GB2141553A - Scatter cells for photosensors - Google Patents

Abstract

A scatter cell comprises a housing having at least four windows or openings for light sources S1 and S2 and detectors D. On-line all purpose intelligent particulate sensors employ the scatter cells and microprocessor means (38) to calculate parameters of fluid media passed through the cells. With a multi-source multi- angle scatter cell a plurality of parameters can be calculated simultaneously and on-line, such as fluid flow rate, particle concentration and colour. <IMAGE>

Description

an on-line cell.

We have now found that a scatter cell even more applicable to use in many industrial flow systems may be achieved if more than one light beam source is employed, together with an appropriate number of detectors. The use of more than one source enables a plurality of parameters of the fluid flow to be monitored at a time. One such multi-source scatter cell is illustrated schematically in Fig. 2. It comprises two light sources, or windows therefor, 1 5 and 16, an array of detectors or windows therefor, 17, 18, 19, and 20, together with a suitably disposed baffle 21 to prevent unscattered light reaching the detectors or windows.

An alternate form of multi-source scatter cell is illustrated in Fig. 3. In this arrangement two sources, or windows therefor, 22 and 23 are spaced axially a distance "d" along the length of the cell housing 23, the flow through which is indicated by the arrow A.

Each source, or window, has a respective set of detectors, or windows, 24, 25 and 26, 27 in alignment therewith. Spacing the sources along the length of the cell is required for some types of optical systems, for determining flow rates, for example. Optical methods for determining flow rates include use of time of flight, power spectra or cross correlation.

Examples of methods employing light beams to determine fluid flow rates are disclosed in our co-pending applications No. 8206028 and No. 8206031 (G.D.Pit; et al 36/3915/18-8/11-2/5 and G.D.Pitt et al 38-1710-4). In application 8206028 the velocity of a fluid flowing in 2 pipe is determined from calculations of the phase-angle difference for different frequency values between two spaced apart (in the direction of flow) optical signals transmitted through the fluid. The slope of the phase-angle curve is related to the transit time of a particular desturbance in the flow between the two optical signals, hence the flow velocity. The electronics to perform the flow velocity calculation may include a microprocessor and a fast Fourier transform.In application No. 8206031 use is made of the fact that a light beam signal is modulated by passage through a fluid stream, as across a fluid flow through a pipe. By detecting the modulated signal and applying it to a fast Fourier transform a power spectrum is obtained. The flow velocity v may be calculated from An expression such as v = Kfc, where K is a constant and fc is the cut-off frequency for the first minimum of the power spectrum.

If the two or more sources of a multi-source multi-angle scatter cell each employ input light of a different wavelegnth then a colour measurement for the fluid may be obtained.

Thus a single sensor having such a multisource multi-angle scatter cell and a microprocessor and other suitable electronics permits the simultaneous measurement on-line of a fluid flow of, for example, particulate concentration, colour and flow rate, since the microprocessor will allow several calculations to be performed.

In such a sensor which is required basically for turbidity or colour measurements, the flow rate measurement could be used to provide a simple alarm for flow "on" or "off". Changes in flow can cause changes in apparent scatter measurement, however with combined flow and concentration measurements, compensation can be automatically carried out at the sensing head.

Since on-line particle sizing is possible with two or three angle detectors, particle size regime changes can be observed and alarms given. For example, this may be employed with hydraulic fluids or fuel oils. In particular darkening when the quality of an oil deteriorates may be indicated, or the sudden appearance of large particles upon filter failure may trigger an alarm.

The electronic circuitry employed with the multi-source multi-angle scatter cell will be determined basically by the type of measurements and calculations required, however it may be substantially the same as that disclosed in our co-pending application No.

8206027 except that the extra sources and detectors will require corresponding circuitry and appropriate computation means will be required for the flow calculations. With microprocessors, fast Fourier transforms and crosscorrelation chips such calculations are now possible by means which occupy a relatively small space and thus can be mounted to a scatter cell to provide an on-line intelligent sensor.

One possible form of electronic circuitry for use with a multi-source multi-angle scatter cell is illustrated schematically in Fig. 4 and employs digital processing techniques. Associated with each light source S1, S2 are three detectors D,1, D,2, D,3 and D21, D22, D23, respectively. The outputs of detectors D, 1, D,2, D,3 are applied to respective sample/hold circuits 30, 31, 32 via respective amplifiers Al, A2, A3. The outputs of detectors D21, D22, D23 are applied to respective sample/hold circuits 33, 34, 35 via respective amplifiers A4, A5, A6.The outputs from the detectors D,1 and D21 associated with the direct paths through the cell are fed back to the respective drive circuits LD1 and LD2 so as to provide automatic gain control feedback loops to provide compensation for both ageing and drifting of the light source, thus enabling continuous calibration of the detector arrangement. The outputs of the sample and hold circuits 30, 31, 32 and an output from drive circuit LD1 are applied to an analogueto-digital converter 36, whereas the outputs of sample and hold circuits 33, 34, 35 and an output from the drive circuit LD2 are applied to an analogue-to-digital converter 37. The SPECIFICATION Sensors and sensor systems This invention relates to sensors and sensor systems and in particular to an all-purpose online intelligent sensor and elemenys thereof.

In our British Patent Specification No.

1588862 (G.D.Pitt 24) there is disclosed a multi-angle scatter cell for use in detecting solid particles, such as rust, as well as liquid particles, such as oil droplets, in water. Such a scatter cell was developed for the monitoring of oil in water in ships ballast and bilge discharge waters, and may also be employed in a system for detecting the concentration of one or more types of oil suspended in water, as described in our co-pending application No.

8206027 (Serial No. ) (G.D.Pitt-B.J.Scott P.Extance 34-5-1X).

According to one aspect of the present invention there is provided a scatter cell including a housing having first and second input windows for respective first and second light beams and a plurality of spaced output windows.

According to another aspect of the present invention there is provided a sensor head comprising a multi-angle scatter cell and incorporating a microprocessor for calculating parameters of a fluid medium passed through the scatter cell in use of the sensor head from output signals of the scatter cell.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 shows somewhat schematically a single source multi-angle scatter cell; Fig. 2 shows somewhat schematically one embodiment of a multi-source multi-angle scatter cell; Fig. 3 shows somewhat schematically a second embodiment of a multi-source multiangle scatter cell; Fig. 4 shows a schematic block circuit diagram, and Fig. 5 shows detection using a fibre optic array.

Referring to Fig. 1 the multi-angle scatter cell 1 comprises a housing 2 through which a fluid medium to be sensed, for example water carrying suspended material, may flow, and provided with an inlet window 3 for incident light and a plurality of outlet windows, 4, 5, 6, for receiving light scattered from the suspended material. The incident light beam may be provided via an infra-red solid state laser (not shown), or an LED or other light source may be employed. A baffle 7 is mounted adjacent the window 3 to ensure that only scattered light reaches the windows 4, 5 and 6. In some applications the baffle may be mounted perpendicularly to the incident light beam. Whereas the cell housing 2 shown is circular in cross-section, square or rectangular cells may alternatively be employed.

Scattered light reaching the windows 4, 5, 6 is fed to respective photodetectors 8, 9, 1 0 which may be mounted to the windows or, and as shown, via optical fibres 11. Advantageously, the scatter cell includes a further output window 1 2 and an associated detector (not shown) whereby the intensity of the incident beam may be monitored and employed, for example, for automatic gain control, as an element 13, of the drive current of the incident light beam source (transmitter), in order to compensate for changes in the incident light beam intensity. The outputs of the photodetectors 8, 9, 1 0 are coupled to (receiver) electronic circuitry, e.g. a microprocessor 14, programmed, for example, to compute the suspended material concentration level from the absolute and relative values of the photodetector outputs.This is possible since the angular scatter of light by the suspended material, for example oil droplets and rust, is a function of the concentration and the particle size distribution. In general smaller particles will scatter light to a larger angle than larger particles. The light intensity may thus be measured at a plurality of angles and the signal strengths calculated such that the relative particulate concentration can be measured. Alternatively approximate particle size information may be determined. Therefore, provided the particle sizes or refractive indices of the stream constituents are sufficiently different, on-line discrimination is possible using multiple forward-angle detection.

Particularly, but not exclusively, in the case of a square or rectangular cell the receiver electronics may be such that the detectors, comprised, for example, by solid state detectors, and their associated pre-amplifiers may be mounted on a single flat printed circuit board mounted directly to the cell. To the same printed circuit board may also be mounted the microprocessor 14, the light beam source and the automatic gain control element 1 3 and any other necessary elements whereby to provide an intelligent sensing head for incorporation in an on-line flow line, for example, and having outputs connectible to display or control elements as required.

Whereas the scatter cells and detecting systems described in our Patent Specification No.

1 588862 and co-pending application No.

8206027 were developed for monitoring oil and particulates such as rust or silt in water, such scatter cells and detecting systems may alternatively be employed for use with any type of flow system through which relatively transparent liquids or fluids are transmitted and because of the rapid response available the cells and systems are particularly appli cable for use on-line in many flow systems.

Relatively transparent liquids or fluids are es sentiai for the light to be transmitted across converter outputs are applied to a suitably programmed microprocessor 38, including any necessary fast Fourier transforms and cross-correlation chips, whose outputs may be applied to display elements and/or control elements as required.

Optical measurement techniques can provide highly stable measurements in difficult enviroments, such as flow and turbulence studies. For example an optical probe can examine smaller volumes in a flow profile than ultrasonics; and thus the bandwidth of information available for processing is larger. The same on-line scatter cells may thus be used to measure continuously, and in parallel, (a) flow rate, (b) three-phase constituent concentrations (e.g. oil, water, rust) and (c) flow profile.

The power spectra analysis, algorithms and control functions can all be accommodated in logic-processing chips (microprocessor).

Particular application where such an on-line intelligent sensor would be advantageous include boiler condensate (oil) monitoring; pure water particulate monitoring in the semiconductor industry; clean room, disk, electronic, computer technology; intravenous liquids monitoring, hospital saline solutions monitoring, outfall monitoring (for example at refineries and power stations); hydrocyclone output control (for example oil concentration measurement can be used to control the output orifice of a hydrocyclone. The degree of separation can depend on the particle size of the oil droplets. The output orifice can be narrowed or opened based on the scatter cell output i.e.

the sensor is used to monitor and control); chemical and food industries; monitoring water in oils and fuels, and monitoring water in gas streams.

The sensor may be employed for monitoring solids in gases and in particular detecting excess salt in a stream of gas to identify situations where salt is likely to crystalise out and affect the measurement of gas flow across an orifice plate. There is a need to detect water in a natural gas stream, typically at a level of 65 lb per mmcf to an accuracy of 6.5 Ib/mmcf, this can be achieved with the intelligent sensor. By using the sensor to detect change in size of particulates ina stream of hydraulic oil there is provided a means of identifying the point at which increased system wear is taking place. In similar application in aircraft engine oil it may be used to detect incipient engine failure. By monitoring water in gas condensates corrosion and other effects may be minimised.Particulates in gas streams may be detected, as for example foliowing well treatment. The sensor is capable of detecting low levels of solids in water, such detection being of importance in semiconductor plant water supplies.

The sensors described above empioy only a small number of discrete detectors or detector windows disposed at the cell housing. Alternatively, however, a full array of fibre optic detectors may be employed around the measurement cell. As well as improving resolution, this will enable additional information to be obtained and overcomes the disadvantage of the relatively large physical dimension of packaged silicon detectors and the difficulty in mounting them close together. Typically optical fibres can have an outer diameter of 200 micrometres, and they can be spaced around the cell as, for example, shown in Fig. 5 in a light guide array 40, together with a detector 41 and scanning system 42. This enables continuous on-line particle size measurement to be achieved, with automatic read-out, over forward and back scatter profiles.

The scatter cells and sensors incorporating them are particularly advantageous for continuous on-line industrial monitoring and control applications in view of their robustness, with no moving parts, and the accuracy and discrimination obtainabie. They allow for the monitoring of the concentration of particles and changes occurring in process strams, and are applicable to a wide range of concentrations e.g. from 0 to 2 ppm (i 0.1 ppm) as in semiconductor process water applications, to 0 to 3000 ppm in tanker ballast discharge, or up to 9000 ppm in very dense streams as in a hydrocyclone.

Claims (11)

1. A scatter cell including a housing having first and second input windows for respective first and second light beams and a plurality of spaced output windows.

2. A scatter cell as claimed in claim 1 and which in use has fluid flow through the housing in a first direction, wherein the input and output windows are disposed around the circumference of the housing in a common plane generally normal to the first direction.

3. A scatter cell as claimed in claim 1 and which in use has fluid flow through the housing in a first direction, wherein the first input window and an associated number of the plurality of output windows are disposed around the circumference of the housing in a first common plane generally normal to the first direction, and wherein the second input window and the remainder of the plurality of output windows are disposed around the cir cumference of the housing in a second common plane generally normal to the first direction, the first and second planes being spaced a predetermined distance apart in the first direction.

4. A scatter cell as claimed in any one of the preceding claims, wherein the ouput windows comprise or are associated with detector means constituted by a plurality of optical fibres in a light guide array disposed around the housing.

5. An on-line intelligent sensor comprising a scatter cell as claimed in any one of the preceding claims and microprocessor means for calculating parameters of a fluid medium passed through the scatter cell in use of the sensor from output signals of the scatter cell.

6. An on-line intelligent sensor as claimed in claim 5, wherein the microprocessor means is mounted to the housing.

7. A scatter cell substantially as herein described with reference to Figs. 2 or 3 with or without reference to Fig. 5.

8. An on-line intelligent sensor substantially as herein described with reference to Fig. 4 of the accompanying drawings.

9. A sensor head comprising a multi-angle scatter cell and incorporating a microprocessor for calculating parameters of a fluid medium passed through the scatter cell in use of the sensor head from output signals of the scatter cell.

10. A sensor head as claimed in claim 9, wherein the scatter cell employs a single light source and the output signals of the scatter cell are derived from light scattered to output windows of the cell comprised by or associated with detecting means constituted by a plurality of optical fibres in a light guide array.

11. A sensor head as claimed in claim 9, wherein the scatter cell employs two or more light sources and the output signals of the scatter cell are derived from light scattered to output windows of the cell comprised by or associated with detecting means constituted by a plurality of optical fibres in a light guide array.

1 2. A sensor head as claimed in claim 11, wherein each.light source emits at a different wavelength.

1 3. A sensor head substantially as herein described with reference to Fig. 1, with or without reference to Fig. 5, of the accompanying drawings.