GB2393783A

GB2393783A - Measuring liquid height, velocity, or water quality using reflection of light from a surface of the liquid

Info

Publication number: GB2393783A
Application number: GB0321229A
Authority: GB
Inventors: Stephen Lindsay Russell; Dene Raymond Marshallsay
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 2002-09-10
Filing date: 2003-09-10
Publication date: 2004-04-07
Anticipated expiration: 2023-09-10
Also published as: GB0321229D0; GB2393783B; GB0220985D0

Abstract

A method of determining whether a liquid is flowing, comprising: directing a light beam from a source 3 such as a diode laser toward a surface 2 of the liquid; detecting the light scattered back 7 from the surface of the liquid to a detector 9 such as a photodiode, or a CCD; using the light power detected to determine whether the liquid is flowing, the velocity of the liquid, flow rate, or liquid heights (18, 17, figure 3). The detector's output power maybe used to estimate sample turbidity, or if water is the liquid in context, water quality. The operating wavelength absorbed by the pure water may be used to suppress scattered and reflected light from a channel floor along which the water may be flowing. The channel may be an open channel and has an intermittent flow. An RMS algorithm or a power spectral density may be applied to detector output signals.

Description

FLOW MONITOR

This invention relates a flow monitor, and more particularly relates to a flow monitor for the detection and measurement of liquid flow.

There are many industrial situations where the monitoring of flow is needed, for example parts of machines, specific security requirements or general vibration monitoring. Many such techniques rely upon attaching a sensor such as an accelerometer. Other techniques, including many security systems detect flow over 10 very large areas.

In some situations it is desirable to detect flow or movement over a very small area from a long distance. An example of this is the monitoring of flows of liquids in open channels. There are numerous techniques for measuring volumetric flow in open 15 channels such as the flume structures described in standards. There are also methods based on velocity area approaches such as using ultrasonic Doppler for velocity and pressure measurement for depth and hence knowing the channel cross section, the area of the flow may be calculated.

20 The drawbacks to these methods are in some cases the direct contact with the liquid, which can cause problems for liquids such as sewage where rags and debris damage or cover the sensors rendering them inoperative. In other cases, the cost is a drawback, for example where for a flume substantial civil engineering works may be needed. In some situations a reliable flow sensor is needed without a high degree of accuracy in the flow measurement. The ouffalls from Combined Sewer Overflows (CSOs) come into this category. CSOs do not carry any flow under normal weather conditions. During storm conditions, the surface water part of the sewer system gather large amounts of 30 storm water in a very short time and this is more than the sewer system can handle.

The CSO provides for the storm water to be spilled into a watercourse such as a river.

The flow from CSO to river may be of short duration but a monitor must be able to operate immediately if the flow is to be detected for environmental or other purposes.

The present invention enables the detection of flow or movement of a very small area to be monitored at a range of several metros, or at greater ranges of tens or even hundreds of metros if required.

5 The present invention provides a method and apparatus for detecting fluid flow. In a preferred embodiment, the method also determines fluid velocity. The fluid is typically in an open channel, and the method and apparatus is particularly useful in situation where the flow is intermittent. The ouffalls from CSOs is an example of one particularly useful application of the invention.

The invention preferably involves the use of a light source for generating a light beam (preferably a collimated light beam), which can be directed towards the surface of the liquid to be measure. The light beam will be scattered by the fluid surface. In a particularly preferred embodiment the wavelength of the scattered light is substantially 15 1300 nanometres (this wavelenth may be +/- 100 nm, or +/- 50nm or +/- 1Onm) A detector is preferably provided detect some of the scattered light. The detector preferably detects the intensity of the scattered light.

20 In a preferred embodiment, the invention involves using the measurements in the changes in the intensity of the detected scattered light to determine whether or not the fluid is flowing. In a preferred embodiment, the changes in the intensity are used to measure flow velocity.

25 The detector can generate a signal output which can be fed to a signal processing means. The signal processing means is provided with the components necessary to perform the determination about fluid flow and velocity. In one example the flow velocity can be measured using a conventional velocity area method. The signal processing means can generate a signal which can be passed to a display means, such as a LCD 30 display, which may display relevant information, such as whether flow is taking place and the flow velocity.

The components of the apparatus, including the light source and the detector, and preferably the signal processing equipment, are preferably housed in a single housing.

The source and detector can be located close together, so that the angle between the light beam and the path of the detected scattered light may be very low, eg less than 5 10 , preferably less than 5 , more preferably less than 1 , still more preferably less than o.5o. The apparatus can be used at a long distance from the fluid, for example, more than one metro, preferably more than 2 metros, more preferably more than 5 metros, still 10 more preferably more than 10 metros. An upper limit of about 50 or 100 metres may be appropriate. The signal processing means can preferably compensate for changes in the distance in the to the liquid surface by appropriate calibration and making use of well known 15 factors, such as the inverse square law relationship between intensity of detected scattered light and the distance to the surface.

The accuracy of the flow estimation can be improved by using an empirical calibration from a depth - velocity flow monitor installed temporarily to log one or more spill 20 events.

The signal from the detector may be passed through amplification and demodulation stages. The detector signal may be processed to convert it to a signal from which the existence of movement can be detected, or flow velocity.

Means to measure the depth of the fluid may be provided (preferably in the apparatus housing), and this additional measurement can enable volumetric flow rate to be determined. 30 Reference is now made to the accompanying drawings, in which: Figure 1 is a schematic diagram showing an embodiment of a flow monitor according to the invention;

Figure 2 is a schematic diagram showing the signal processing for the monitor shown in figure 1; and 5 Figure 3 shows a further embodiment of a flow monitor according to the invention, which can also measure displacement of a target area.

Referring to figure 1, a flow monitor 1 is mounted above a moving surface 2, for example a liquid surface such as a water surface. Light from a collimated light source 3, 10 for example a diode laser, is directed through a window 4 in the end of the flow monitor 1 to strike the moving surface 2, resulting in reflected light 5 and refracted light 6. When the surface 2 is moving ripples and turbulence on the surface 2 will cause the collimated light to be reflected at many different angles, and some of these such as 7 will be reflected back towards the flow monitor 1. A lens 8 collects this light and focuses 15 it upon the detector 9, which may be, for example, a silicon photodiode.

We have found that with the typical surface flow velocities in a CSO of 0. 2 to 3 metros per second the frequency of the detected signal is predominantly in the range 5 to 100 Hz. As the surface velocity increases, the power of the reflected signal increases, thus 20 providing a means to infer surface flow velocity.

The signal processing is shown schematically in figure 2. The collimated light source 3 is modulated at a frequency well above the 50 and 100 Hz flicker of mains light sources using an oscillator 10. The signal from the detector 9 is amplified using a conventional 25 transimpedance amplifier 11 and then AC coupled to a further gain stage 12. The signal is then demodulated using demodulator 13 to obtain the amplitude of the received radiation at the frequency of the oscillator 10. The signal then passes through a low pass filter 14 operating at 100 Hz and is AC coupled to a RMS to DC converter 15. The RMS to DC converter is a convenient way to obtain a signal which increases with the 30 AC power generated by the reflected light. Other methods such as total AC power derived from Fourier analysis could be used. The functions of the signal-processing scheme can be implemented digitally with an analogue to digital converter taking the signal from the second amplifier 12.

Referring to figure 1, it is apparent that the light intensities received by the detector 9 will vary with the distance between the flow monitor 1 and the liquid surface 2, and that this will follow an inverse square law. Typically the distance will be in the range 1 to 3 5 metros, and so to normalise the signal to a standard range a multiplier will be needed in the signal processing chain, for example to normalise to a 1 metre range signals measured at a range of 3 metros will need to be multiplied by a factor of 9.

In order to measure volumetric flow it is necessary to measure the cross section of the 10 flowing liquid and to perform a velocity-area calculation. There is abundant literature on how this may be done both in textbooks and standards (see for example ISO 748-1979 (E) Liquid flow measurement in open channels - Velocity-area methods). In order to make the calculation it is necessary to know both the geometry of the flow structure and the liquid level. The liquid level may be measured by means of a modification to the 15 flow monitor.

Referring to figure 3, the single detector is replaced by a position sensitive detector 16.

Whereas in the basic flow monitor the optical axes of the collimated source and the lens and detector may be parallel or close to parallel provided that the image of the light 20 spot on the liquid surface is fully imaged onto the detector through the working range of the instrument, it is necessary for level measurement for the 2 axes to be non-parallel.

It is also necessary that the focusing lens 8 is either well corrected for aberrations, or corrections are made in software if a simple lens such as a single piano-convex lens is used. As the liquid level varies between a low level 17 and a high level 18 the image of 25 the collimated light spot moves across the detector. The depth of flowing liquid can be inferred from a knowledge of the range from the flow monitor 1 to the liquid surface 2, the angle of the monitor axis with the horizontal 19 and an initial depth measurement when the system is installed.

30 A further development of the flow monitor may be used to obtain a measure of the turbidity of the flowing liquid. For liquids such as sewage or treated sewage this would give an indication of the polluting load of the discharge. The light received by the detector includes a varying component due to the surface ripples and turbulence of the

flowing liquid and variations in light scattered back from the refracted beam if the liquid contains particles or bubbles. In a turbid liquid the scattered light from the refracted beam is always present and provides a steady light intensity at the detector which will increase with increasing turbidity.

Referring to figure 2, this steady signal is available at the output from the low pass filter 14. This steady light intensity will also include light scattered by fog and mist in the light path from the collimated source to the liquid surface and light scattered back from the floor of the flow structure. Provided that steps are taken to minimise the steady 10 contribution to detected light from mist and fog and from the floor of the structure, a useful measure of the sample turbidity can be made under most conditions. To avoid most of the backscatter from mist and fog, it is necessary to use an optical arrangement where the detector aperture excludes the path of the collimated light source beam for most of its length. To avoid back scatter from the floor of the channel the collimated 15 light beam wavelength can be chosen so that it is strongly absorbed by the flowing liquid. The optimum wavelength is a trade-off between allowing sufficient penetration of the liquid to obtain useful turbidity information but with the attenuation needed so that at typical liquid depths the back scattered light from the floor is negligible in comparison with the turbidity contribution. For example with aqueous samples a wavelength of 980 20 nanometres is strongly absorbed by pure water and this would suppress back scatter from the channel floor for depths of 100 millimetres or more. If shallower depths are expected a wavelength further into the infrared may be used such as 1300 nanometres where stronger absorbance occurs.

25 It will be appreciated that the invention can be modified.

Claims

l CLAIMS

1. A method of determining whether a liquid is flowing, comprising directing a light beam towards a surface of the liquid, detecting light from said light beam scattered 5 back from the surface, and using the detected scattered light to determine whether the liquid is flowing.

2. A method according to claim 1, further comprising using the detected scattered light to determine flow velocity.

3. A method according to claim 1 or 2, comprising applying to a signal generated from the detector an RMS algorithm or power spectral density over approximately 5 -

100 Hz frequency range to produce.

15 4. A method according to any preceding claim, wherein the light beam source and the light beam sensor are disposed 1-3 metres from the liquid surface.

5. A method according to any preceding claim, comprising measuring the power of the light scattered back to the detector.

6. A method according to any preceding claim, comprising using a velocityarea method in conjunction with the detected scattered light to estimate volumetric flow rate.

7. A method according to any preceding claim and the comprising using a velocity 25 area method in conjunction with the detected scattered light to estimate flow velocoty and measuring sample depth to determine volumetric flow rate using a position sensitive detector or detector array in the monitor.

8. A method according to any preceding claim, wherein a normalization factor is 30 applied to a signal representative of the detected scattered light intensity to correct for the variations in the range of the sample using the inverse square law or empirical measured relationship.

9. A method according to any preceding claim, wherein a steady or DC component of the detected scattered light is used to estimate the sample turbidity, which is a measure of quality for a wastewater.

5 10. A method according to any preceding claim, wherein an operating wavelength absorbed by pure water is used to suppress scattered and reflected light from the channel floor, for example 1300 nanometres.

11. A method according to any preceding claim, wherein the liquid flow is along an 10 open channel and is intermittent.

12. An optical non-contact monitor comprising means for directing a light beam onto a moving fluid surface, detection means for detecting light from said light beam scattered back from the surface, and flow measuring means for determining whether 15 the liquid is flowing, from the variation in the detected light intensity.

13. An optical non-contact monitor according to claim 1, wherein the flow determining means also determines the liquid flow velocity.

20 14. A method of determining whether a liquid is flowing substantially as herein described, with reference to and as shown in the accompanying drawings.

15. An optical non-contact monitor substantially as herein described, with reference to and as shown in the accompanying drawings.