Ltd
Priority date: 2018-12-13
Filing date: 2018-12-13
Publication date: 2019-09-18
Anticipated expiration: 2038-12-13
Also published as: GB201908272D0; GB2572090B

Abstract

A sample 900 for use in an optical sensor for monitoring corrosion in a water system comprises a metal element 112 having a first surface for exposure to the water of the water system, and a second surface opposite the first surface for receiving and reflecting light. A portion of the first surface is provided with a corrosion-resistant coating 144 for providing a location for forming a seal between the sample and the optical sensor. The element may be a disc coupon and the coating is annular. A transparent element 120 through which light passes in use may be used with the element.

Description

The present invention relates to an optical sensor for detecting and/or monitoring corrosion in water systems such as closed water systems and systems in the process industry where water only passes through the system once.

Corrosion in water systems is a known problem which can lead to the system requiring costly maintenance, or replacement incurring large direct and indirect costs. By way of example, closed water systems can include heating, ventilation, and air conditioning systems (HVAC), chilled water systems (CHW), and low temperature hot water systems (LTHW) and are characterised by the system water being retained in the system for a long time and circulating past the components of the system many times (fresh water is added in a top up or a flush scenario, but these are relatively rare). Other water systems such as through flow industrial process systems may be arranged that water only flows past a given point once, i.e. each part of the system receives fresh water.

A major cause of corrosion in water systems is dissolved oxygen present in the water of the water system. The dissolved oxygen is reduced at a cathode and causes a corresponding oxidation at an anode. The oxidation causes metal loss (corrosion) of the anode. Typically, the cathode may be copper and the anode may be steel, although any case where two different metals with different nobilities are exposed to the water within the water system can be affected in this way. Also anodic and cathodic sites can form on the surface of the same metal due to small defects or localised environmental differences (e.g. differential aeration).

Pinhole or pitting corrosion is distinguished from other forms of corrosion in that where other forms of corrosion cause a broadly uniform loss of thickness from exposed surfaces, pinhole corrosion is extremely localised and can result in deep cavities in relatively compact areas of e.g. pipes of a water system. These narrow pits can extend a significant way into walls of the system, in the time it takes other, uniform types of corrosion to remove only a thin layer from the outside of the system. Clearly a problem occurs when pitting pinhole becomes so deep that it penetrates entirely through a pipe wall (that is, it creates a hole all the way through the pipe wall), becoming a pinhole. In some other cases, the pitting may penetrate only most of the way through a pipe wall, with the system pressure causing the final rupture. Of course, it is not just pipes which suffer from the effects of pinhole corrosion, since other elements, such as valves, joints, pumps, etc. can all be damaged by pinhole corrosion causing leakage.

Pinhole or pitting corrosion is a particularly pernicious problem as failure modes can occur much quicker than with other corrosion methods, since far less material needs to be removed before a pinhole corrodes through a given thickness of metal. Moreover, traditional corrosion detection methods tend to rely on the actual amount of material lost, either directly by detecting weight loss or somewhat more indirectly by measuring e.g. galvanic currents, which are indicative of the number of metal atoms lost. Pinhole corrosion is hard to quantify by these methods, since the actual lost mass of metal is small, so it can be hard to measure, and even harder to correlate with a risk of system failure, because a risk assessment as to how close a pipe is to rupturing necessarily must take account of the small scale details of the pinhole corrosion such as the width of the pinholes. Without this information a simple measure of lost mass of metal is insufficient to determine the likelihood of imminent system rupture.

There is a clear need to be able to monitor the corrosion of metals (e.g. carbon steel, brass, copper, aluminium etc.) in water systems. Existing sensors are prohibitively expensive for commercial use in monitoring systems aimed at the commercial HVAC market, for example. A typical corrosion rate sensor capable of determining the corrosion rate of just one metal can be very expensive. Desired is a low-cost corrosion sensor capable of withstanding environmental conditions such as temperature and pressure found in commercial water systems. In particular, a sensor adapted to detect and monitor pinhole corrosion is particularly desirable.

The present invention seeks to solve some or all of the problems set out above.

Disclosed herein is an optical sensing apparatus for mounting in a water system and for monitoring corrosion in the water system, comprising: a metal sample having a uniform thickness and a first planar surface and a second planar surface opposite the first planar surface, wherein the first planar surface is arranged to be in contact with water within the water system; a light source configured to emit light towards the second planar surface of the metal sample; and a light sensor configured to receive light reflected by the second planar surface of the metal sample, and output a signal indicative of the intensity of the reflected light.

Since the metal sample is of uniform thickness, the appearance of pinhole corrosion on that sample provides a warning that pinholes of that depth are present in the system. It can be hard to predict where on a sample pinhole corrosion will occur, as it is driven by localised variations such as minor imperfections of the metal on a small scale or gaps in passive layers. Consequently, as well as providing a realistic representation of e.g. pipes in a system (which have a uniform thickness), providing an area of metal of uniform thickness, with one side exposed to the system water, means that wherever on the first planar surface the pitting starts, it will have the same thickness of metal to travel through before it becomes visible on the second planar surface, thereby causing a change in the light reflected from the second surface. The detection of a change in reflectivity in this way provides an indication that pinholing has occurred and that the depth of pitting in the system is at least as deep as the thickness of the metal sample. Indeed, the time at which pinholing is first detected indicates the time at which the pitting reaches that depth. This in turn allows monitoring of the system health in that a comparison may be made against a maximum safe pit depth for the system, which may take into account factors such as: temperature of the water in the system; pressure of water in the system; materials from which the system is constructed (e.g. to assess strength); thickness of components of the system (e.g. pipe wall thickness); and so forth. Should the safety threshold be exceeded, then part or all of the pipes or other components of the system can be flagged for replacement, repair, or any other appropriate remedial action. It should be noted that the terminology “pinhole” usually refers to a hole formed entirely through a wall, while “pit” usually refers to a hole which extends part way through a wall.

Putting this another way, as corrosion progresses, the film will gradually pin-hole changing the visual appearance of the inner surface. This visual change manifests itself as an increase in the number and/or size of dark circles as corrosion debris forms and grows from the pin-hole edge. This change in appearance can be detected by changes in reflectivity of the light emitted from the light source and reflected from the metal sample, as detected by the light sensor as described above.

An estimate of the rate of pitting can be obtained by dividing the thickness of the metal sample by the amount of time which has elapsed since the sample was installed in the system water. This estimate can be used to provide an advanced warning of when the system is expected to reach an unsafe level of pinhole corrosion, so that maintenance can be scheduled in advance of the system requiring such maintenance, thereby improving efficiency. Indeed, in some cases attention can be directed to the causes of corrosion such as high dissolved oxygen or unfavourable pH.

Sensors for detecting uniform corrosion exist which have a wedge-shaped sample. As corrosion progresses, a uniform amount of metal is removed from a surface of the metal sample exposed to system water. This uniform corrosion is monitored by such a sensor because a uniform thickness of lost metal results in complete loss of metal at the thin end of the wedge. As uniform corrosion progresses, more and more of the wedge is completely dissolved. There is therefore a linear relationship between uniform thickness lost across the wedge and amount of light reflected by the metal because where metal is lost, light is not reflected by the wedge-shaped sample. While such systems seem to provide a convenient method of measuring uniform corrosion, they are entirely unsuited to monitoring pinhole corrosion. It is also expected that some corrosion information might be lost due to the physical arrangements of the light source and detector. This is because the non-uniform thickness of a wedge-shaped sample will show pinhole corrosion at the thinner end first. Unless efforts are made to correlate the existence of a pinhole with the location of the pinhole and the original thickness of the wedge at that location, then no meaningful information on the depth of pitting in the system can be gleaned. Typically such systems are incapable of determining the location of pinholes with sufficient accuracy to correlate this with a thickness of metal, and therefore to provide a useful indication of the severity of pitting and pinholing in the system.

Pinholing shows up primarily in two ways. First, the pinhole extends through the metal, causing a small hole of missing metal. Reflection occurs from the metal of the second surface, so where a pinhole appears this manifests itself in the absence of reflection in that area. However, as noted, pinholes tend to be relatively small in diameter (leading to a small area of metal missing for reflection), and therefore this effect is not large. The second evidence for pinholing is tarnishing of metal around the pinhole. This occurs because system water can penetrate through the metal to contact parts of the second surface. This contact causes parts of the second surface to corrode, causing discolouration and further metal loss. In addition, the side walls of the cylindrical pinhole undergo standard uniform corrosion, which causes discolouration of the area of the second planar surface around the pinhole, even where direct contact between system water and the second planar surface is not possible. After time, the uniform corrosion of the side walls of the cylindrical pinhole causes widening of the pinhole and a larger area of missing metal, thereby enhancing the first effect described above.

It will be appreciated that the present invention is also suitable for detecting uniform corrosion inasmuch as where pinholing does not dominate, the complete loss of a sample can be an indication that a uniform thickness of metal (i.e. the thickness of the sample) has been lost from the system.

As used herein, the term “metal” includes elemental metals such as iron, copper and aluminium, as well as alloys such as brass and stainless steel. In order to improve the signal to noise ratio further, the reflectivity of the second surface of the metal sample can be provided with an increased reflectivity, for example by polishing that surface. This allows as much incident light as possible to reflect from the second surface, and thereby maximising the difference between parts of the sample which are reflecting the light and those which are not, whether due to tarnishing or outright absence. A large difference such as this results in a larger change in received light, and consequently makes even small changes easier to detect. To ensure accuracy and capturing of all corrosion effects on the thin film surface, the inner walls of the housing (i.e. an optical cavity, where the light source and the light sensor are housed), is coated by a highly diffuse reflective paint. This gives light emitted by the light source more interaction with the surface of the metal sample. It also gives the system the ability to work with different metal sample surface finishes.

Optionally, the optical sensing apparatus further comprises a transparent element disposed at least partly between the light source and the metal sample. For example, this may be formed from glass or transparent plastic or the like. The fact that the element is transparent means that it does not block the transmission of light to or from the second planar surface, thereby ensuring that the operation of the device is not impeded. In addition, the metal samples of the system may be selected to have a specific thickness in order that pinholing of that thickness indicates pitting has occurred to that depth. This thickness is selected independently of the required strength of components which contact the water of the water system (e.g. independently of any requirement to withstand a particular system pressure). Therefore, the transparent element can help to provide strength to the metal sample. The transparent element can be selected to be a suitable thickness to withstand the pressure of any given system. “Transparent” in this context means that the element blocks little or none of the light emitted by the light source. That is to say, the element may be chosen so that its absorption and/or reflection spectrum is relatively low at wavelengths of light emitted by the sensor and/or reflected from the second planar surface.

Optionally the transparent element has a third planar surface arranged adjacent to the second planar surface of the metal sample. This transparent element protects the second planar surface from exposure to the water of the system, thereby ensuring that the type of corrosion which results in a detectable change in the reflected intensity is primarily that due to pinholing. The third planar surface can be arranged in contact with the second planar surface, or otherwise in such a way as to form a seal to prevent the second planar surface being in contact with system water. The transparent element may have the same shape and size as the metal sample to assist in this.

Optionally, the optical sensing apparatus further comprises a seal for protecting the second planar surface from water in the water system. As noted above, this seal may be formed in part by a transparent element. Alternatively this may include O-rings or other suitable sealing means. Sealing the second planar surface from the system water can help to ensure that the corrosion must corrode through the entire thickness of the sample before it becomes detectable, thereby preferentially focussing the measurement on pinhole corrosion. In addition, since the light source and light sensor are arranged to respectively shine light at and receive reflected light from the second planar surface, many arrangements position these components on the same side of the metal sample as the second planar surface. In other words, the metal sample may be located between the system water and the light emission and sensing parts of the apparatus. Arrangements which seal the second planar surface against the system water can also have the effect that the light emission and detection means are also protected from system water. This can help to protect e.g. electronic components from damage by exposure to system water. In some cases, both the metal sample and the transparent element may have a seal to prevent system water from flowing past them. The metal sample may be sealed to protect the second planar surface from system water (at least until a pinhole corrodes entirely through the metal sample). The transparent element may be sealed so that even when pinholes have corroded holes through the entire thickness of the metal sample, the light source and sensor are protected from system water and its associated damage. In some cases, both seals may be provided by a single seal.

Optionally the seal may be located adjacent to the first planar surface. This allows a single seal to be used to protect the light source, light sensor and second planar surface. Further optionally, the metal sample may be provided with a corrosionresistant coating on the first planar surface in the vicinity of the seal. It has been found that corrosion is concentrated in locations near to a seal due to what is known as crevice corrosion. This distorts the amount of corrosion detected by the system by artificially creating a preferential site for corrosion. It has been found that a thin layer of corrosionresistant coating between the metal sample and the seal prevents preferential crevice corrosion in the vicinity of the seal. The corrosion-resistant coating should be selected for its durability. Since the device is anticipated to leave a sample in contact with system water for many years, the corrosion-resistant coating should: withstand temperatures encountered in water systems (typically 5°C to 100°C); resist corrosion from the water, dissolved oxygen, inhibitors, sterilisation products (e.g. chlorine), impurities (dissolved metal ions, salt, etc.); survive in such environments for a long period of time (e.g. 5 to 10 years); prevent corrosion of the underlying metal sample; be compatible with the underlying metal sample; and be applicable to the underlying metal sample without causing oxidation or otherwise reacting with the sample, in order to not alter the chemical properties of the metal sample at the edges of the corrosion-resistant layer, which can distort the corrosion.

The metal sample may be representative of one or more metals used in the water system. For example, the pipework may be made from carbon steel, stainless steel, copper, etc. Pumps and valves, etc. may be made from stainless steel, aluminium or brass, while solders formed from alloys of many metals (tin, copper, bismuth, indium, zinc, silver, manganese, antimony, lead, cadmium, aluminium, etc.) may be used to form joints in the system. By choosing the sample to reflect the metals or alloys used in the system, a realistic picture of the actual corrosion happening to that metal in the environment of the system can be obtained. It is important to emphasise that the corrosion which occurs to the metal sample is directly representative of the corrosion which occurs to elements in the water system formed from the same metal as the metal sample because they are both exposed to the same water (the system water).

The uniform thickness of the metal sample may be 1 mm or less, for example from 0.025mm to 1mm, or from 0.05mm to 0.5mm. Since pipe wall thicknesses in systems tend to be 1mm or more, the use of metal samples of 1mm or less can provide advanced warning of the failure of such systems, so that remedial action can be taken in advance of an expensive failure.

Optionally, the apparatus may include a plurality of metal samples. This can allow corrosion of different types of metal to be monitored by providing samples of different metals. In other cases, a series of samples of different thicknesses may be provided. This can provide a more compact sensor than providing a separate sensor for each thickness of metal and/or each different metal type. Of course, an alternative arrangement is to provide a separate sensor for each metal and/or thickness. Where a single sensor has a plurality of metal samples, the sensor may share various measurement components. In some cases, the plurality of metal samples includes N metal samples and wherein the sensing apparatus comprising fewer than N light sources and/or fewer than N light sensors. The sensor may use multiplexing to correlate the received light with the plurality of metal samples. For example the multiplexing may include one or more of: time division multiplexing; wavelength or frequency division multiplexing; special division multiplexing; and/or polarisation division multiplexing.

For example, a single light source (or in general fewer light sources than there are samples) can be arranged to shine onto the second surface of multiple metal samples, thereby ensuring that each sample is illuminated with the same light, so allowing a meaningful comparison of the received light intensity, as the light received by the second surface of each sample can be controlled to be the same in each case. In other cases, there may be a single light sensor and multiple light sources. For example, each light source may emit light in a narrow wavelength range and the sensor may be configured to detect the received intensity broken down by received wavelength, thereby allowing the sensor to correlate intensity in a wavelength with a particular sample (known as wavelength division multiplexing), and thus monitor several samples using only one sensor, or more generally using fewer sensors than there are samples. An alternative method of sharing light sources and light sensors is to use a directed light source and/or light sensor and illuminate the second planar surface of different samples at different times (known as time division multiplexing), and synchronise the sensor with the light source, so that light received at a given time is correlated with a given metal sample.

An alternative method of sharing light sources and light sensors is to use a directed light source and/or a light sensor having a narrow angular range of sensitivity and illuminate the second planar surface of different samples at different times (known as time division multiplexing), and synchronise the sensor with the light source, so that light received at a given time is correlated with a given metal sample. The light source and/or light sensor may be arranged to direct or receive (respectively) light in narrow angular ranges, and to change the regions to which the light is directed or received over time. For example, by physically rotating a light source, or manipulating mirrors, lenses, etc. to direct the emitted light to a different location. This can be used to shine light at different metals samples at different times, thereby providing spatial multiplexing. In some examples, polarised light may be used to multiplex the signals (polarisationdivision multiplexing). As an example, a light source may be split into two orthogonal polarisation directions and each different polarisation directed towards a different metal sample. Light sensors can be fitted with polarisers to ensure that only a particular polarisation is detected by the sensor. Not only does this reduce the number of light sources required, but it can help to reduce cross-talk as light polarised in an orthogonal direction to a given sensor’s polariser cannot enter the sensor.

In yet another case, other aspects such as processors for interpreting the received intensity data or housings for enclosing the sensors (both of which are described in more detail below) may be shared between multiple metal samples. In general, it will be seen that multiplexing in time, wavelength, etc. can be used to monitor multiple samples while sharing some of the monitoring equipment such as light sources, light sensors and/or processors. While wavelength division multiplexing can be more expensive to provide, it can provide more subtlety as the required sensors can help to determine type of corrosion as well as simply detecting some form of corrosion. A cheaper alternative is to match the light source with the light detector based on prior experimental data and use the matched pair for each surface. On the other hand, costs and complexity may be kept down by using an appropriate time division multiplexing. Is some cases, it may be helpful to combine different types of multiplexing to achieve the desired effect.

In some cases the plurality of metal samples each have a different thickness, as set out below, in order to determine the maximum pitting corrosion depth. The samples may be provided as a series of samples of thicknesses, for example, 0.025mm, 0.05mm, 0.075mm, 0.1mm, 0.125mm and so forth. As set out below, this allows the maximum pitting depth to be estimated to the nearest 0.025mm.

In some examples the (or each) metal sample is replaceable. This may be achieved by use of access hatches or the like for accessing the metal sample, removing the sample, and replacing the metal sample. In other cases, the apparatus may be mounted on a bypassable section of pipe, e.g. with another portion of pipe running in parallel to the section of pipe having the sensor and with valves for directing flow along either section of pipe. In normal use the system water is directed to flow past the portion of pipe to which the apparatus is mounted. When the sample in the sensor is to be replaced, the flow can be diverted along the bypass pipe (the section parallel to the portion with the sensor), and optionally the section of pipe having the sensor apparatus can be drained of system water. It is now possible to remove the sensor from the system without causing a leak. Once the apparatus has been removed, the metal sample can be removed from the sensor and replaced with a fresh one, a different thickness, one formed from a different metal, etc. This arrangement can also be used in the initial installation of the apparatus, and for removing the apparatus itself for cleaning, maintenance, etc.

The apparatus may further comprise a processor configured to receive the signal indicative of the intensity of the reflected light from the light sensor. This can be used to determine the level of corrosion in the water system. In some cases the processor is configured to relate the signal indicative of the intensity of the reflected light to corrosion of the metal sample. For example the processor may be configured to determine that a decrease in the intensity of the reflected light corresponds to an increase in the corrosion of the metal sample, for the reasons set out above. The processor may be remote, in the sense that the signals are fed to e.g. a centralised monitoring station, where signals from multiple sensors are collected and analysed together or individually as part of a holistic health check of the water system. In any case, the reflected intensity from the (or each) sample can be provided as an analogue output, or it can be converted to a universally compatible format such as Modbus for easy transport and centralised processing, for example.

The light sensor may be configured to output a plurality of signals indicative of the intensity of the reflected light over time and wherein the processor is configured to determine a rate of corrosion of the metal sample from the plurality of signals indicative of the intensity of the reflected light over time received from the light sensor. In other words, by monitoring the reflected light signal over time, it is possible to detect how quickly the corrosion is progressing, and thereby provide e.g. updated estimates of the time until the system reaches an unsafe level of corrosion or to predict the remaining lifetime of particular components of the system, so allowing advanced action to be taken. In other cases, the sensor may be configured to issue (and the processor configured to receive) signals at predetermined (e.g. periodic) times. This can allow an ongoing assessment of whether corrosion is occurring and how severe it is at each reading.

Where there are multiple samples present, the processor may be configured to receive a plurality of signals indicative of an amount or a rate of corrosion of a corresponding plurality of metal samples. Thus a series of signals, each corresponding to a particular metal and/or thickness, may be received and the processor can be configured to determine which samples have corrosion (and how much). As above, each set of samples of the same metal can be used to determine a maximum pitting depth forthat metal.

The plurality of metal samples may be formed from the same metal as one another, wherein each of the plurality of metal samples has a different thickness and wherein the processor is configured to determine a range of maximum pitting corrosion depths in the water system from the plurality of received signals. In other words, the system may be configured to “digitise” the outputs to determine that the pitting corrosion depth in the system is at least as deep as the thinnest sample which shows pinholing, but is not as deep as the thickest sample which does not show pinholing, with the exact depth between these limits being unknown. By selecting the various samples to be relatively close in thickness, the accuracy of the determination of the depth of pitting in the metals of the system can be improved.

Where multiple samples of different thickness are present, the expected situation is that all samples up to and including a certain thickness show pinholing, while all samples above that certain thickness do not. In some cases however, especially those with samples of very closely spaced thicknesses, it may be that there are gaps in the samples which show pinholing, for example the thinnest three samples may show pinholing, the next (fourth) sample does not, the fifth sample does show pinholing, and all thicker samples do not. This can result from the random nature of pinhole corrosion, for example. Nonetheless, the thickest sample which shows corrosion should be used as an estimate for how far through the metal the corrosion has penetrated. This is primarily for safety reasons, as this sample shows definitive evidence that pitting corrosion has penetrated to this depth in metals of the system.

Of course, it is possible to have a series of thicknesses of samples for each representative metal in the system in some cases. For each series of thicknesses of the same metal, the above considerations apply.

The apparatus may further comprise an optical element configured to direct light emitted by the light source towards the second planar surface of the metal sample, and/or configured to direct the reflected light towards the light sensor. This can be in the form of a lens, a fibre optic cable, a mirror, a prism, etc. In cases where there is a transparent element on the second planar surface of the metal sample, the transparent element can function as the optical element. The purpose of the optical element is to ensure that the light emitted by the light source is directed to the second planar surface, and provides a uniform coverage of light to the second planar surface. This in turn ensures that where it will result in the same change in reflected light intensity, irrespective of where the corrosion occurs on the second planar surface.

The light source and/or the light sensor may be mounted in a housing forming a closed opaque cavity. This can inhibit ambient light from affecting the sensor and providing false positive results. In such cases, a portion of the opaque walls is the second planar surface for receiving and reflecting light emitted from the light source. That is to say, the metal sample forms an opaque portion of the wall. In some of these cases, as described above, the metal sample may be behind a transparent element, meaning that while the metal sample is not technically an internal wall of the housing, it nevertheless provides an opaque surface to prevent light entering the housing.

Internal walls of the housing may have diffuse highly reflective inner surfaces for providing an optical integrating cavity. Optical integrating cavities are chosen so that all light emitted by the light source eventually reaches the light sensor. They have the effect of smoothing out the light intensity in the interior of the housing so that each portion receives approximately the same light intensity. This prevents inhomogeneities in the angular distribution of light emitted from the light source from causing different parts of the sample to be illuminated with different intensities of light and ensures that the background light levels are highly consistent. Such a situation could lead to different parts of the sample showing different effects when corrosion occurs, if the inhomogeneities are severe enough. Of course, providing a highly homogeneous light source is another solution to this. Where the internal walls of the cavity are said to be diffuse reflective surfaces, this does not necessarily apply to the second planar surface of the metal sample, and certainly does not apply to the transparent element, where such an element is present.

The optical sensing apparatus may further comprise a baffle for blocking direct light transmission between the light source and the light sensor. The baffle can prevent the sensor from outputting a false negative, in the sense that if a significant amount of light is transmitted directly from the source to the sensor without reflecting from the metal sample, then corrosion of the metal sample will have less of an overall effect on the received intensity as a portion of the received intensity will always come directly from the light source, irrespective of corrosion of the sample. A baffle can be as simple as an opaque protrusion on an internal surface of a housing positioned between the light source and the light sensor. In other cases, the light source and/or the light sensor may be mounted in a respective recess in order to prevent direct light transmission between the source and the sensor. In such examples, the recess will be arranged so as to provide a path for light from the source to shine onto the second planar surface and/or to provide a path for light reflected from the second surface to arrive at the sensor.

The optical sensing apparatus may further comprise a second light sensor for directly sampling the light emitted from the light source to provide a reference value. By directly sampling light emitted by the light source, the light received by the first light sensor (that is the light reflected form the second planar surface) can be normalised to the intensity (and indeed spectral composition) of the emitted light. This technique provides an effective light intensity referencing scheme to avoid light intensity variation due to drift and temperature changes. As noted above, it is anticipated that the sensing apparatus remains installed in a water system for many years. In some cases, the light source will remain on for this entire time, in other cases, the light source will be switched on and off many times as part of a periodic measurement schedule. In either scenario, the light source is liable to change its output intensity and/or spectral composition over time. By comparing the received intensity at the first detector (after reflecting from the second planar surface) to the received intensity at the second detector (sampled directly from the light source), changes relative to the actual light output can be detected, thereby accounting for the effect of changes in light output over time.

The second sensor may receive light directly from the light source in a variety of ways. For example, the light source may be arranged to emit light into a fibre-optic bundle. Some fibres in the bundle may then be split out of the bundle some way along the length of the bundle and fed to the second sensor, while the remainder of the bundle directs the light towards the second planar surface. In other cases, a mirror may be used in much the same way, by reflecting a part of the light emitted from the light source, while the unreflected part of the beam is unaffected by the mirror and proceeds instead to the metal sample as described above. A related idea is to use a beam splitter (partsilvered mirror, pair of prisms joined to form a cube, etc.) to split the emitted light into two portions, one of which is directed towards the metal sample, and the other towards the second (i.e. reference) sensor.

In some examples the light emitted by the light source has a spectrum selected based on the metal from which the metal sample is made and/or the expected corrosion mode of the metal from which the metal sample is made. For example, since different metals have different colours (that is reflect/absorb different wavelengths of light), the light source may be configured to emit light in a particular wavelength range. For example, a copper sample reflects strongly in the red and orange part of the spectrum (around 600 to 700nm), but much less strongly in the green, blue and violet parts of the spectrum (around 400 to 550nm). This means that the contrast between a light source reflecting perfectly from a copper surface and not reflecting (due to tarnishing of the metal, or indeed complete corrosion of the metal) is far less pronounced when the light is in the blue/green part of the spectrum than when it is in the red part of the spectrum. The efficiency of the device can therefore be improved by emitting light only in the regions of the spectrum which produce a high contrast effect. This can include tailoring the emitted light to the wavelength-dependent reflection of the sample, i.e. limiting to red parts of the spectrum for copper, yellow parts for brass, providing uniform wavelength distribution for silvery metals such as stainless steel, aluminium, etc.

In any of the examples presented herein, light sources can be any suitable source, such as incandescent bulbs, halogen bulbs, fluorescent bulbs, LEDs, lasers and the like. Where emission at specific wavelengths or ranges of wavelengths is required, white light sources can be filtered, or inherently limited wavelength sources such as LEDs or lasers can be selected as appropriate. Indeed, while the above discussion makes use of readily understood terms in the context of visible electromagnetic radiation for clarity and ease of understanding, it will be appreciated that the sensor may operate outside of the visible part of the electromagnetic spectrum, for example where specific wavelength regions provide a strong contrast between clean and tarnished metal, for a given metal. In some cases, for example, infra-red or ultra-violet light may be used in the measurement (using a suitable light source and light sensor pairing). In other cases, regions of the electromagnetic spectrum yet further from the visible parts may be used.

In any of the examples presented herein, light sensors can be any suitable sensor, such as photodiodes, phototransistors, photoresistors, CCD devices and the like. Where the light emitted from the light source is limited to a particular wavelength of range of wavelengths, or indeed, the metal sample preferentially reflects particular wavelengths of light, the sensor should be configured to be able to detect these wavelengths, and preferably is particularly sensitive to such wavelengths. In some cases, as set out below, it may be advantageous to filter the particular wavelengths out of light entering the sensor, or to configure the sensor to be particularly insensitive to particular wavelengths. In yet more cases, it may be preferable for the sensor to be able to detect colours (e.g. using an RGB or more complex CCD arrangement) to provide a more nuanced view of corrosion in the metal sample.

In some cases, the spectral composition of the light emitted from the light source may be tailored to the type of corrosion which the metal sample is expected to undergo. This may be in addition to or instead of tailoring the spectrum to the metal itself, as described above. For example taking the copper example above, copper is known to form a green-blue patina under some conditions, where the copper(ll) (Cu²⁺) ion forms and binds to chemical species (acetate, carbonate, chloride, etc.). Where this corrosion route is anticipated (from the chemical and physical environment in the water system) red light may be a particularly suitable choice for emission from the light source. This is because the red light will reflect strongly from the untarnished (reddish-orange) surface. Where the surface corrodes entirely, or tarnishes to blue-green, the red light will be reflected only weakly or indeed not at all. In this example, limiting to light emission in the red parts of the electromagnetic spectrum improves the ability of the system to detect corrosion (and particularly pinhole corrosion) by causing a large drop in reflected light intensity because not only does the (relatively small) pinhole fail to reflect light, but the (usually larger) tarnished region also reflects little or no light. In cases where the metal is silver coloured, such as stainless steel, the uncorroded/untarnished reflection profile may be relatively uniform across visible wavelengths and tailoring the light to the expected corrosion routes of that metal may be the best way to ensure there is a high contrast between uncorroded/untarnished and corroded/tarnished states.

In other examples, the light emitted by the light source may be selected to be in one or more wavelength ranges which reflect strongly from both untarnished and tarnished metal. By providing a sensor which is sensitive to each of these wavelengths (and can distinguish between them), it is possible to determine the relative proportions of the metal which are untarnished, tarnished and completely eroded. For example, where the light received is full strength (optionally normalised to the emitted value as described above) in the “untarnished” part of the spectrum, then no corrosion or tarnishing has occurred. As parts of the second surface become tarnished, the proportion of received light starts to split between untarnished and tarnished (with the end being full strength in the “tarnished” part of the spectrum). Where the entire second surface is either tarnished or untarnished, the sum of [proportion of light in the tarnished part of the spectrum] and [proportion of light in the untarnished part of the spectrum] should add to 100%. Where this sum does not equal 100%, the difference between the measured value and 100% gives an indication of the proportion of the second surface which shows complete corrosion. For example, where the normalised value of the received light in the tarnished part of the spectrum is 32%, and the normalised value of the received light in the untarnished part of the spectrum is 51%, then approximately 51% of the surface is untarnished, approximately 32% is tarnished and approximately 100% - (32% + 51%) = 17% is completely corroded. In some cases, rather than selecting the light source to output two or different wavelengths or ranges of wavelengths, the metal sample may be illuminated with white light, and the separation into reflected wavelength components may be performed by the sensor which can be configured to detect two or more reflected wavelengths or ranges of wavelengths and thereby determine the proportion of the sample which is untarnished, corroded, or tarnished. In some cases, the tarnished category may even be broken down into sub-categories, indicating different types of corrosion. The specific type of corrosion may be used, e.g. to alert a user that the water in the system contains unexpected contaminants, and thereby allow a user to take corrective action.

The apparatus may further comprise control electronics for controlling the light source and/or the light sensor. The electronics can be used to periodically trigger emission of light and a corresponding measurement of the light reflected from the second metal surface. Where there is a second sensor, this too can be controlled by the control electronics. The control electronics can also be used to adjust the brightness or spectral composition of the light emitted by the light source, for example to optimise the contrast between untarnished and tarnished metal for the metal sample in the sensor. The control electronics can also be used to control the sending of the received signals or a determination of the extent or type of corrosion in the metal sample(s) to a remote location.

The apparatus may further comprise an electrical power source for providing electrical power to the light source and the light sensor. In some cases, there is a processor, which is also supplied with electrical power by the power source. As noted, the apparatus may be remote from e.g. a centralised monitoring system, which collates the information from many distributed systems. It may be impractical to transmit power to sensors which are located in a remote location, so providing the apparatus with a power supply such as a battery allows the apparatus to function.

Also disclosed herein is an apparatus for monitoring a plurality of parameters for monitoring and/or determining overall system health in a water system, comprising one or more of the sensors of any preceding claim. Pitting corrosion and/or pinhole corrosion is a big problem, and the sensor described above provides a convenient way of monitoring corrosion and particularly pitting/pinhole corrosion. A plurality of sensors can be used as part of such a system, as described above to provide an estimate of the maximum depth of pitting corrosion in the system. In particular, monitoring a plurality of aspects of system health in situ, using sensors in contact with system water provides a very accurate and reliable assessment of the health of the system.

Also disclosed herein is a method of monitoring corrosion in a water system, the method comprising: mounting a metal sample in a water system, the metal sample having a uniform thickness and including a first planar surface and a second planar surface opposite the first planar surface, wherein the first planar surface is arranged in contact with water of the water system; emitting light towards the second planar surface of the metal sample; receiving light reflected by the second planar surface of the metal sample; generating a signal indicative of the intensity of the reflected light; and correlating the intensity of the reflected light to corrosion of the metal sample.

Since the metal sample is of uniform thickness, the appearance of pinhole corrosion on that sample provides a warning that pinholes of that depth are present in the system. It can be hard to predict where on a sample pinhole corrosion will occur, as it is driven by localised variations such as minor imperfections of the metal on a small scale or gaps in passive layers. Consequently, as well as providing a realistic representation of e.g. pipes in a system (which have a uniform thickness), providing an area of metal of uniform thickness, with one side exposed to the system water, means that wherever on the first planar surface the pitting starts, it will have the same thickness of metal to travel through before it becomes visible on the second planar surface, thereby causing a change in the light reflected from the second surface. The detection of a change in reflectivity in this way provides an indication that pinholing has occurred and that the depth of pitting in the system is at least as deep as the thickness of the metal sample. Indeed, the time at which pinholing is first detected indicates the time at which the pitting reaches that depth. This in turn allows monitoring of the system health in that a comparison may be made against a maximum safe pit depth for the system, which may take into account factors such as: temperature of the water in the system; pressure of water in the system; materials from which the system is constructed (e.g. to assess strength); thickness of components of the system (e.g. pipe wall thickness); and so forth. Should the safety threshold be exceeded, then part or all of the pipes or other components of the system can be flagged for replacement, repair, or any other appropriate remedial action.

An estimate of the rate of pitting can be obtained by dividing the thickness of the metal sample by the amount of time which has elapsed since the sample was installed in the system water. This estimate can be used to provide an advanced warning of when the system is expected to reach an unsafe level of pinhole corrosion, so that maintenance can be scheduled in advance of the system requiring it, thereby improving efficiency. Indeed, in some cases attention can be directed to the causes of corrosion such as high dissolved oxygen or unfavourable pH.

The method optionally further comprises generating a plurality of signals indicative of the intensity of the reflected light over time, and determining a rate of corrosion from the plurality of intensities of the reflected light over time. In other words, by monitoring the reflected light signal over time, it is possible to detect how quickly the corrosion is progressing, and thereby provide e.g. updated estimates of the time until the system reaches an unsafe level of corrosion or to predict the remaining lifetime of particular components of the system, so allowing advanced action to be taken. In other cases, the sensor may be configured to issue (and the processor configured to receive) signals at predetermined (e.g. periodic) times. This can allow an ongoing assessment of whether corrosion is occurring and how severe it is at each reading.

Optionally, the light is emitted from a light source and/or the reflected light is detected by a light sensor. Moreover, the light source and the light sensor may be mounted in a housing forming a closed opaque cavity. This can inhibit ambient light from affecting the sensor and providing false positive results. In such cases, a portion of the opaque walls is the second planar surface for receiving and reflecting light emitted from the light source. That is to say, the metal sample forms an opaque portion of the wall. In some of these cases, as described above, the metal sample may be behind a transparent element, meaning that while the metal sample is not technically an internal wall of the housing, it nevertheless provides an opaque surface to prevent light entering the housing.

Internal walls of the housing may have diffuse reflective inner surfaces for providing an optical integrating cavity. Optical integrating cavities are chosen so that all light emitted by the light source eventually reaches the light sensor. They have the effect of smoothing out the light intensity in the interior of the housing so that each portion receives approximately the same light intensity. This prevents inhomogeneities in the angular distribution of light emitted from the light source from causing different parts of the sample to be illuminated with different intensities of light and ensures that the background light levels are highly consistent. Such a situation could lead to different parts of the sample showing different effects when corrosion occurs, if the inhomogeneities are severe enough. Of course, providing a highly homogeneous light source is another solution to this. Where the internal walls of the cavity are said to be diffuse reflective surfaces, this does not necessarily apply to the second planar surface of the metal sample, and certainly does not apply to the transparent element, where such an element is present.

In some examples of the method a baffle for blocking direct light transmission between the light source and the light sensor may be provided. The baffle can prevent the sensor from outputting a false negative, in the sense that if a significant amount of light is transmitted directly from the source to the sensor without reflecting from the metal sample, then corrosion of the metal sample will have less of an overall effect on the received intensity as a portion of the received intensity will always come directly from the light source, irrespective of corrosion of the sample. A baffle can be as simple as an opaque protrusion on an internal surface of a housing positioned between the light source and the light sensor. In other cases, the light source and/or the light sensor may be mounted in a respective recess in order to prevent direct light transmission between the source and the sensor. In such examples, the recess will be arranged so as to provide a path for light from the source to shine onto the second planar surface and/or to provide a path for light reflected from the second surface to arrive at the sensor.

The method may be executed on an apparatus further comprising an optical element configured to direct light emitted by the light source towards the second planar surface of the metal sample, and/or configured to direct the reflected light towards the light sensor. This can be in the form of a lens, a fibre optic cable, a mirror, a prism, etc. In cases where there is a transparent element on the second planar surface of the metal sample, the transparent element can function as the optical element. The purpose of the optical element is to ensure that the light emitted by the light source is directed to the second planar surface, and provides a uniform coverage of light to the second planar surface. This in turn ensures that where it will result in the same change in reflected light intensity, irrespective of where the corrosion occurs on the second planar surface.

The method may further comprise a second light sensor for directly sampling the light emitted from the light source to provide a reference value, wherein correlating the intensity of the reflected light to corrosion of the metal sample includes comparing the reference value to the received light reflected from the second planar surface. In other words, the second light sensor is used to normalise the light received at the first light sensor which has been reflected by the second planar surface of the metal sample. By directly sampling light emitted by the light source, the light received by the first light sensor (that is the light reflected form the second planar surface) can be normalised to the intensity (and indeed spectral composition) of the emitted light. This technique provides an effective light intensity referencing scheme to avoid light intensity variation due to drift and temperature changes. As noted above, it is anticipated that the sensing apparatus remains installed in a water system for many years. In some cases, the light source will remain on for this entire time, in other cases, the light source will be switched on and off many times as part of a periodic measurement schedule. In either scenario, the light source is liable to change its output intensity and/or spectral composition over time. By comparing the received intensity at the first detector (after reflecting from the second planar surface) to the received intensity at the second detector (sampled directly from the light source), changes relative to the actual light output can be detected, thereby accounting for the effect of changes in light output over time.

The method may be performed on a plurality of metal samples are mounted in the water system and wherein each metal sample has a uniform thickness and includes a first planar surface and a second planar surface, wherein the first planar surface of each sample is arranged in contact with water of the water system and wherein: light is emitted towards the second planar surface of each metal sample; light is reflected by the second planar surface of each metal sample and received by a sensor; a signal is generated corresponding to each metal sample, the signal being indicative of the intensity of the reflected light from each metal sample; and corrosion of each metal sample is correlated with the intensity of the reflected light.

The plurality of metal samples may be formed from the same metal as one another, wherein each of the plurality of metal samples has a different thickness to the thickness of the other metal samples. This can allow corrosion of different types of metal to be monitored by providing samples of different metals. In other cases, a series of samples of different thicknesses may be provided. This can provide a more compact sensor than providing a separate sensor for each thickness of metal and/or each different metal type. Of course, an alternative arrangement is to provide a separate sensor for each metal and/or thickness. Where a single sensor has a plurality of metal samples, the sensor may share various measurement components. In some cases, the plurality of metal samples includes N metal samples and wherein the sensing apparatus comprising fewer than N light sources and/or fewer than N light sensors. The sensor may use multiplexing to correlate the received light with the plurality of metal samples. For example the multiplexing may include one or more of: time division multiplexing; wavelength or frequency division multiplexing; special division multiplexing; and/or polarisation division multiplexing.

For example, a single light source (or in general fewer light sources than there are samples) can be arranged to shine onto the second surface of multiple metal samples, thereby ensuring that each sample is illuminated with the same light, so allowing a meaningful comparison of the received light intensity, as the light received by the second surface of each sample can be controlled to be the same in each case. In other cases, there may be a single light sensor and multiple light sources. For example, each light source may emit light in a narrow wavelength range and the sensor may be configured to detect the received intensity broken down by received wavelength, thereby allowing the sensor to correlate intensity in a wavelength with a particular sample (wavelength division multiplexing), and thus monitor several samples using only one sensor, or more generally using fewer sensors than there are samples. In yet another case, other aspects such as processors for interpreting the received intensity data or housings for enclosing the sensors (both of which are described in more detail below) may be shared between multiple metal samples. An alternative method of sharing light sources and light sensors is to use a directed light source and/or light sensor and illuminate the second planar surface of different samples at different times (known as time division multiplexing), and synchronise the sensor with the light source, so that light received at a given time is correlated with a given metal sample.

In yet another case, other aspects such as processors for interpreting the received intensity data or housings for enclosing the sensors (both of which are described in more detail below) may be shared between multiple metal samples. In general, it will be seen that multiplexing in time, wavelength, etc. can be used to monitor multiple samples while sharing some of the monitoring equipment such as light sources, light sensors and/or processors. While wavelength division multiplexing can be more expensive to provide, it can provide more subtlety as the required sensors can help to determine type of corrosion as well as simply detecting some form of corrosion. A cheaper alternative is to match the light source with the light detector based on prior experimental data and use the matched pair for each surface. On the other hand, costs and complexity may be kept down by using an appropriate time division multiplexing. Is some cases, it may be helpful to combine different types of multiplexing to achieve the desired effect. The method may further comprise determining a range of maximum pitting corrosion depths in the water system from the each of the signals indicative of the intensity of the reflected light from each metal sample. In other words, the system may be configured to “digitise” the outputs to determine that the pitting corrosion depth is at least as deep as the thinnest sample which shows pinholing, but is not as deep as the thickest sample which does not show pinholing, with the exact depth being unknown. By selecting the various samples to be relatively close in thickness, the accuracy of the determination of the depth of pitting in the metals of the system can be improved.

The metal sample may be removeable and/or replaceable and the method may include removing and/or replacing the metal sample. This may be achieved by use of access hatches or the like for accessing the metal sample, removing the sample, and replacing the metal sample. In other cases, the apparatus may be mounted on a bypassable section of pipe, e.g. with another portion of pipe running in parallel to the section of pipe having the sensor and with valves for directing flow along either section of pipe. In normal use the system water is directed to flow past the portion of pipe to which the apparatus is mounted. When the sample in the sensor is to be replaced, the flow can be diverted along the bypass pipe (the section parallel to the portion with the sensor), and optionally the section of pipe having the sensor apparatus can be drained of system water. It is now possible to remove the sensor from the system without causing a leak. Once the apparatus has been removed, the metal sample can be removed from the sensor and replaced with a fresh one, a different thickness, one formed from a different metal, etc. This arrangement can also be used in the initial installation of the apparatus, and for removing the apparatus itself for cleaning, maintenance, etc.

The method may further include controlling one or more of: power; intensity; and/or spectral weight of the emitted light.

Also disclosed herein is a sample for use in a sensor for monitoring optical corrosion in a water system, the metal sample comprising a metal element having: a first surface for exposure to the water of the water system; and a second surface opposite the first surface for receiving and reflecting light; wherein a portion of the first surface is provided with a corrosion-resistant coating for providing a location for forming a seal between the sample and the sensor. As noted above, it has been found that a seal formed directly on the surface of a metal sample can give an unrealistic depiction of the amount of corrosion in the system, as system water tends to disproportionately corrode the sample near to the seal, since it forms a crevice and is therefore susceptible to crevice corrosion, which enhances the rate of corrosion in the crevice regions. By placing a corrosion-resistant coating on the sample, the seal can be formed over the corrosion-resistant coating, which helps to mitigate this effect. In other words, the corrosion-resistant coating allows a seal to be formed between the sample and a sensor which does not alter the corrosion environment of the sample adjacent to the seal. Other parts of the first surface (those without a corrosion-resistant coating) are exposed to system water and corrode in a manner which is the same as other metals in the system, so providing an accurate measure of the corrosion of other instances of that metal where such instances occur in the water system.

The metal element may be planar and/or has a uniform thickness. As noted above at length, a planar and/or uniform thickness sample can be particularly suited to detecting and/or monitoring pitting and pinhole corrosion.

The corrosion-resistant coating may be applied to the edges of the metal element. This placement allows the sample to be mounted using a seal at the edges. This in turn allows a single seal to be used to hold the sample in place on the sensor and to seal the sensor from the system water, thereby protecting sensitive electronic components.

The corrosion-resistant coating may extend between 0.5mm and 5mm inward from the edge of the first surface. The metal element may have a width of between 10mm and 50mm. The corrosion-resistant coating covers no more than 10% of the first surface. These numbers provide a good balance between preventing unrealistic enhancements of corrosion while leaving a sufficiently large area of metal exposed to system water to provide a representative surface for monitoring corrosion.

The metal element may be disc shaped. Optionally the corrosion-resistant coating is annular. These shapes provide a sample which is easy to seal securely to the sensor.

The metal element may be formed from carbon steel, stainless steel, copper, brass, aluminium, or other materials representative of metals in the water system. By forming the sample from representative metals in the system, an accurate picture can be gleaned of how such metals are faring in other parts of the system.

The corrosion-resistant coating may be stable for at least 5 years when submerged in water of the water system. Additionally or alternatively, the corrosionresistant coating may be stable when submerged in system water of at least 85°C. These parameters reflect the intended use case, where the sample is used in system water for long periods of time.

Any method feature as described herein may also be provided as an apparatus, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently.

The invention extends to methods, system and apparatus substantially as herein described and/or as illustrated with reference to the accompanying figures.

One or more aspects will now be described, by way of example only and with reference to the accompanying drawings, in which:

Figure 1 shows a schematic of an example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system;

Figure 2 shows a schematic of another example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system, having a plurality of metal samples of different thicknesses;

Figure 3 shows a schematic of another example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system, having a replaceable metal sample;

Figure 4 shows a schematic of another example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system, having a light source and light sensor housed in a separate unit;

Figure 5 shows a schematic of another example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system, having a baffle between the light source and the light sensor;

Figure 6 shows a schematic of an example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system, having a second light sensor for directly sampling light emitted by the light source;

Figure 7 shows a schematic of another example embodiment of an optical sensor for detecting and/or monitoring corrosion in a water system, having a second light sensor for directly sampling light emitted by the light source;

Figure 8 shows a flow chart illustrating an exemplary method according to the present disclosure;

Figure 9A shows a schematic of a plan view of a metal sample for use in the sensor;

Figure 9B shows a side view of the metal sample for use in the sensor; and

Figure 10 shows a schematic of an arrangement of multiple metal samples in a sensing apparatus.

The Figures will now be described in more detail. In each case, similar elements are labelled with the same number. The devices presented operate along the same broad principles, and consequently the overall operation will not be described in detail in each case. Instead, the differences between each Figure will be emphasised, and it is to be understood that operational principles are generally transferrable between each Figure, except where this would cause a contradiction.

Figure 1 shows an example embodiment of an optical sensor 100 for detecting corrosion in a water system. The optical sensor 100 comprises a sensor housing 102, which, in use, is partially positioned within a water system. As shown, the sensor 100 is mounted to a pipe 104 of the system, which is filled with system water 106. The sensor includes a metal sample 112 (for example a thin film or foil of metal) in contact with the system water 106. In particular, a first surface of the metal sample 112 is in contact with the water 106 of the water system. The contact between the first surface of the metal sample 112 and the system water 106 leads to corrosion of the metal sample 112. As the metal sample 112 corrodes, corrosion debris and/or tarnishing will appear on a second surface opposite surface to the first surface (and separated from the first surface by the thickness of the metal sample 112), which will decrease the reflectivity of the surface. In some cases, the metal sample 112 will corrode away entirely in parts, for example due to pinhole corrosion progressing through the entire thickness of the sample 112, or due to uniform corrosion of such severity that the parts of the metal sample 112 are entirely corroded away.

The optical sensor 100 is configured to illuminate the metal sample 112 with a beam of light 110 emitted from a light source 108. The optical sensor 100 is also provided with an optical sensor 116 to receive the reflected light 114 and determine the intensity of that reflected light beam 114. The reflectivity of the metal sample 112 can be determined from the intensity of the reflected light beam 114 received, which is related to the amount of corrosion debris on the second surface of the metal sample 112, tarnishing due to contact with the system water 106 and outright missing metal. Consequently, the change in reflected light intensity is related to the amount (and in some cases, the type) of corrosion of the metal sample 112. Note that the optical sensor 116 can detect changes in reflected light in this way whether or not the reflection is diffuse or specular. In either case, the amount of reflected light changes and a change in received light intensity can be detected.

A common cause of corrosion in water system is dissolved oxygen within the system water 106. This causes corrosion of metals within the water system, as described above. The metal sample 112 is used as a sacrificial sample to monitor and/or detect corrosion. If the water 106 in the water system 102 is corrosive, for example it contains high levels of dissolved oxygen or other corrosive components (acids, bases, organic chemicals, etc.), then corrosion of the metal sample 112 can occur. This may take the form of pinhole corrosion, also known as pitting corrosion, where pinhole-sized holes form in a metal. Due to surface irregularities and imperfections, a small pit forms in the first surface in contact with the water due to pitting corrosion. Pinhole corrosion is particularly troubling because it results in very little loss of metal, so is hard to assess severity using weight loss or cumulative galvanic current studies, yet it causes damage to the deep structure of metal, and can corrode holes entirely through metal (e.g. through pipe walls, causing leaking). Worse still, pinhole corrosion is often obscured by corrosion debris such as tubercles or tarnishing, meaning that it is often not clear whether a tarnished portion represents mere surface corrosion or if one or more pinholes have caused much deeper problems.

The sensor 100 has an O-ring seal 118 to prevent system water 106 from entering the interior of the housing 102. This seal also helps to ensure that the system water 106 cannot contact the second surface of the metal sample 112 by leaking around the outer edge of the metal sample 112. The O-ring seal can be formed from any suitable material for making a watertight seal which can withstand the conditions found in the water system. For example, temperatures up to around 100°C, high pressures, and exposure times of 5 to 10 years. In addition, a transparent plate 120 is provided on the second surface of the metal sample 112, thereby ensuring that the system water 106 cannot contact the second surface, other than by corroding through the metal sample 112. The transparent plate 120 can be formed from plastics, glasses, etc. and be suitable for withstanding the temperatures and pressures set out above for the timescales of intended use. Since the metal sample is relatively thin (often thinner than the thickness of pipes, etc. in order to provide advanced warning of a corrosion problem), it may not be sufficiently strong to withstand the pressures in the water system. The transparent plate 120 can provide structural support to the metal sample 112. The transparent plate 120 does not affect the operation of the system, since (being transparent) light emitted 110 from the light source 108 travels through the transparent plate 120, reflects from the metal sample 112, travels back through the transparent plate

120 and is received by the light sensor 116. In any case, the sensor can be calibrated to ensure the range of output is indicative of 0% to 100% loss of material. In some cases, there may be no need for a transparent element 120 in this location, for example, where the metal sample 112 is strong enough to resist the system pressures, and where there is nothing on the interior of the housing 102 which would be adversely affected by system water 106 leaking into the housing 102, as would be the case if the metal sample 112 is penetrated by corrosion and there is no transparent element 120.

Since only the first surface of the metal sample 112 is exposed to the system water 106, the only way for the second surface to corrode (i.e. for tarnishing or loss of metal to show on the second surface) is for the corrosion to travel through the whole thickness of metal. As shown, the metal sample 112 has a uniform thickness, which means that detection of a pinhole corroding through the sample 112 is not dependent on the location of the corrosion. In other words, pitting corrosion occurring anywhere on the metal sample 112 must travel through the same thickness of metal in order to form a pinhole, and has the same effect on the reflected light beam 114.

As the metal sample 112 is chosen to be thin, pitting can form a small pinhole through the metal sample 112. Corrosion debris spreads from pinholes on the second surface of the metal sample 112. The corrosion debris decreases the reflectivity of the surface, which provides a detectable decrease in intensity of reflected light 114.

The metal sample 112 is chosen to be a metal typically found elsewhere in the water system 102. Typically, the metal sample 112 may be formed from carbon steel, copper, stainless steel, brass, or aluminium. By choosing a metal that is present in the water system, determining the amount of corrosion or rate of corrosion of that metal within the optical sensor 100 provides an indication of the corrosion of that metal elsewhere in the system.

The metal sample 112 is chosen to be a thin film such that corrosion occurs quicker than at sections of metal in the rest of the system 102. This allows a fast diagnosis of corrosion without waiting for corrosion of parts of the actual system to progress to unsafe levels. This can prevent significant damage to the system, savings costs by identifying corrosion without waiting for catastrophic failure.

The metal sample 112 is sealed by seals 118 such that the first surface of the metal sample 112 is exposed to the water while the second surface is not. The seals 118 prevent water from leaking around the metal sample 112 while it is held in position. For example, the seals 118 may be in the form of rubber seals, or a glue / resin between the metal sample 112 and the walls of the sensor housing 102. The seals 118 prevent the water from being in contact with the electronics and sensing apparatus, which will now be described.

The optical sensor 100 comprises a light source, for example the light source 108 in Figure 1 may be a light-emitting diode (LED) 108. The LED 108 is configured to emit light onto the metal sample 112. In particular, the LED 108 is configured to emit light onto the second surface of the metal sample 112. The second surface is the opposite surface to the surface in contact with the water. The emitted light is shown in Figure 1 by arrows 110. The metal sample 112 is reflective, and therefore reflects the light from the LED 108. The reflected light is shown by arrows 114.

In other examples, the light source 108 may be a laser, or a fluorescent light bulb or any other suitable light source.

The optical sensor 100 also comprises a light sensor 116, for example, the light sensor 116 shown in Figure 1 may be a photodetector. The photodetector 116 is configured to detect the light emitted from the LED 108. In particular, the photodetector 116 is configured to detect the reflected light 114 from the second surface of the metal sample 112.

The photodetector 116 is positioned adjacent to the LED 108 such that light from the LED 108 directed towards the metal sample 112 (light beam 110), which is then reflected (light beam 114) by the second surface of the metal sample 112, and is received by the photodetector 116. The photodetector 116 and the LED 108 are on the same side of the metal sample 112, facing the second surface of the metal sample 112. This allows the light source and the light sensor to be isolated from the water within the water system 102, for example by the housing 102 and the seals 118.

In some examples, the photodetector 116 is configured to not detect light emitted directly from the LED 108 that has not been reflected by the metal sample 112, that is no direct light path between the LED 108 and the photodetector 116 exists. For example, an opaque barrier may be placed between the LED 108 and the photodetector 116 to prevent stray light affecting the readings, as set out in more detail with regard to Figure

5. In other examples, the output of the photodetector 116 photodiode is calibrated to take into account the inner optical properties of the cavity including the reflectivity of the metal sample 112. In other words the photodetector 116 may be calibrated to remove the effects of the direct LED light on the readings, and only detect changes due to changing reflectivity of the metal sample 112. In yet further examples, the light source 108 and light sensor 116 may have relatively narrow angular ranges of emission and/or detection respectively, so that there is no direct transfer of light from the light source 108 to the light sensor 116.

The photodetector 116 is configured to transmit a signal corresponding to the detected light reflected by the metal sample 112. For example, the photodetector 116 outputs a signal which is related to the intensity of the detected light, in the form of an analogue signal. For example a current or voltage signal with its amplitude related to the received intensity. In other examples the sensor 100 may output a digital signal in a standardised format, such as Modbus.

The sensor 100 may further comprise a processor (not shown), configured to receive the signal from the light sensor 116 corresponding to the detected light reflected by the metal sample 112. For example, the light sensor 116 is configured to measure the intensity of the received light, and transmit the signal corresponding to the intensity towards the processor (which may be remote, for example for collating many measurements from a variety of sensors). The signal is transmitted to the processor via an electrical, wireless, fibre-optic, etc. connection.

The processor may comprise a data acquisition system for storing the received data from the light sensor 116. For example, the data acquisition system may be a data logger or other memory controllable by the processor.

The processor may be configured to process the data received from the light sensor 116. For example, the processor receives periodic measurements of the intensity from the light sensor 116. The processor is configured to convert the measurement signal from the light sensor 116 into intensity of light. The processor is configured to store the periodic intensity values in a table, and output to a user. For example, the data may be outputted to a display device for user interaction by displaying the table of values, or displaying a graph of the intensity values over time.

The processor is configured to detect changes in the values of intensity. For example, if the intensity of light decreases substantially over time, this may correspond to a decrease in the reflectivity of the metal sample 112. This decrease in reflectivity is caused by corrosion of holes through the metal sample 112 and/or subsequent spread of corrosion debris over the surface reducing the intensity of light reflected by the surface. This data may be extrapolated to estimate the time remaining until one or more pipes in the system corrode through and start to leak.

Upon detecting a decrease in intensity, the processor may be configured to trigger an alert. A threshold may be present to indicate corrosion of the metal sample 112. For example, a decrease from an initial high value of intensity by a certain amount, below a threshold, may trigger an alert. In another example, the threshold may be related to the gradient of the decreasing intensity, or a percentage decrease from a highest value of intensity. If a threshold is exceeded, an alert such as a message to a user may be triggered.

The processor may be configured to control the light source 108 and/or the light sensor 116. For example, the intensity or spectral range of light emitted by the light source 108 or the spectral range at which the light sensor 116 is most sensitive may be controlled electronically to gain more information about the type and severity of corrosion in the system.

Consider now Figure 2, which shows another example embodiment of an optical sensor, 200 having a plurality of metal samples 112. The sensor 200 is similar to that shown in Figure 1, but has three metal samples 112a, 112b, 112c. Each of these is formed from the same metal in Figure 2, and has a different (uniform) thickness. Each sample has its own respective light source 108a, 108b, 108c and light sensor 116a, 116b, 116c for detecting corrosion in the manner set out above. Since each metal sample 112 has a different thickness, knowledge of which sample has corroded provides information on the depth of metal which has corroded in the system and also the depth of metal which has not corroded. For example, if the thinnest metal sample 112a is

0.025mm thick, with the next sample 112b being 0.05mm thick and the thickest sample 112c being 0.075mm thick, then if the first sample 112a shows tarnishing or other signs of corrosion, while the other two 112b, 112c do not, then it can be inferred that the corrosion depth is between 0.025mm and 0.05mm. This determination can be performed locally using a processor (not shown), or the readings can be transmitted to a remote location for processing.

In some cases, instead of metals of different thicknesses, the multiple samples 112 may be formed from different metals and/or alloys. This can provide a measure of how different metals present in the system are faring under exposure to system water 106. Of course, in some cases, there may be a combination of different metals and different thicknesses to provide an estimate of the maximum pitting corrosion depth in several metals at once.

While each metal sample 112 is shown with a corresponding light source 108 and light sensor 116, in some cases a single light source 108 may be configured to shine light onto a plurality of metal samples 112, and/or a single light sensor 116 may be configured to receive light from a plurality of metal samples 112. Thus, associating many samples 112 with a single housing 102 as shown in Figure 2 may allow a reduction in the amount of electronics in the system, thereby saving costs.

The plurality of metal samples 112 may be formed from the same metal as one another, wherein each of the plurality of metal samples 112 has a different thickness to the thickness of the other metal samples 112. This can allow corrosion of different types of metal to be monitored by providing samples 112 of different metals. In other cases, a series of samples of different thicknesses may be provided. This can provide a more compact sensor than providing a separate sensor for each thickness of metal and/or each different metal type. Of course, an alternative arrangement is to provide a separate sensor 100 such as that in Figure 1 for each metal and/or thickness. Where a single sensor 200 has a plurality of metal samples 112, the sensor 200 may share various measurement components. For example, a single light source 108 (or in general fewer light sources 108 than there are samples) can be arranged to shine onto the second surface of multiple metal samples 112, thereby ensuring that each sample 112 is illuminated with the same light, so allowing a meaningful comparison of the received light 114 intensity, as the light received 110 by the second surface of each sample 112 can be controlled to be the same in each case. In other cases, there may be a single light sensor 116 and multiple light sources 108. For example, each light source 108 may emit light 110 in a relatively narrow wavelength range and the sensor 116 may be configured to detect the received intensity broken down by received wavelength, thereby allowing the sensor 116 to correlate intensity in a wavelength with a particular sample (wavelength division multiplexing), and thus monitor several samples 112 using only one sensor 116, or more generally using fewer sensors 116 than there are samples. In yet another case, other aspects such as processors for interpreting the received intensity data or housings 102 for enclosing the sensors 116 may be shared between multiple metal samples 112. An alternative method of sharing light sources 108 and light sensors 116 is to use a highly directed light source 108 and/or light sensor 116 and illuminate the second planar surface of different samples 112 at different times (time division multiplexing), and synchronise the sensor 116 with the light source 108, so that light received 114 at a given time is correlated with a given metal sample 112. In yet further examples, baffles such as that shown in Figure 5 (element 134) may be used to prevent direct transmission from a light source 108 to a light sensor 116, and also to prevent transmission of light from one “sensing unit” (light source 108, metal sample 112 and light sensor 116) to another sensing unit, i.e. to prevent cross talk between measurements on each sample 112.

Turning now to Figure 3, a further example sensor 300 is shown, which has a bypass arrangement. The sensor 300 operates in a broadly identical manner to that shown in Figure 1. In this case, however, the sensor 300 is mounted on a section of pipe 104 having two flow paths in parallel. The main pipe 104 branches into a sensor section 104a having the sensor 300 mounted to it, and a bypass section 104b, having no sensor. System water 106 is able to flow through the main pipe, and then either through the sensor section 104a or the bypass section 104b. The water 106 is controlled by two valves, each having a first position 122 where they block water from flowing into the bypass section 104a and a second position 124 where they block water from flowing into the sensor section 104b. Thus, where water is intended to flow through the sensor section 104a, the valves are each set to their first positions 122, and water is blocked from flowing into the bypass section 104b and instead flows past the sensor 300 via the main section 104a of pipe. Similarly, the water 106 can be arranged to miss out the sensor section 104a of pipe by diverting water 106 through the bypass section 104b or pipe, which is achieved by setting the valves to their second positions 124. In some cases, the two valves can be connected such that triggering one valve to change between its first 122 and second 124 positions causes the other one to change. That is, rather than having each valve being independently controllable (leading to four distinct configurations, two of which are effectively to block flow through the element entirely), there are simply two configurations: bypass and sensor.

After traversing the sensor 104a or bypass 104b section of pipe the system water 106 re-joins the main pipe 104. The sensor may be supplied with such a pipe section, in which case the main pipe 104 may be provided with solderable joints, screw threads, etc. for connecting into a main water system.

In any case, the purpose of this arrangement is to provide a means for installing the sensor and for replacing the metal sample 112. For example such a pipe arrangement may be fitted to the water system, and the valves set to their second positions 124 so that system water 106 flows through the bypass section 104b, and not through the sensor section 104a. In some cases, no sensor 300 need be fit to the sensor section during installation. For example, this pipe arrangement may be installed with a view to fitting a sensor at a later date. Since the system water flows through the bypass section 104bm there is no danger that the system water will leak out of a hole in the sensor section 104a of the pipe, since there is no water flow through that section. In other cases, the sensor section 104a may be provided with a cover for plugging the hole where the sensor 300 is intended to be installed.

In any case, when a sensor is to be fit to the system, the valves are set to their second positions 124 to isolate the sensor mounting hole. It may be necessary at this stage to drain the sensor section 104a of any system water 106 in the sensor section. In other examples, it may be desirable not to drain the sensor section 104a, e.g. to avoid introducing air into the system, which can reduce pumping efficiency and increase the rate of corrosion. In any case, once the sensor section 104a no longer contains water under pressure, any cap can be removed from the hole for mounting the sensor 300 and the sensor 300 slotted into place. The sensor can be sealed in place using any suitable means such as using O-rings and clamps, screw threads, etc. In some cases, solder or welding may be used, but this can make removal of the sensor 300 e.g. for replacement of the metal sample 112, difficult.

Once the sensor 300 has been mounted to the sensor section 104a, the system water can be directed back though the sensor section 104a by setting both valves to their first positions 122. This causes system water 106 to flow past the metal sample 112 and thereby provides an in-situ measurement of corrosion in the water system.

In case the metal sample 112 needs replacing, for example if it corrodes entirely away, or simply to the point where it is difficult to gain any further information on the corrosion state of the sample 112, a corresponding process can be followed:

1. Set both valves to their second positions 124 to direct system water 106 down the bypass pipe 104b.

2. Remove the sensor 300 from the sensor pipe 104b.

3. Replace the metal sample 112 (and/or clean the sensor 300, perform maintenance, etc.)

4. Replace the sensor 300 in the sensor section 104a and seal the sensor 300 in place.

5. Set both valves to their first positions 122 to direct system water 106 down the sensor pipe section 104a.

The sample 112 may be held to the sensor 300 using a screw thread, which can help to press the seal 118 to form a watertight fit.

Consider now Figure 4. This shows a sensor arrangement 400 similar to that shown in Figure 1. Here, however, the housing 102 does not contain the light source 108 or the light sensor 116. Instead, the light source 108 and the light sensor 116 are housed in a second housing 126, separated from the housing 102. A fibre optic cable 128 transmits the light from the light source 108 to the interior of the housing and, once the light has reflected from the metal sample 112, another fibre optic cable transmits the light back to the light sensor 116. This arrangement means that even in the event that system water leaks into the housing 102, the electronic parts of the sensor 400 are protected from damage due to system water 106.

This design is sealed from the system water 106 by double O-ring seals 118. One seal is located in contact with the metal sample 112 and the system water 106. The second seal is located behind the transparent plate 120. This means that even in the case where the system water has corroded through the entire thickness of the metal sample 112, system water 106 is prevented from entering the housing 102 by the second seal 118 forming a seal with the transparent plate 120 and the housing 102. It should be borne in mind that the Figures are only schematic. For example, in order to provide uniform light intensity on the second surface of the metal sample 112, it may be preferable for the fibre optic cable 128 which transmits light from the light source 108 to the interior of the housing 102 to occupy substantially all of the upper surface of the housing 102, and the other fibre optic cable 128 to take up less of the upper surface of the housing 102, or to exit the housing 102 from a different surface. In other words, the fibre optic cables 128 may be differently sized relative to each other and the housings 102, 126 than they appear in Figure 4, which is not to scale.

In Figure 5, yet another sensor apparatus 500 is shown, in this case using a lens 130 to focus light on the second surface of the metal sample 112. Similarly to the sensing apparatuses described above, the sensor 500 in Figure 5 has a housing 102 containing a light source 108 and a light sensor 116. The light 110 emitted from the light source 108 is directed not directly towards the metal sample 112, but instead to a lens 130. The lens 130 directs the emitted light 110 towards the second planar surface of the metal sample 112. The lens can be configured to redirect the emitted light 110 in such a way as to provide a homogenous light intensity over the entire second surface of the metal sample 112. In other words, the lens can be used to smooth out any inhomogeneities in the emitted light 110 to ensure that the sensor detects approximately the same loss of reflected light intensity irrespective of the location at which corrosion occurs on the metal sample 112.

The light output 132 from lens 130 is directed to the metal sample 112, whereupon it is reflected 132 and enters the lens 130 again (in some cases, there may be a second lens for receiving the reflected light). The lens 130 directs the light towards the light sensor 116. This can help to ensure that as much of the reflected light 114 as possible is directed to the light sensor 116, thereby improving the sensitivity of the sensor 500 to small changes in reflectivity. In some cases, the lens 130 may be replaced with one or more prisms, fibre optic arrangements, mirrors, etc. or a combination of these to achieve the same effect.

Additionally, the sensing apparatus 500 has a baffle 134 to block direct transmission of light 110 emitted from the light source 108 to the light sensor 116. This reduces the occurrence of false negatives, as it prevents situations in which the reflected light 114 is reduced due to corrosion, but this is not detected by the light sensor 116 because the signal is swamped by directly transmitted light.

In some cases, it may be advantageous to combine the features shown in Figures 4 and 5, for example to provide a sensor with both a lens 130 and fibre optic cables 128 running to a second housing 126. As noted above the fibre optic cable 128 transmitting light to the housing 102 should be relatively large to ensure that the light received by second surface of the metal sample 112, is broadly uniform. An alternative way to achieve this is to mount a lens 130 inside the housing 102, as shown in Figure 5, to provide a more even distribution of light emitted by the fibre optic cable 128 onto the second surface of the metal sample 112.

In Figure 6, a further example of a sensing apparatus 600 is shown. This example has a light source 108 which is housed in a mounting 140 attached to the internal walls of the housing 102. The mounting 140 has a recess housing the light source 108, so that the mounting 140 operates a little like the baffle 134 shown in Figure

5.

The light source 108 in Figure 6 is arranged to emit light into a fibre-optic bundle 136. A first portion 136a of the fibre optic bundle is directed towards a reference sensor 138, the reference sensor also being housed in the mounting 140. The first portion 136a may be only a single fibre of the fibre-optic bundle 136, or it may be a plurality of fibres. The first portion 136a directs a portion of the light emitted by the light source 108 to the reference sensor 138. A second portion 136b of the fibre optic bundle (in this case, the entire remainder of the bundle 136) transmits the light towards the metal sample 112. In other words, the end of second portion 136b of the bundle 136 which directs light towards the metal sample 112 acts a little like the light source 108 in other examples, in the sense that the second portion 136b of the bundle emits light 110 towards the metal sample 112. This light is reflected and received and the intensity interpreted in the manner described above.

The reference sensor 138 receives light directly from the light source 108, so detects any variations in the emitted light intensity and/or spectral range without the emitted light 110 being affected by environmental factors, such as the change in reflectivity of the metal sample 112. This means that a drop in intensity of the reflected light 114 detected by the light sensor 116 can be correlated with the intensity measured by the reference sensor 138. Where the intensity of the emitted light 110 drops, the reference sensor 138 and the light sensor 116 will both detect a drop of approximately the same magnitude as each other, indicating a false positive which can be discounted. In cases where the light sensor 116 detects a change in the intensity of reflected light 114 which does not correlate with a drop in emitted light 110 detected by the reference sensor 138, or where the light sensor 116 detects a drop larger than would be expected for a corresponding drop detected by the reference sensor 138, then the event can be logged as representing corrosion.

Figure 7 shows another example of a sensing apparatus 700 having a reference sensor 138. In this case, however, rather than using a fibre-optic bundle, the reference sensor 138 is provided with a reference beam of light 110a by virtue of a beam splitter 142. As before, the light source 108 emits a light beam 110. This is directed towards the beam splitter 142 which splits the emitted light beam 110 into two beams 110a and 110b. The first of these beams 110a is directed to the reference sensor 138 and the reference sensor 138 operates in much the same manner as discussed above in relation to Figure 6. The second beam 110b is directed towards the second surface of the metal sample 112 and the beam is then reflected as a reflected beam 114 and enters the light sensor 116 in the manner set out above. Similarly to the situation described in respect of Figure 6, the use of a reference sensor 138 allows variations in the intensity and/or spectral range of the emitted light 110 to be accounted for, so improving the reliability of measurements provided by the sensing apparatus 700.

The beam splitter 142 can be selected so that it splits any proportion of the light to the reference sensor 138 as desired. Typically, only a small proportion of light (e.g. no more than 20%) should be diverted for reference sampling, so that the actual measurement is performed with a reasonable intensity of light. Naturally, this reduced absolute magnitude of the reference measurement can be adjusted to account for the smaller proportion of light being received by the reference sensor 138.

Consider now Figure 8 which shows a flow chart 800 describing the operation of the method as set out herein. The method starts at step 150 where a metal sample is mounted in a water system, the metal sample having a uniform thickness and including a first planar surface and a second planar surface opposite the first planar surface, wherein the first planar surface is arranged in contact with water of the water system. As noted above, this exposes only one surface of the metal sample to the system water, and consequently allows a determination of the extent and sometimes the type of corrosion by monitoring corrosion which penetrates throughout the uniform thickness of the metal sample.

The method continues at step 152 in which light is emitted towards the second planar surface of the metal sample. As noted above, a light source may be supplied to emit the light, and is arranged to direct the emitted light towards the second planar surface. The light source may be adjustable in the sense that the intensity or spectral composition of the emitted light may be adjusted or controlled as part of the method. Indeed, the light source may be controlled in the sense that it emits no light most of the time, and is switched on periodically to emit light and provide a reading.

Next, at step 154, the light reflected by the second planar surface of the metal sample is received, for example at a light sensor. The light reflects from the second planar surface in a known way, for example changing intensity and/or spectral range in a known way in response to an entirely clean surface, and changing intensity and/or spectral range in a known way, different to the first known way, in response to tarnishing or other signs of corrosion. The received light is analysed by factoring in the difference between the known composition of reflected light from a clean surface and the known composition of reflected light from a tarnished and/or otherwise corroded surface to arrive at an indication that the surface is showing signs of corrosion. Optionally, the sensor can determine the extent and/or type of corrosion. The sensor can be configured to synchronise with the light source to only detect reflected light at times when the light source is on, thereby reducing power consumption. Indeed, the signals from the sensor can also be analysed only when the light source is on, in order that resources are not used analysing data when no light is supplied to the metal sample. In other examples, the spectral range and/or intensity of the sensor are/is selected to conform to the expected intensity and/or spectral range of light emitted by the light source and reflected from the metal sample. Optionally, as set out above, the method may make use of a reference sensor to normalise the measurement and help rule out false positive results.

Next, at step 156 the system (e.g. the light sensor) generates a signal indicative of the intensity of the reflected light. This may be an analogue signal, e.g. where the magnitude of a current or voltage output by the sensor is representative of the received light intensity. In other cases, the output may be digitised, for example to encode light levels as a digital signal, optionally, wherein different wavelength bands are separately encoded to allow for a spectral analysis. In one example, this may include sending digitised intensity values for each of a red, green and blue (RGB) band, similarly to how digital images are stored. In the RGB system, it is common for the peak intensity of the bands to be located at wavelengths of approximately: Red: 650nm; Green: 525nm; Blue: 440nm, for example. In other cases, different spectral bands may be used, as appropriate, including more than three bands, for example or extending beyond the visible range of the electromagnetic spectrum.

In step 158 the intensity of the reflected light is correlated with corrosion of the metal sample. This step relates, as set out above, a change in light intensity with the onset or progression of corrosion in the metal sample. Such a determination can be used to e.g. provide an alert to a maintenance team that pipes of a certain thickness and made from a particular metal are likely to fail soon and should be replaced.

Modifications to the general method 800 set out in Figure 8 may be made, for example by using some of the features of the examples set out respect of the other Figures.

Consider now Figures 9A and 9B, which show a plan and side view respectively of a metal sample system 900 for use in the sensors described herein. The sample system 900 comprises a metal disc 112, having a layer of corrosion-resistant material 144 having an annular shape around the edges of the metal disc 112. As shown in e.g. Figure 1, where a metal sample 112 is mounted on a sensing apparatus 100, there is a portion at the edges where a seal 118 contacts the metal sample 112. It has been found that the presence of the seal disproportionately increases the rate of corrosion near to the point of contact between the seal 118 and the metal sample 112. The corrosionresistant coating 144 is positioned to align with the region where the seal 118 contacts the metal sample 112. The corrosion-resistant coating may be wider than the footprint of the seal 118 to allow a user to position the sample 112 and the seal 118 with a reasonably low degree of accuracy. There is a balance between allowing a user to be inaccurate in their positioning of the sample and ensuring that a reasonable area of metal is exposed to the system water.

As shown in Figure 9B, the metal sample 112 may be supplied with a corresponding transparent disc 120. This may, for example, be adhered to the metal disc (using a compatible and transparent adhesive), or it may be supplied loose for holding in place by the clamping action of the seals 118. Indeed, since the metal sample systems 900 are intended to be replaceable, once a sensing apparatus has been sold, users may find it useful to buy replacement metal sample systems only. Since the transparent element 120 does not usually become corroded or damaged, the metal sample 112 with its corrosion-resistant coating 144 may be supplied as a separate element that is the metal sample 112 may be supplied without a transparent element 120.

As an example, the metal disc 112 may be formed from a metal representative of metals in the water system into which the metal disc 112 is to be mounted, for example, carbon steel, aluminium, brass, copper, stainless steel, etc. The disc 112 may be approximately 10 to 24mm in diameter, and the corrosion-resistant coating 144 may extend inwardly from the edge by approximately 3mm. The metal disc 112 may be supplied in various thicknesses, for example 0.025mm, 0.05mm, 0.075mm, 0.01mm, etc.

Turning to Figure 10, which shows an arrangement for multiple metal samples

112 in a sensing apparatus 200. Here a plurality of metal samples 112 is arranged in a grid. Horizontal rows are all formed from the same metal, and are shown separated by dashed lines simply to guide the eye. A first set of metal samples 112a is formed from a first metal representative of metals in the system, for example copper. A second set of metal samples 112b is formed from a second metal representative of metals in the system, for example brass. A third set of metal samples 112c is formed from a third metal representative of metals in the system, for example aluminium. A fourth set of metal samples 112d is formed from a fourth metal representative of metals in the system, for example stainless steel. An arrow (A) shows the direction in which thickness increases for each set of metal samples 112a - 112d. In some cases, for example, the first metal sample 112 (the leftmost sample) in each set of samples 112a - 112d may have the same thickness, e.g. 0.025mm. The next samples moving to the right in the direction of arrow (A) may have respectively thicknesses of 0.05mm, 0.075mm and 0.1mm. In other examples, the metals in each set of metal samples 112a - 112d may have different thicknesses, for example based on the likelihood of corrosion of that metal type, the typical thicknesses of components made from that metal, and the degree of accuracy required for that metal. In general, metals which corrode quicker may have thicker metal sample 112 sizes than metals which corrode slower. Similarly, where components tend to be made thicker (or tend to be able to withstand severe pitting), thicker metal samples 112 may be chosen. As noted above, the difference in thickness between adjacent metal samples 112 is related to the accuracy with which the pitting depth can be determined. Therefore, the progression of thickness of samples 112 can be selected to provide the desired resolution for that metal.

While four thicknesses of each type of metal are shown in the Figure, some cases may have more or fewer samples 112. Indeed, in some case each metal type may have a different number of thicknesses, depending on the information a user wishes to obtain. While four metal types are shown in this example, different examples may provide more or fewer metal types.

While not shown here, the sensing apparatus has the features described above for making the measurement, such as at least one light source 108, at least one light sensor 116, etc. Multiplexing may be used as described above to provide fewer light sources 108 and/or light sensors 116 than the number of metal samples 112 (i.e. fewer than 16 light sources 108 and/or sensors 116 in the example shown in Figure 10).

In any of the above examples of sensing apparatuses, internal walls of the housing may have diffuse reflective inner surfaces for providing an optical integrating cavity. Optical integrating cavities are chosen so that all light emitted by the light source eventually reaches the light sensor. They have the effect of smoothing out the light intensity in the interior of the housing so that each portion receives approximately the same light intensity. This prevents inhomogeneities in the angular distribution of light emitted from the light source from causing different parts of the sample to be illuminated with different intensities of light and ensures that the background light levels are highly consistent. Such a situation could lead to different parts of the sample showing different effects when corrosion occurs, if the inhomogeneities are severe enough. Of course, providing a highly homogeneous light source is another solution to this. Where the internal walls of the cavity are said to be diffuse reflective surfaces, this does not necessarily apply to the second planar surface of the metal sample, and certainly does not apply to the transparent element, where such an element is present.

It will be appreciated from the above description above that many features of the different examples are interchangeable with one another. The disclosure extends to further examples comprising features from different examples combined together in ways not specifically mentioned. Indeed, there are many features presented in the above examples and it will be apparent to the skilled person that these may be advantageously combined with one another. Examples of features which are combinable with other features in this way include: the baffle 134 shown in Figure 5; the use of multiple metal samples 112 in a single sensing apparatus as shown in Figure 2; the use of multiplexing with multiple samples to reduce the number of light sources and/or sensors in an apparatus; the use of the double O-ring seal arrangement of Figures 4 to 7 for improving the seal; the use of fibre optic cables 128 as in Figure 4 to mount the light source 108 and light sensor 116 in a second housing 126; and the use of a lens 130 as shown in Figure 5.

While the present invention is defined by the appended claims, further examples of the disclosure of this application are set out in the following clauses, which should not be confused with the claims which follow these clauses:

(a) An optical sensing apparatus for mounting in a water system and for monitoring corrosion in the water system, comprising:

a metal sample having a uniform thickness and a first planar surface and a second planar surface opposite the first planar surface, wherein the first planar surface is arranged to be in contact with water within the water system;

a light source configured to emit light towards the second planar surface of the metal sample; and a light sensor configured to receive light reflected by the second planar surface of the metal sample, and output a signal indicative of the intensity of the reflected light.

(b) An optical sensing apparatus according to clause (a), further comprising a transparent element disposed at least partly between the light source and the metal sample.

(c) An optical sensing apparatus according to clause (b), wherein the transparent element has a third planar surface arranged adjacent to the second planar surface of the metal sample.

(d) An optical sensing apparatus according to any preceding clause, further comprising a seal for protecting the second planar surface from water in the water system.

(e) An optical sensing apparatus according to clause (d), wherein the seal is located adjacent to the first planar surface and wherein the metal sample is provided with a corrosion resistant coating on the first planar surface in the vicinity of the seal.

(f) An optical sensing apparatus according to any preceding clause, wherein the metal sample is representative of one or more metals used in the water system.

(g) An optical sensing apparatus according to any preceding clause, wherein the metal sample is a film having a thickness of 1 mm or less.

(h) An optical sensing apparatus according to any preceding clause, comprising a plurality of metal samples.

(i) An optical sensing apparatus according to clause (h), wherein the plurality of metal samples comprise metal samples of different thicknesses.

(j) An optical sensing apparatus according to clause (h) or clause (i), wherein the plurality of metal samples comprise metal samples of different metals.

(k) An optical sensing apparatus according to any one of clauses (h) to (j), wherein the plurality of metal samples includes N metal samples and wherein the sensing apparatus comprising fewer than N light sources and/or fewer than N light sensors.

(l) An optical sensing apparatus according to clause (k), wherein the apparatus uses multiplexing to correlate the received light with the plurality of metal samples.

(m) An optical sensing apparatus according to clause (I), wherein the multiplexing includes one or more of:

time division multiplexing;

wavelength or frequency division multiplexing;

spatial division multiplexing; and/or polarisation division multiplexing.

(n) An optical sensing apparatus according to any preceding clause, wherein the or each metal sample is replaceable.

(o) An optical sensing apparatus according to any preceding clause, further comprising a processor configured to receive the signal indicative of the intensity of the reflected light from the light sensor.

(p) An optical sensing apparatus according to clause (o), wherein the processor is configured to relate the signal indicative of the intensity of the reflected light to corrosion of the metal sample.

(q) An optical sensing apparatus according to clause (o) or clause (p), wherein the light sensor is configured to output a plurality of signals indicative of the intensity of the reflected light over time and wherein the processor is configured to determine a rate of corrosion of the metal sample from the plurality of signals indicative of the intensity of the reflected light over time received from the light sensor.

(r) An optical sensor according to any one of clauses (o) to (q), wherein the processor is configured to receive a plurality of signals indicative of an amount or a rate of corrosion of a corresponding plurality of metal samples.

(s) An optical sensor according to clause (r), wherein the plurality of metal samples are formed from the same metal as one another, wherein each of the plurality of metal samples has a different thickness and wherein the processor is configured to determine a range of maximum pinhole corrosion depths in the water system from the plurality of received signals.

(t) An optical sensing apparatus according to any preceding clause, further comprising an optical element configured to direct light emitted by the light source towards the second planar surface of the metal sample, and/or configured to direct the reflected light towards the light sensor.

(u) An optical sensing apparatus according to any preceding clause wherein the light source and the light sensor are mounted in a housing forming a closed opaque cavity.

(v) An optical sensing apparatus according to clause (u), wherein internal walls of the housing have diffuse reflective inner surfaces for providing an optical integrating cavity.

(w) An optical sensing apparatus according to any preceding clause, further comprising a baffle for blocking direct light transmission between the light source and the light sensor.

(x) An optical sensing apparatus according to any preceding clause, further comprising a second light sensor for directly sampling the light emitted from the light source to provide a reference value.

(y) An optical sensing apparatus according to any preceding clause, wherein the light emitted by the light source has a spectrum selected based on the metal from which the metal sample is made and/or the expected corrosion mode of the metal from which the metal sample is made.

(z) An optical sensing apparatus according to any preceding clause, further comprising control electronics for controlling the light source and/or the light sensor.

(aa) An optical sensing apparatus according to any preceding clause, further comprising an electrical power source for providing electrical power to the light source and the light sensor.

(bb) An apparatus for monitoring a plurality of parameters for monitoring overall system health in a water system, comprising one or more of the sensors of any preceding clause.

(cc) A method of monitoring corrosion in a water system, the method comprising:

mounting a metal sample in a water system, the metal sample having a uniform thickness and including a first planar surface and a second planar surface opposite the first planar surface, wherein the first planar surface is arranged in contact with water of the water system;

emitting light towards the second planar surface of the metal sample;

receiving light reflected by the second planar surface of the metal sample at a light sensor;

generating a signal indicative of the intensity of the reflected light; and correlating the intensity of the reflected light to corrosion of the metal sample.

(dd) A method according to clause (cc), further comprising generating a plurality of signals indicative of the intensity of the reflected light over time, and determining a rate of corrosion from the plurality of intensities of the reflected light over time.

(ee) The method according to clause (cc) or clause (dd), wherein the light is emitted from a light source and wherein the light source and the light sensor are mounted in a housing forming a closed opaque cavity.

(ff) The method according to clause (ee), wherein internal walls of the housing have diffuse reflective inner surfaces for providing an optical integrating cavity.

(gg) The method according to clause (ee) or clause (ft), further comprising a baffle for blocking direct light transmission between the light source and the light sensor.

(hh) The method according to any one of clauses (cc) to (gg), further comprising an optical element configured to direct light emitted by the light source towards the second planar surface of the metal sample, and/or configured to direct the reflected light towards the light sensor.

(ii) The method according to any one of clauses (cc) to (hh), further comprising a second light sensor for directly sampling the light emitted from the light source to provide a reference value, wherein correlating the intensity of the reflected light to corrosion of the metal sample includes comparing the reference value to the received light reflected from the second planar surface.

(jj) The method according to any one of clauses (cc) to (ii) wherein a plurality of metal samples are mounted in the water system and wherein each metal sample has a uniform thickness and includes a first planar surface and a second planar surface, wherein the first planar surface of each sample is arranged in contact with water of the water system and wherein:

light is emitted towards the second planar surface of each metal sample; light is reflected by the second planar surface of each metal sample and received by a sensor;

a signal is generated corresponding to each metal sample, the signal being indicative of the intensity of the reflected light from each metal sample; and corrosion of each metal sample is correlated with the intensity of the reflected light.

(kk) An optical sensing apparatus according to clause (jj), wherein the plurality of metal samples includes N metal samples and wherein the sensing apparatus comprising fewer than N light sources and/or fewer than N light sensors.

(II) An optical sensing apparatus according to clause (kk), wherein the apparatus uses multiplexing to correlate the received light with the plurality of metal samples.

(mm) An optical sensing apparatus according to clause (II), wherein the multiplexing includes one or more of:

time division multiplexing;

wavelength or frequency division multiplexing;

spatial division multiplexing; and/or polarisation division multiplexing.

(nn) The method according to any one of clauses (jj) to (mm), wherein the plurality of metal samples are formed from the same metal as one another, wherein each of the plurality of metal samples has a different thickness to the thickness of the other metal samples.

(oo) The method according to any one of clauses (jj) to (nn), further comprising determining a range of maximum pinhole corrosion depths in the water system from the each of the signals indicative of the intensity of the reflected light from each metal sample.

(pp) The method according to any one of clauses (cc) to (oo), wherein the metal sample is removeable and/or replaceable and wherein the method includes removing and/or replacing the metal sample.

(qq) The method according to any one of clauses (cc) to (pp), further comprising controlling one or more of:

power;

intensity; and/or spectral weight of the emitted light.

Claims

1. A sample for use in a sensor for monitoring optical corrosion in a water system, the sample comprising:

a metal element having:

a first surface for exposure to the water of the water system; and a second surface opposite the first surface for receiving and reflecting light; wherein a portion of the first surface is provided with a corrosion-resistant coating for providing a location for forming a seal between the sample and the sensor.

2. The sample of claim 1, wherein the metal element is planar and/or has a uniform thickness.

3. The sample of claim 1 or claim 2, wherein the corrosion-resistant coating is applied to the edges of the metal element.

4. The sample of claim 3, wherein the corrosion-resistant coating extends between 0.5mm and 5mm inward from the edge of the first surface.

5. The sample of any one of the preceding claims, wherein the metal element is disc shaped.

6. The sample of claim 5 as dependent on one of claims 3 or 4, wherein the corrosion resistant coating is annular.

7. The sample of any one of the preceding claims, wherein the metal element has a width of between 10mm and 50mm

8. The sample of any one of the preceding claims, wherein the metal element is formed form stainless steel, copper, brass, aluminium, or other materials representative of metals in the water system.

9. The sample of any one of the preceding claims, wherein the corrosionresistant coating covers no more than 20% of the first surface.

10. The sample of any one of the preceding claims, wherein the corrosion resistant coating is stable for at least 5 years when submerged in water of the water system.

5

11. The sample of any one of the preceding claims, wherein the corrosionresistant coating is stable when submerged in system water of at least 85°C.

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5 11. The sample of any one of the preceding claims, wherein the corrosionresistant coating is stable when submerged in system water of at least 85°C.

Amendments to the claims are as follows:

1. A sample for use in an optical sensor for monitoring corrosion in a water system, the sample comprising:

a metal element having:

5 a first surface for exposure to the water of the water system; and a second surface opposite the first surface for receiving and reflecting light; wherein a portion of the first surface is provided with a corrosion-resistant coating for providing a location for forming a seal between the sample and the sensor.

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3. The sample of claim 1 or claim 2, wherein the corrosion-resistant coating

15 is applied to the edges of the metal element.

20 5. The sample of any one of the preceding claims, wherein the metal element is disc shaped.

7. The sample of any one of the preceding claims, wherein the metal element has a width of between 10mm and 50mm.

8. The sample of any one of the preceding claims, wherein the metal

30 element is formed from stainless steel, carbon steel, copper, brass, aluminium, or other materials representative of metals in the water system.