ES2695247A1 - SYSTEM AND METHOD OF MONITORING THE STATE OF A FLUID (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

System and method of monitoring the state of a fluid. A monitoring system (3, 13) for the inspection of a fluid (2, 71, 72) contained in a tank (1) by inserting it in a socket (5) of the tank (1), which comprises: an area of measure (20, 120) configured to circulate therein a sample of said fluid (2, 71, 72); light emitting/receiving means (41) consisting of a lighting system (411) and a light detection system (412) located on the same side of the monitoring system (3, 13) with respect to said area of measurement (20, 120); an optical window (44) disposed between said light emitting/receiving means (41) and said measuring area (20, 120); and a rear optical element (45, 451, 452, 453) located on the other side of the monitoring system (3, 13) with respect to said measurement area (20, 120). The lighting system (411) is configured to emit optical radiation to said measurement zone (20, 120), and the light detection system (412) is configured to detect an optical radiation comprising the light (i) reflected by said fluid (2, 71, 72) and/or light (ii) transmitted through said fluid (2, 71, 72) and reflected in said rear optical element (45, 451, 452, 453). The monitoring system (3, 13) further comprises an electronic subsystem (150) comprising processing means (51) configured to control the activation/deactivation of the lighting system (411) and to process the signals obtained from the detector system of light (412). (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

SYSTEM AND METHOD OF MONITORING THE STATE OF A FLUID

FIELD OF THE INVENTION

The present invention belongs to the field of fluid monitoring methods and systems, such as oils, and in particular lubricating oils, to determine their general state (degradation, content of particles, etc.). More specifically, the invention pertains to the field of the measurement of the state of fluids, such as oils, by colorimetry or optical spectroscopy.

BACKGROUND OF THE INVENTION

The industrial machinery often uses lubricating fluids for the correct functioning of the components of the machine in question. Examples of these fluids include lubricants and oils that may be based on hydrocarbons, synthetic and / or petroleum based products, as well as hydraulic fluids. These fluids must be maintained within a preferred range of composition and cleanliness for efficient machine performance. For example, the unwanted addition of water or waste can cause the machine to lose efficiency or suffer damage. That is, the industrial machinery often suffers failures or unexpected interruptions caused by problems associated with lubrication. These failures or interruptions can reduce the service life of the machinery, as well as unnecessary maintenance costs. It is therefore necessary to monitor the fluid (normally, oil) used for the lubrication and to determine the state of said fluid.

A conventional way of monitoring the condition of the lubricating oil is by "off-line" measurement, that is, by analyzing oil samples in the laboratory, however, the "off-line" techniques do not provide a sufficiently early detection of the process. degradation because they are not performed often enough due to the human and material effort that the sampling and analysis of these samples requires. For example, it is common that when taking the sample the lubricant is mixed with sediment, complicating the control of the oil. It can also happen that the sampling requires the machine to stop or even to empty the lubricant, causing a loss of production of the machine.

To overcome the inherent drawbacks of "off-line" analysis techniques, "on-line" techniques have been developed to analyze the state of the fluid in question during its own operation, in motion, without the need to extract samples from it for its subsequent analysis and without temporary loss of production. For example, the Spanish patent ES2455465B1 describes a system for inspecting the degradation of an "on-line" lubricating fluid in a measuring cell by means of an optical sensor.For the "on-line" inspection, the system needs external elements of flow control to stop the fluid, deaerate the measuring cell and ensure that the fluid sample does not present bubbles. In addition, this system requires a pressure difference between the input and output of the sensor, which limits its use to bypass configurations and installations.

In another example, the international patent application WO2012032197A1 describes a system for detecting the degradation of an oil from an analysis of its spectral absorption characteristics. This system also needs to be installed in bypass in a lubrication system, immersed in the fluid under inspection.

However, there are scenarios in which it is not possible to perform monitoring in bypass mode. Such is the case, for example, of environments with little measuring space, such as retro-fitting systems, not designed to be instrumented with sensors. Or environments in which, at most, there is a single point of sampling. Examples of this type are some lubricated mechatronic systems, such as robots and cranes. Many of these systems require the monitoring of thousands of points of interest in a plant. For example, in a car production plant, each of the many robots has two or three hydraulic systems to be monitored. Or, in an automotive assembly plant, each articulation of each robot (of which there may be thousands) incorporates a gear system that in turn incorporates its own lubrication microsystem (approximately one liter of oil). To monitor a system of such magnitude, it is desirable to be able to use low cost systems that provide automatic data collection of interest. In sum, sometimes it is desirable to be able to perform the analysis of a fluid in a simple shot, that is, not requiring a pressure difference.

On the other hand, a conventional system for monitoring the degree of degradation of a fluid (for example, a lubricating oil) is based on illuminating the fluid with a light source and observing the light that is transmitted through it. According to the state of the fluid (typically darker the more degraded the fluid), the transmitted light varies.

Using various techniques and implementations (visible or non-visible light, spectral or intensity analysis, etc.), more or less detailed information on fluid status can be obtained. However, when working in transmission, when the fluid is or becomes very dark, the transmitted light is so scarce that it is not possible to measure properly. This is the case of lubricating fluids, which experience a substantial change in color due to their use as lubricants. Therefore, it has been observed that, although on-line oil monitoring devices based on the principle of transmittance (that is, they work in transmission mode), they are effective when the oil under supervision is very translucent (ie, normally at the beginning of its use, when the oil is relatively clean), these monitoring devices lose effectiveness as the oil becomes opaque due to its continued use. An example of these systems working in transmittance is that of the aforementioned patent ES2455465B1. It would be possible to perform the same measurements analyzing the reflected light in a diffuse way instead of analyzing the light transmitted by the fluid. However, in this case, instead of having the problem with dark fluids, the problem arises with clear or translucent fluids, which do not reflect, in a diffuse way, enough light to have an adequate measure.

The international patent application WO2016 / 080824A1 proposes a system for monitoring the dynamics of the color of a fluid, formed by a probe submergible in the fluid under analysis. The probe is coupled to a video camera to capture images of the fluid sample. The system uses two light sources: a light source in the plane of the video camera (frontal lighting system) to work in diffuse reflection mode for the supervision of opaque samples; and a light source facing the video camera (backlighting system) to work in transmission mode for the supervision of transparent or translucent samples. This system is complex, requiring two light emitters, in addition to fragile, to incorporate one of the light sources in the part of the device most exposed to the fluid pressure under analysis. In addition, the system is designed to be introduced into the container that houses the fluid perpendicular to it, and therefore perpendicular to the evacuation of air by deaeration, so that said evacuation is difficult and therefore the movement of fluid in inspection area, thus preventing the renewal of samples under analysis.

DESCRIPTION OF THE INVENTION

The present invention provides a system for the monitoring of a fluid in a single intake, that is, without requiring a pressure difference between two points of access to the fluid under analysis, which solves the drawbacks of previous proposals. The system of the invention facilitates the analysis of a fluid in tank in a very compact and low cost way. With respect to the systems that make measurements in transmittance (proposed for example in the patent ES2455465B1), the system of the present disclosure represents an evolution to a system of reflection / transmission, facilitating the measurement both in opaque fluids as in translucent or transparent fluids , but minimizing the implementation of active optical elements, specifically light emitters, both in number and disposition. This allows to monitor with greater resolution -and at the same time with simplicity of design- both fluids that from their initial clean state are opaque (ie, whose absorbance in said wavelength is greater than 1.0, as more than 2.0 or greater than 3.0), as fluids that in their useful life pass from being translucent (ie, whose absorbance at a certain wavelength is less than 1.0, such as less than 0.5, or less than 0, 2 or less than 0.1) to opaque (ie, whose absorbance at said wavelength is greater than 1.0, as greater than 2.0 or greater than 3.0). It offers a compact, simple and autonomous solution for fluid monitoring without the need to extract samples of it from its operating environment.

The proposed system also has a configurability that allows adapting to different typologies and / or state of degradation of the fluid under supervision, selecting one or another implementation for its back cover (rear plane).

In the context of the present disclosure, opaque fluids are understood to be those that have an absorbance in a given spectral band greater than 1.0. From an absorbance in the working spectral band greater than 3.0, a fluid is considered very opaque. Also, translucent fluids are those that have an absorbance in a spectral band less than 1.0, and very translucent if their absorbance is less than 0.1. In general, light does not penetrate opaque fluids, but is absorbed at a depth close to the surface and the part that is not absorbed can leave the sample again in the form of diffuse reflection due to scattering produced by the internal reflections in the fluid in the molecules that form it or particles that may be. Figure 8 illustrates these phenomena. This effect is more likely to exist in opaque fluids than in translucent fluids, but it depends strongly on the absorbance and molecular composition and on the particles present in it. Regardless of the scattering, in general terms, the greater the absorbance, the lower the amount of light that the fluid passes through, propagating to through it. In the case of translucent fluids, in general, light passes through (is transmitted) through it.

In a first aspect of the invention, a monitoring system for inspecting a fluid contained in a tank is provided by inserting said monitoring system into a socket of said tank, comprising: a measurement zone configured to circulate for it a sample of said fluid. The monitoring system further comprises: light emitting / receiving means consisting of a lighting system and a light detection system located on the same side of the monitoring system with respect to said measuring area; an optical window disposed between said light emitting / receiving means and said measuring zone; and a rear optical element located on the other side of the monitoring system with respect to said measurement zone. The lighting system is configured to emit optical radiation towards said measurement zone. The light detector system is configured to detect an optical radiation comprising the light reflected by said fluid circulating in said measurement zone and / or the light transmitted through said fluid and reflected in said rear optical element. The monitoring system further comprises an electronic subsystem comprising processing means configured to control the activation / deactivation of the lighting system and to process the signals obtained from the light detection system.

In embodiments of the invention, the rear optical element is implemented by an absorbent element from the optical point of view, said rear optical element being configured to prevent reflection of the light transmitted through said fluid.

In embodiments of the invention, the rear optical element is implemented by a reflective element from the optical point of view, said rear optical element being configured to favor the reflection of the light transmitted through said fluid. This reflexive element from the optical point of view can be a flat reflexive element. Alternatively, the reflective element from the optical point of view can be a concave reflexive element, in order to concentrate the reflected rays.

In embodiments of the invention, the rear optical element is interchangeable, so that depending on the absorbance of the fluid under inspection, a rear absorbing or reflective optical element is chosen.

In embodiments of the invention, the monitoring system further comprises at least one control photodiode configured to measure the intensity emitted by the lighting system.

In embodiments of the invention, the monitoring system is comprised in a housing, wherein said light emitting / receiving means are located in a portion of said housing and said rear optical element is located in another portion of said housing, where said carcass portions define said measurement zone, said back optical element and said optical window delimiting the measurement zone.

In embodiments of the invention, the height of said measurement zone is adjustable to guarantee the renewal of the fluid sample within said measurement zone.

In embodiments of the invention, the optical window has an inclination with respect to the plane defined by the rear optical element to prevent the accumulation of air in the measurement zone.

In embodiments of the invention, the monitoring system comprises a diffuser disposed between said lighting system of said light emitting / receiving means and said at least one control photodiode.

In another aspect of the invention, there is provided a method of monitoring a fluid contained in a reservoir, comprising: inserting a monitoring system into a single intake of said reservoir, wherein said monitoring system comprises: a measurement zone configured for that a sample of said fluid circulates through it; light emitting / receiving means consisting of a lighting system and a light sensing system located on the same side of the monitoring system with respect to said measuring area; an optical window disposed between said light emitting / receiving means and said measuring zone; and a rear optical element located on the other side of the monitoring system with respect to said measurement zone; to influence an optical radiation from said lighting system towards said measurement zone; detecting by said light detector system an optical radiation comprising the light reflected by said fluid circulating in said measurement zone and / or the light transmitted through said fluid and reflected in said rear optical element; in an electronic subsystem comprised in said monitoring system, controlling the activation / deactivation of the lighting system and processing the signals obtained from the light detection system.

In embodiments of the invention, when said fluid has an absorbance greater than 1.0 in its initial state, said rear optical element is an absorbing rear optical element.

In embodiments of the invention, when said fluid has an absorbance of less than 1.0 in its initial state, said rear optical element is a reflective rear optical element.

In embodiments of the invention, the method further comprises adjusting the height of said measuring zone to ensure the renewal of the fluid sample within said measurement zone.

In embodiments of the invention, the method further comprises tilting said optical window with respect to the plane defined by the rear optical element to prevent the accumulation of air in the measurement zone.

Advantages and additional features of the invention will be apparent from the description in detail that follows and will be pointed out in particular in the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

To complement the description and in order to help a better understanding of the characteristics of the invention, according to an example of practical realization of the same, is accompanied as an integral part of the description, a set of figures in which character illustrative and not limiting, the following has been represented:

Figure 1A schematically illustrates a system for monitoring a fluid by inserting it into a simple intake of a reservoir containing said fluid, according to a possible embodiment of the invention. Figures 1B and 1C show respective views (front and profile, respectively) of the monitoring system according to a possible embodiment of the invention, in which the channel through which the sample flows under inspection can be observed. A part of the fluid contained in the tank in which the monitoring system is coupled circulates through this channel, at which time the samples are taken. Figure 1D shows a profile view of a monitoring system according to another embodiment of the invention, in which the channel that is formed on the outside of it can be observed, as a narrowing of the housing that covers the system.

Figure 2A shows a diagram of the system for monitoring the state of a fluid by spectrometry, according to a possible embodiment of the invention. Figure 2B shows a possible housing in which the structure or support in which the optical and electronic elements of the system are placed is inserted. Figure 2C shows the electronics housed inside the housing shown in Figure 2B, inside which can be seen the optical and electronic elements schematized in Figure 2A. Figure 2D shows in detail the arrangement of the light detector system and of the at least one lighting source, both of the same side or portion of the system with respect to the channel.

Figures 3A-3C schematize the path of the optical radiation emitted by a source of illumination when traveling towards a fluid (transmitting through it and / or reflected in the volume near the surface thereof and / or in the rear element, depending on certain parameters of the fluid and the characteristics of the rear element) and the light detected by a light detector system arranged in the same plane as the source of illumination.

Figures 4A and 4B depict the operation of the monitoring system in operation transmission / reflection mode.

Figure 5 illustrates the problem of the introduction of the monitoring system in a tank in a totally vertical manner, making it difficult to evacuate air in the measuring channel.

Figure 6 shows a monitoring system with height of the variable measuring channel, according to a possible embodiment of the invention.

Figure 7 shows a monitoring system whose optical window is inclined with respect to the plane defined by the rear plate, according to a possible embodiment of the invention.

Figure 8 illustrates the phenomena of transmission, absorption, scattering and diffuse reflection that can take place in a medium depending on the characteristics thereof in terms of absorbance, transmittance and reflectance. Note that the effect of specular reflection has been ignored.

DESCRIPTION OF A MODE FOR CARRYING OUT THE INVENTION

Figure 1A illustrates a schematic of a possible scenario for the application of a monitoring system (or measurement system) 3 of a fluid by inserting or coupling the monitoring system 3 into a standard socket (single socket) 5 of a tank, tuberla, or in general, tank 1 in which said fluid 2 is located, according to a possible realization of the invention. The monitoring or measurement system is sometimes referred to as a "sensor" throughout the present disclosure.The monitoring or measurement is performed by spectrometry.The monitoring system 3 of the invention is designed to be integrated into a reservoir 1, coupling to the same through a simple intake, such as a standard hydraulic intake, without the need to perform a bypass through ducts or pipes, which divert the fluid for monitoring.The monitoring system 3 is designed to be introduced in tank 1, so that the system 3 can take measurements of the fluid without the need to extract a sample of fluid from reservoir 1. Applying a series of algorithms for the interpretation of the measurements taken relative to various parameters, such as the color of fluid 2, information is obtained on the fluid, for example on its degree of degradation, and therefore it is possible to act on the fluid in question or make decisions based on an example in its degree of deterioration. The monitoring system 3 can take periodic or non-periodic measures (for example, at the request). The fluid 2 collected in reservoir 1 is preferably an industrial lubricating oil.

The monitoring system 3 schematized in Figure 1A has a series of optoelectronic elements (not illustrated in the scheme of Figure 1A) integrated in a housing, sheath or encapsulation, which constitutes the external part of the system or sensor. Figures 1B and 1C represent front and profile views, respectively, of a possible implementation of the monitoring system 3. In it, the housing, sheath or encapsulation has a shape that allows the passage of fluid 2 between four external surfaces 31 32 33 34 of the housing. That is, fluid 2 passes through an outer zone 20 to the housing. This outer zone is a kind of tunnel, channel, conduit or measuring zone between the outer surface of a first portion 35 of the housing and the outer surface of a second portion 36 of the housing facing the first portion 35 of the housing, both portions defining the measurement conduit or zone 20 for the fluid. The second portion 36 is U-shaped, this shape defining the measurement channel or zone 20 through which the fluid flows when the monitoring system 3 has been coupled to the fluid reservoir 1 under supervision.

Figure 1D represents a profile view of another possible implementation of the monitoring system 3. As in the previous case, the housing, sheath or encapsulation of the system or sensor has a shape that allows the passage of fluid 2 between three external surfaces 31 ' 32 '33' of the housing. These three surfaces 31 '32' 33 'define the measurement channel or zone between the outer surface of a first portion 35 of the housing and the surface exterior of a second portion 36 of the casing facing the first portion 35 of the casing, both portions defining the measurement conduit or zone 20 for the fluid. That is to say, as seen in Figure 1B, the casing has a certain thickness, thickness or background "z1" in the first and in the second portions, and experiences a narrowing in its thickness "z2" in an intermediate portion 37 enters the first and second portions 35, so that the housing is divided into two portions joined by a narrow part 37 of the housing, leaving a gap or channel 20 through which the fluid flows when the monitoring system 3 has been coupled to the tank 1 of fluid 2 under supervision. Although in FIGS. 1B-1D the surface delimiting the measurement zone 20 is illustrated as flat walls or surfaces (31-34, 31'-33 '), other implementations of the housing could be made with curved surfaces, for example a substantially curved surface, in which therefore a clear differentiation between said surfaces 31-34 or 31'-33 'can not be established.

Depending on the configuration of the reservoir 1 that collects the fluid 2 under analysis, the intake or coupling 5 in reservoir 1, through which the sensor or monitoring system 3 is inserted or coupled in reservoir 1, may be in a side of the tank 1, as is the case of the configuration shown in figure 1A, or it may be in the upper part (for example, cover) of the tank 1. In the first case, the monitoring system 3 is inserted in the deposit 1 obliquely to it. In the second case, the monitoring system 3 is inserted into the tank 1 perpendicular to it.

In Figure 2A, the monitoring system 13 is illustrated in greater detail according to a possible implementation of the invention. In relation to this figure, optoelectronic elements of the system are described, which are integrated or supported in a structure that in turn is inserted in the housing, sheath or encapsulation described above. Figure 2B shows a possible housing in which the structure or support (shown in Figure 2C) is inserted in which the optical and electronic elements of the system are placed. The monitoring system has an optical part (optical subsystem) 140 and an electronic part (electronic subsystem) 150. The optical subsystem 140 occupies the second portion 136 of the system housing and a portion of the first portion 135 of the housing, being understood for first and second portions referenced as 35, 36 in the previous Figures 1B-1D. In particular, of the first portion 135 of the housing, the optical subsystem 140 occupies the part closest to the external channel 120 to the monitoring system 13, through which the fluid flows under supervision. Figure 2A shows said channel 120. The arrow along the channel 120 represents the fluid flow in the channel 120. The electronic subsystem 150 occupies the part of the first portion 135 of the housing more remote to channel 120 defined by the outer surface of the housing (31-34 in Figure 1B, 31'-33 'in Figure 1D).

The optical subsystem 140 is formed, among other elements, by a light-emitting / receiving means 41. The light-emitting / receiving means 41 are composed of a lighting system 411 and a light-sensing system 412. The lighting system Illumination 411 is formed by at least one source of illumination. The light detector system 412 is formed by one or more light detectors. The light-emitting / receiving means 41 are located in the first portion 135 of the housing, which is the most robust portion of the housing taking into account, for example, its dimensions, opposite the second portion 136 of the housing, more exposed to the total volume of fluid occupied by tank 1 (see Figures 1A-1D). By way of example, the first portion 135 of the housing can have a diameter ranging between 25 and 30 mm (millimeters, 10-3 meters) and a height ranging between 40 and 45 mm, while the second portion 136 of the The housing can have a diameter that varies between 20 and 25 mm and a height that varies between 7 and 12 mm. That is, the lighting system 411 and the light sensing system 412 are located on the same side with respect to the measurement area 20, 120. Non-limiting examples of lighting sources 411 are one or more light emitting diodes (LEDs) ), one or more tungsten lamps (lamp comprising tungsten in its filaments), one or more halogen lamps, one or more mercury vapor lamps, among others. The source or sources of illumination 411 can be broadband (as is the case, for example, of the halogen lamp), which offers stable spectrum from the ultraviolet to the far or deep infrared. In a possible implementation, the at least one illumination source 411 is one or more LEDs emitting white light to illuminate the fluid flowing through the channel 120. Non-limiting examples of light detection systems 412 are ultraviolet (UV) light detectors. , visible light detectors (VIS), light detectors in the near infrared (NIR) and combinations thereof. In a possible implementation, at least one color sensor is used, for example an RGB color sensor (configured to capture visible light in the red band (R, network), green (G, green) and blue (B, blue)). In order to avoid or minimize diafonla (more commonly known as crosstalk, for its term in English), that is, part of the intensity emitted by the at least one source of illumination arrives back to the receiver without effect of the sample, the Light emitting / receiving means 41 may incorporate screening means 413, implemented for example as a separating wall made for example of an absorbent material or of a reflective material at the working wavelengths, between the at least one source of light. lighting 411 and the at least one light detector system 412.

The light sensing system 412 is preferably arranged in the same plane as the at least one illumination source 411 (both on the same side or portion of the system with respect to the channel 20, 120), as illustrated in Figure 2D. As explained below, the light sensing system 412 is configured to detect light reflected at a depth close to the surface of the fluid (diffuse reflection) and to detect light transmitted through the fluid and reflected in a rear optical element that it is described later.

The optical subsystem 140 may also include one or more control photodiodes 43, configured to be paired to the at least one illumination source 411 of the light emitting / receiving means 41. The function of the one or more control photodiodes 43 is to measure the intensity emitted by the at least one source of illumination 411, in order to control said emitted intensity. As the system temperature increases, the light intensity emitted by the light source decreases and, therefore, the amount of light incident on the sample decreases. This decrease in the amount of light that affects the sample can cause the measurement to be incorrect. To avoid this effect, a closed-loop control of the emission power of the lighting source is preferably implemented. This closed-loop control can be implemented as follows: based on the amount of light received in the control photodiode 43, the intensity value to which the lighting source has to be turned on is calculated in order to achieve that the light intensity emitted is the adequate to perform the measurement. The appropriate intensity value is obtained by a calibration process that is preferably carried out in manufacturing. In Figure 2A, the angle $ represents the emission angle of the lighting source 411, the angle a represents the reception angle of the control photodiode 43 and the angle p represents the reception angle of the light detection system 412. In addition , Pd-c (V) refers to the value of the measurement made by the control photodiode 43, RGB (mA) represents the measurement value of the light detector system 412 and LED (mA) represents the value of the current to the at least one emitter of the illumination source 411 has to be turned on. The at least one illumination source 411, the at least one light sensing system 412 and the at least one control photodiode 43 are controlled from the subsystem electronic 150. For example, from the electronic subsystem 150 the necessary current for the supply of the at least one illumination source 411 is provided, in the electronic subsystem 150 the signal detected by the at least one light detection system is received and processed. 412 and it receives and processes the signal provided by the photodiode control 43.

Preferably, between the at least one illumination source 411 and the at least one control photodiode 43 there is arranged a diffuser 48 whose main mission is the diffusion of the amount of light emitted by the at least one illumination source 411 to achieve an illumination. homogeneous throughout the area under inspection (zone 120 occupied by the fluid, such as oil, under analysis). The diffuser 48 is of a material substantially transparent to the working wavelengths but having the function of scattering light. Thus, the at least one illumination source 411 can adequately illuminate the fluid circulating in the measurement zone 120. In a possible embodiment, the diffuser 48 is a crystal, for example a frosted crystal.

Optical subsystem 140 also includes an optical window 44. In Figure 2B, reference 44 refers to the gap occupied by this optical window 44, which in Figure 2B is not illustrated. That is, the surface of the second portion 35, 135 of the sensor housing or monitoring system, which is one of the surfaces of the system defining the channel 20, 120, and therefore one of the surfaces in contact with the sample. of fluid passing through the channel 20, 120, is hermetically sealed by a transparent protection window 44 (transparent to the working wavelength). The sealed surface corresponds to the wall 33 in FIG. 1B and to the wall 33 'in FIG. 1D of the second portion 35 of the sensor housing or monitoring system. The source of illumination 411 is oriented towards the channel 20, 120 through which the fluid flows. The transparent protection window 44 is located between the light emitting / receiving means 41 and the area 20, 120 through which the fluid flows. In a possible non-limiting embodiment, this transparent protection window 44 is made, for example, of borosilicate glass (optical glass BK7) or of a plastic material, such as PMMA. Through this optical window 44 the light emitted by the illumination source 411 travels to the fluid found in the gap, groove or channel 120. The optical window 44 allows substantially all the light reaching it to be transmitted by its interior towards channel 20, 120.

On the other side of the channel 120, that is to say, on the second portion 136 of the system housing, there is another element of the optical subsystem 140. It is a rear optical element or rear plate 45. This rear optical element 45 forms at least part of the surface of the first portion 36, 136 of the housing defining the measuring channel 20, 120. For example, in the scheme of Figure 1B, the rear optical element is integrated into the wall 31, or in the scheme of Figure 1D, the rear optical element is integrated in the wall 31 '. Depending on various factors, such as the type of fluid (such as oil) used or the degree of predictable degradation of the fluid under analysis, the rear optical element 45 is designed to provide a certain (greater or lesser) degree of reflection . In possible implementations of the invention, the rear optical element 45 is implemented as a black surface, or as a flat mirror or as a concave or parabolic mirror configured to concentrate the reflected rays in the light detecting system 412. That is, the region of fluid in which the measurement is made is defined by the channel 20, 120, the optical window 44 and the rear optical element 45.

In turn, the electronic subsystem 150 has a processing means 51 for the tasks of activating / deactivating the lighting (control of the lighting sources) and for processing and calculating the signals obtained, coming from the detector system. of light, to obtain indicators of oil degradation in function of the measures taken. In a possible realization, not limitation, the processing means 51 are implemented by means of an embedded microcontroller, programmed to perform said functions of activation / deactivation of illumination and of calculation and processing of signals. The processing means 51 house the following algorithms (whose specific content is outside the scope of the present invention):

- one or more lighting control algorithms 511, which ensure that the intensity of the light emitted by the at least one lighting source 411 is adjusted to a lighting setpoint; the input parameters of this algorithm are the measurement of the control photodiode 43 (Pd_c (V)) and a lighting setpoint, while the output parameter is the current to be applied to the lighting system (Led (mA));

- one or more oil degradation calculation algorithms 512, designed to calculate a degradation indicator from the RGB reading made in the light detecting system 412; Y

-one or more 513 calibration algorithms, designed for an adjustment of work intensities and reference point.

The electronic subsystem 150 also has other elements, such as communication drivers 52 that allow the system 13 to communicate with external equipment to receive commands (instructions for making measurements, performing calibration, etc.) and for transmitting measurements or results of the processing performed in the media. of processing 51; power source 53, designed to power all the electronic devices of the system 13; memory means 54, configured to store measurement results and parameters of the algorithms; and temperature sensor 55, to know and monitor the temperature of the system 13.

The monitoring system 13 also comprises connection means, which can be wired, such as, for example, the connector 60 shown in FIG. 2A or the wiring 4 shown in FIG. 1A, or wireless, for communication with external equipment or for receiving power. external in case it is necessary.

Depending on the circumstances, such as the type of industrial system under monitoring, or the type (and therefore, the absorbance and reflectance) of fluid-preferably lubricating oil-that is being monitored, or the predictable rate of degradation that of said fluid can be expected, one can choose between different implementations of the rear optical element 45. As illustrated in Figure 2A, the fluid flowing in the channel 20, 120 can be a fluid considered opaque (i.e., a fluid of absorbance greater than 1.0) or a fluid considered translucent (ie, an absorbance fluid less than 1.0). In the case of lubricating oils for industrial machinery, such as gas engines, there are opaque oils (absorbance greater than 1.0, such as greater than 2.0 or greater than 3.0 at the working wavelength). when they are clean, that is, before they start to be used and therefore degrade; and there are also oils that initially (for example when they are clean) are translucent (absorbance less than 1.0, such as less than 0.5 or less than 0.2 or less than 0.1 at the working wavelength) and that, throughout its useful life, its absorbance increases with the passage of time and the use as lubricant of the machinery in question, until it reaches a high opacity at the end of its useful life (for example, presenting an absorbance around 1.0 or higher, such as 2.0 or 3.0 in the measured wavelength). In any case, as they are used as lubricants, their absorbance increases due to oxidation processes, among others.

The monitoring system of the present disclosure is preferably implemented so that the rear optical element 45 is interchangeable, to adapt to the type of fluid that it is desired to monitor. Thus, with the design of the light emitting / receiving means 41 and the rear optical element 45, it is possible to cover different use cases, selecting the rear optical element 45 among different possible options. Figures 3A-3C schematize the path of the optical radiation emitted by the at least one illumination source 411 when traveling towards a fluid occupying the channel 20, 120 and the light detected by the at least one light detector system 412 arranged in the same side as the at least one illumination source 411 (both on the same side of the channel 20, 120) and, preferably, in the same plane to facilitate the optical design. In these figures, the role played by the rear optical element 45 with respect to the optical radiation coming from the at least one light source 411 that reaches said rear optical element 45 (continuous arrow) is explained. The dotted arrows represent the diffuse reflection in the fluid, present to a greater or lesser extent depending on the type of fluid.

The predominant mode of operation is determined by the type of back optics chosen and the characteristics of the monitored fluid. Thus, in FIG. 3A, in which an absorbent material has been used from the optical point of view (ie, a material that substantially does not reflect any light at wavelength (or wavelengths) of light) implementing the rear optical element 451, it is observed how this absorbent material does not reflect anything, so that substantially all the signal contribution received in the light detector system 412 corresponds (dotted line) to the diffuse reflection signal in the sample. Non-limiting examples of absorbent materials are black nylon and anodized aluminum, among others. The configuration of Figure 3A is recommended when you want to ensure that only diffuse reflection generated in the sample is measured. In this case, the predominant mode of operation is reflection mode, since the main optical radiation measurement made by the at least one light detector system 412 is that relative to diffuse reflection (dashed arrow).

For its part, in Figure 3B, in which the rear optical element 452 has been implemented by a reflective material not curved from the optical point of view (ie, a material that at the wavelength (or wavelengths) of work reflects substantially all the light incident on said reflective material), it is observed as the rear optical element 452 reflects everything (continuous arrow that part of the rear optical element 452), so that the light detector system 412 receives reflection intensity diffuse (dotted line) and transmission signal that corresponds to the portion of light transmitted through the fluid and reflected on the back plate 452 that is transmitted back through the fluid (continuous line arriving at the light detector system 412) . Non-limiting examples of reflective materials are polished aluminum, white nylon, and in general, any material that acts as a mirror. In this case, the rear optical element 452 acts as a virtual emitter, since the behavior is as if there were a virtual emitter on the other side of the fluid occupied by the channel 20, 120. The continuous arrow coming from the rear element 452 refers to the radiation reflected by said rear element 452. In this case, the predominant mode of operation is transmission mode, since the main optical radiation measurement made by the at least one light detector system 412 is that relative to the optical radiation from the rear optical element 452 that acts as a virtual emitter (continuous arrow).

Finally, in Figure 3C, in which the rear optical element 45 has been implemented by means of a curved reflective material, focused reflective or curved reflector 453, it concentrates the rays coming from the at least one source of illumination 411 that reach the back plate 453, in the active area of the light detector system 412, to maximize the received signal. That is to say, the rear optical element 45 has been implemented by means of a material that at the wavelength (or wavelengths) of work reflects substantially all the light that falls on said reflective material and is also designed to concentrate the light in the light detector system 412. Non-limiting examples of implementations such as this are implementations using curved mirrors (parabolic or concave). As in the case of FIG. 3B, in FIG. 3C, the light detector system 412 receives diffuse reflection intensity (dotted line) and transmission signal that corresponds to the portion of light transmitted through the fluid and reflected on the plate. rear 453 that is transmitted back through the fluid (continuous line that arrives at the light detector system 412), the latter optimized (it arrives focused from the virtual transmitter). That is, as in the case of Figure 3B, the predominant mode of operation is transmission mode, since the main optical radiation measurement made by the at least one light detector system 412 is that relative to the optical radiation from the optical element. rear 453 that acts as a virtual emitter (continuous arrow).

These configurations (Figures 3B and 3C) are recommended to measure the transmittance level of the sample. However, if the fluid to be monitored has a high absorbance (opaque or very opaque fluids), it will hardly reach the back plate and the behavior of the system will be similar to that described in the case of Figure 3A, that is, it works in reflective mode despite having a reflective rear plate 452, 453.

That is, thanks to the location of the optical emission / reception means 41 on the same side of the fluid, and of the rear optical element 45 located on the other side of the fluid with respect to the optical emission / reception means 41, a of operation in reflection or in transmission that will allow to monitor a wide range of fluids in terms of absorbance (transmittance) and reflectance.

Figures 4A and 4B depict the operation of the monitoring system. Specifically, figure 4A represents the optical behavior of the rays transmitted by the at least one source of illumination 411 on contacting the fluid that fills the channel 20, 120 when the fluid filling the channel 20, 120 is a high opacity fluid 72, that is, when the fluid presents a absorbance greater than 1.0, such as greater than 2.0 or greater than 3.0 (lower part of Figure 4A); and when the fluid filling the channel 20, 120 is a low opacity fluid 71, that is, when the fluid has an absorbance of less than 1.0, such as less than 0.5 or less than 0.2 or less of 0.1 (upper part of figure 4A).

When an optical radiation is emitted that impinges on a high opacity fluid 72, practically none of the light gets through the fluid 72 due to the opacity of this, since the light is absorbed in a depth close to the surface and the part that it is not absorbed it can leave the sample again in the form of diffuse reflection due to the scattering produced by the internal reflections in the fluid (see figure 8). In this case, the predominant mode of operation is the mode in reflection. Under these circumstances, the behavior of the light rays in the fluid is substantially the same irrespective of the type of back plate used, since practically no ray reaches the back plate.

On the contrary, when an optical radiation is emitted that impinges on a low opacity fluid 71, a high proportion of the light emitted by the source passes through the fluid. In this case, the predominant mode of operation is the transmission mode, because the light radiation is able to cross the fluid and the component due to scattering is in a much smaller proportion (it must also be taken into account that the scattering effect depends on the characteristics of the fluid). In case the rear plate of the system is reflective 452, 453, all the light that reaches this surface will be reflected and will return back through the fluid. If, on the other hand, the back plate is absorbent 451, all the light that hits this surface will be absorbed and nothing will return back through the fluid.

The choice of back plate 451, 452, 453 depends on the characteristics of the fluid to be monitored and its evolution during use. The use of an absorbent back plate 451 allows obtaining information on the level of diffuse reflection due to scattering in the sample, which can give information on the appearance of particles in it, can indicate for example the appearance of varnishes. Whereas a reflective rear plate 452, 453 allows for example to monitor the absorbance level of the sample, changes in color, etc. All these effects are indicative of the state of oil degradation.

As indicated, the lubricating fluid in its initial state and its evolution during its use condition the choice of the rear plate 45, so that a lubricating fluid considered opaque in the initial (clean) state could determine the choice of an absorbent back plate 451, while a lubricating fluid considered translucent in initial state, could determine the choice of a reflective rear plate 452, 453. Figure 4B shows in detail the main components of the optical subsystem 140 and the predominant modes (mode reflexion and transmission mode) in function of the opacity of the fluid occupied by the channel 20, 120 and of the rear plate or optical reflection element chosen. Reference (i) refers to light rays reflected in the vicinity of the fluid surface (diffuse reflection) which are detected in the light-sensing system 412, while reference (ii) refers to light rays passing through the fluid and that are reflected by the rear optical element 45 (when it acts as a virtual emitter), so that they travel back through the fluid. Thus, when an absorbing back plate 451 is chosen, the light detecting system 412 substantially only receives the rays (i), that is, the rays of light reflected by diffuse reflection, independently of whether the fluid has more or less opacity, since that the rays emitted by the illumination source 411 that have been able to pass through the fluid, if any, are absorbed by the absorbing rear optical element 451. The system works in this case predominantly in reflection mode, because most of the radiation detected by detector 412 is component (i) reflected in the fluid. On the other hand, when a reflective rear plate 452, 453 is chosen, the light detector system 412 receives the rays (i) (diffuse reflection) plus the reflected rays (ii) by the rear reflective optical element 452, 453 that have achieved traverse the fluid back. In this case, if the fluid is opaque, that is, absorbance greater than 1.0, such as greater than 2.0 or greater than 3.0, component (i) may be greater than (ii), because the opaque fluid 72 allows the passage of little reflected radiation. In this case, the system works in a predominant way in reflection mode (diffuse). Conversely, if the fluid is translucent (absorbance less than 1.0, such as less than 0.5 or less than 0.2 or less than 0.1), component (i) may be less than (ii) ), because the fluid of low opacity 71 allows the passage of greater amount of radiation reflected by the back plate 453, 453. The system works predominantly in transmission mode in this case. In general, of the two light components (i), (ii) which are detected by the light detecting system 412, the smallest of them can become null.

On the other hand, the fact of working with a monitoring system designed to fit in a compact and simple way in a simple take of a deposit and work for both in submerged mode, it is necessary to overcome certain drawbacks: On the one hand, it must be ensured that there is an effective renewal of the fluid within the channel 20, 120. Note that the flow of the fluid along the measuring channel 20, 120 depends on the small pressure differences that exist in tank 1 due to internal turbulence, fluid viscosity and fluid temperature. For example, it has been obtained by simulation that with a pressure increase of 17.07 Pascals between the input and output of the monitoring system, for a channel 20, 120 of 2 mm thickness (distance between window 44 and back plate 45). ) a volumetric flow rate of 0.053 ml / s is achieved.

Therefore, the renewal of fluid in the measuring channel 20, 120 is guaranteed under the described pressure conditions. On the other hand, it must be guaranteed that when introducing the monitoring system 3, 13 in the tank 1, the monitoring system 3, 13, evacuate the air that it may contain in its measuring cavity (measurement channel) 20, 120. The probability of not evacuating the air contained in the measuring channel 20, 120 is greater the more vertically the monitoring system 3, 13 is introduced in the tank 1, and the more viscous the fluid under monitoring. The greatest risk of accumulation of air in the measuring channel 20, 120 is that in which the monitoring system is introduced completely vertically in a tank containing a highly viscous fluid (for example, 320 cSt), where cSt they are centiStokes, that is, units of viscosity, being 1cSt = 10 "6 m2 / s3.

Figure 5 illustrates the problem of the introduction of the monitoring system 3 in a tank 1 in a completely vertical manner, in which case the air bubbles accumulated in the measuring channel have difficulties to evacuate the channel and, therefore, it is not filled with the fluid that you want to monitor 2.

To guarantee the renewal of the fluid sample within the measuring channel 20, 120, thus ensuring that the monitoring system takes measurements on different samples of fluid over time, in embodiments of the invention a measuring channel is designed. , 120 adjustable height for each type of fluid. Figure 6 shows a monitoring system with height H of the measuring channel 20, 120 variable. In embodiments of the invention, the height H of the channel 20, 120 can vary between 0.5 mm and 5 mm, for example between 0.75 mm and 4 mm, or between 1 mm and 3.5 mm, or between 1, 5 mm and 3 mm, or between 1.75 mm and 2.75 mm.

In order to prevent the accumulation of air in the measuring channel 20,120, that is to encourage the evacuation of air from said channel 20, 120, in embodiments of the invention, the connection between the optical window 44 and the body or housing of the system is designed. of monitoring without edges. In other embodiments of the invention, said optical window 44 is designed, which is preferably flat, with a certain inclination with respect to the plane defined by the rear optical element 45. This inclination favors the evacuation of air from the channel 20, 120. The figure 7 shows a monitoring system whose optical window 44 is inclined with respect to the plane defined by the rear optical element 45. In embodiments of the invention, the inclination p can vary between 5 and 15 °, for example between 6 and 14 °, or between 7 and 13 °, or between 8 and 12 °, or between 9 and 11 °. In embodiments of the invention, the connection between the optical window 44 and the body or housing of the monitoring system without edges is designed, and furthermore the optical window 44 is inclined at an angle p with respect to the plane defined by the rear optical element. .

A prototype of the described monitoring system has been constructed. The prototype consists of a white light LED emitter, a RGB color detector and a channel width (distance between optical window and back plate) of 2 mm. The following table shows the values obtained when measuring with different fluids (fluid 1 to fluid 6) and different configurations of the system (absorbing or reflective back plate) versus data obtained in the laboratory. In all measurements made with the prototype and with the same rear plate configuration, the emitter sends the same amount of light. This value is determined during the sensor calibration process (monitoring system) under vacuum conditions, that is, without fluid in the channel. Under these conditions, the value measured in the detector is 80% with respect to its full scale. This value has been chosen for reasons of sensor resolution, such as to avoid situations of saturation of the detector. The last two columns refer to the measurement taken by the RGB color detector in the presence of fluid in the channel. Fluid 1 is Beslux degraded. Fluid 2 is Beslux intermediate degraded. The fluid 3 is Beslux Reference. The fluid 4 is Cepsa degraded. The fluid 5 is Cepsa intermediate degraded. The fluid 6 is Cepsa reference.

A bsorbanc ia m ed R eed up m edida m ed D etector r M ectio n Laboratory D etector in Labra tory (P laca rear (Rear view mirror) Absorben) F luido 1 3 4 32 % 66% Fluid 2 2 3,25 39% 62% Fluid 3 1 4,5 44% 66% Fluid 4 0,5 5,25 55% 67% Fluid 5 0,2 6 63% 68% Fluid 6 0, 04 6.75 79% 69%

As can be seen, when using an absorbent back plate 451 (first column on the right), and therefore substantially all the radiation that may have passed through the fluid is absorbed by the back plate 451, for any of the samples (fluid 1 to fluid 6) the detector measures a value around 60-70 % of your fund of scale. In this case the system works in reflection mode. On the contrary, when using a reflective back plate 452, 453 (second column on the right), the measurement obtained by the RGB color detector varies a lot depending on the absorbance of the fluid, from scale values close to 80% in very translucent fluids (fluid 6) up to values close to 30% in very opaque fluids (fluid 1). In this case, the system works in predominant transmission mode.

As can be seen, with respect to monitoring systems in a simpler manner of the state of the art, such as, for example, the one described in the international patent application WO2016 / 080824A1, the present disclosure simplifies and cheapens the design, manufacture and maintenance of the monitoring system. , because a single source of lighting is needed, which is also located in the most robust area of the monitoring system. In short, the monitoring system of the present disclosure is implemented as a 'plug-in' sensor (which can be coupled to a simple intake of a tank) that can deliver an oil degradation output (or its lubrication capacity), as well as other Parameters indicative of the oil, such as its level of antioxidant additives, its degree of acidity, its level of oxidation and / or its level of presence of varnishes, whether the fluid is opaque or translucent. The system can be threaded, or in any other form of simple coupling, which does not require a bypass channel in the tank for the selection of the fluid sample. The system allows the measurement in reflection and / or transmission of opaque and transparent fluids, passing through the different degrees of translucency. Various rear optical elements can be used, either absorbent or reflective material, and in this case both planes and curved to focus the rays. The system has been optimized so that, when entering the tank, the air that may be in the measuring channel is eliminated, avoiding the presence of air in the measurement area. In addition, it has been optimized so that the samples on which the measurements are taken are regenerated.

In this text, the word "comprises" and its variants (such as "understanding", etc.) should not be interpreted in an exclusive manner, that is, they do not exclude the possibility that the description includes other elements, steps, etc.

On the other hand, the invention is not limited to the specific embodiments that have been described but also covers, for example, the variants that can be made by the average expert in the field (for example, regarding the choice of materials, dimensions, components, configuration, etc.), within what is clear from the claims.

Claims (16)

1. A monitoring system (3, 13) for the inspection of a fluid (2, 71, 72) contained in a tank (1) by inserting said monitoring system (3, 13) into a socket (5). ) of said deposit (1), comprising: a measurement zone (20, 120) configured so that a sample of said fluid (2, 71, 72) circulates therethrough; the monitoring system (3, 13) being characterized in that it comprises: light-emitting / receiving means (41) consisting of a lighting system (411) and a light-sensing system (412) located on the same side of the monitoring system (3, 13) with respect to said area of measurement (20, 120); an optical window (44) disposed between said light emitting / receiving means (41) and said measuring area (20, 120); and a rear optical element (45, 451, 452, 453) located on the other side of the monitoring system (3, 13) with respect to said measurement area (20, 120), said lighting system (411) being configured to emit optical radiation towards said measurement zone (20, 120), said light detection system (412) being configured to detect an optical radiation comprising the light (i) reflected by said fluid (2, 71, 72) circulating in said measurement zone (20, 120) and / or light (ii) transmitted through said fluid (2, 71, 72) and reflected in said rear optical element (45, 451, 452, 453), further comprising said monitoring system (3, 13) an electronic subsystem (150) comprising processing means (51) configured to control the activation / deactivation of the lighting system (411) and to process the signals obtained from the detector system of light (412). 2. The monitoring system (3, 13) of the revindication 1, wherein said rear optical element (45, 451) is implemented by an absorbent element from the optical point of view, said rear optical element (45, 451) configured to prevent reflection of the light transmitted through said fluid (20, 120). 3. The monitoring system (3, 13) of the revindication 1, wherein said rear optical element (45, 452, 453) is implemented by a reflective element from the optical point of view, said rear optical element ( 45, 452, 453) configured to favor the reflection of the light transmitted through said fluid (20, 120). 4. - The monitoring system (3, 13) of claim 3, wherein said optically reflective element (452, 453) is a planar reflective element (452). 5. - The monitoring system (3, 13) of claim 3, wherein said optically reflective element (452, 453) is a concave reflective element (453). 6. - The monitoring system (3, 13) of claim 1, wherein said rear optical element (45, 451, 452, 453) is interchangeable, so that depending on the absorbance of the fluid under inspection (2). , 71, 72), a rear absorbent (451) or reflective (452, 453) optical element is chosen. 7. The monitoring system (3, 13) of any of the preceding claims, further comprising at least one control photodiode (43) configured to measure the intensity emitted by the lighting system (411). 8. The monitoring system (3, 13) of any of the preceding claims, said monitoring system (3, 13) being comprised in a housing, wherein said light emitting / receiving means (41) are located in a portion (136) of said housing and said rear optical element (45, 451, 452, 453) is located in another portion (135) of said housing, wherein said housing portions (135, 136) define said measurement zone (20, 120), said rear optical element (45, 451, 452, 453) and said optical window (44) delimiting the measurement area (20, 120). 9. The monitoring system (3, 13) of any of the preceding claims, wherein the height (H) of said measurement zone (20, 120) is adjustable to guarantee the renewal of the fluid sample (2, 71). , 72) within said measurement zone (20, 120). 10. - The monitoring system (3, 13) of any of the preceding claims, wherein said optical window (44) has an inclination with respect to the plane defined by optical rear element (45, 451, 452, 453) to avoid accumulation of air in the measurement area (20,120). 11. - The monitoring system (3, 13) of any of the preceding claims, comprising a diffuser (48) arranged between said lighting system (411) of said light emitting / receiving means (41) and said at minus a control photodiode (43). 12. - A method of monitoring a fluid (2, 71, 72) contained in a tank (1), comprising: inserting a monitoring system (3, 13) into a single socket (5) of said tank (1), wherein said monitoring system (3, 13) comprises: an area of measurement (20, 120) configured so that a sample of said fluid (2, 71, 72) circulates therethrough; light-emitting / receiving means (41) consisting of a lighting system (411) and a light-sensing system (412) located on the same side of the monitoring system (3, 13) with respect to said area of measurement (20, 120); an optical window (44) disposed between said light emitting / receiving means (41) and said measuring area (20, 120); and a rear optical element (45, 451, 452, 453) located on the other side of the monitoring system (3, 13) with respect to said measurement zone (20, 120); to influence an optical radiation from said lighting system (411) towards said measurement zone (20, 120); detecting by said light detector system (412) an optical radiation comprising the light (i) reflected by said fluid (2, 71, 72) circulating in said measurement zone (20, 120) and / or light (ii) ) transmitted through said fluid (2, 71, 72) and reflected in said rear optical element (45, 451, 452, 453); in an electronic subsystem (150) comprised in said monitoring system (3, 13), controlling the activation / deactivation of the lighting system (411) and processing the signals obtained from the light detection system (412). 13. - The method of claim 12, wherein when said fluid (2, 71, 72) has an absorbance greater than 1.0 in its initial state, said rear optical element (45, 451, 452, 453) is an absorbent rear optical element (451). 14. - The method of claim 12, wherein when said fluid (2, 71, 72) has an absorbance of less than 1.0 in its initial state, said rear optical element (45, 451, 452, 453) is a reflective rear optical element (452, 453). 15. - The method of any of claims 12 to 14, further comprising adjusting the height (H) of said measurement zone (20, 120) to ensure the renewal of the fluid sample (2, 71, 72) within of said measurement zone (20, 120). 16. - The method of any of claims 12 to 115 further comprising tilting said optical window (44) with respect to the plane defined by rear optical element (45, 451, 452, 453) to prevent the accumulation of air in the area of measurement (20,120).