GB2082764A

GB2082764A - Measuring Particles in Fluids

Info

Publication number: GB2082764A
Application number: GB8027232A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-08-21
Filing date: 1980-08-21
Publication date: 1982-03-10
Also published as: GB2082764B

Abstract

An arrangement for detecting and measuring solid or liquid particles (13) suspended in a moving fluid includes a light source (11) and a photodetector disposed so as to receive light from said source through the moving fluid. Suspended particles eg contaminants in water passing the detector (12) cause a transient pulse reduction in its output. These output signals are fed via a threshold circuit (15) to a pulse counter (16) coupled via an integrator (17) to a switch (18) which operates an alarm or cuts off the water supply when the pulses reach a predetermined rate. <IMAGE>

Description

SPECIFICATION Measuring Solids in Fluids This invention relates to arrangements for detecting and measuring particles suspended in fluids.

According to one aspect of the invention there is provided an arrangement for detecting and measuring solid or liquid particles suspended in a moving fluid, the arrangement including a collimated light source, a photodetector disposed so as to receive light from said source through the moving fluid, the source and detector being so arranged that the light path therebetween is not co-linear with the fluid velocity vector, and means for detecting the magnitude and means frequency of attenuation signals produced by the detector in response to the passage of said particles through the light beam.

According to another aspect of the invention there is provided a water supply monitoring arrangement adapted to respond to contaminant particles suspended in a water stream the arrangement including a light source for generating a substantially parallel beam, a photodetector disposed so as to receive at least a portion of the parallel beam via the water, the source and detector being so arranged that the light path therebetween is not co-linear with the fluid velocity vector, a threshold circuit coupled to the detector output and arranged to produce an output pulse when the detector output falls to a predetermined value corresponding to the presence of a said particle in the light path between the source and the detector, and means for counting said pulses so as to provide an indication of the number of said particles per unit volume of water.

All fluids to a greater or lesser extent contain suspended particles which cast a shadow when illuminated with a substantially parallel light beam. The precise nature of this shadow is determined by the relative refractive indices of the fluid and the particles, and the ratio of the particle diameter to that of the light beam. The limit of resolution is fixed by the wavelength of the light and the noise of the system. In general the attenuation of a light beam is, to a first approximation, proportional to the square of the average particle diameter.

We have found that a detectable shadow can be produced from a particle containing fluid which moves with uniform velocity past a collimated light beam which is incident on a detector. The shadow of a suspended particle reduces the output of the detector from the value obtained with an unobscured beam.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic view of a particle detector and measurement arrangement; and Fig. 2 illustrates the detector output signal of the arrangement of Fig. 1.

Referring to Fig. 1, the particle measurement arrangement includes a light source 11 typically a semiconductor laser from which a parallel light beam is directed to a photodetector 12, which photodetector produces an output voltage proportional to the intensity of the incident light beam. Advantageously the light sensitive element of the detector 12 comprises a PIN diode. Each suspended particle 13 casts a shadow 14 on the detector 12 as it is carried past the detector by the moving fluid thus causing a temporary drop in the output of the detector, i.e. a negative pulse (Fig. 2) superimposed on the detector output.The depth AV of the output pulse is given by the expression: AV=K(d/D)2 Where D is the diameter of that portion of the detector 11 on which the collimated beam is incident, d is the particle diameter and K is a constant, approximately equal to unity, which is determined by calibration of the system with particles of known size. Hence, once the calibration constant has been determined, the output pulse depth AV gives a direct measure of the size of particles suspended in the fluids.

The concentration of particles in the fluid is determined by measuring the average rate at which pulses are produced by the detector. The detector output is feed via an adjustable threshold circuit or pulse height discriminator 14 to a pulse counter circuit 1 5. By suitable adjustment of the threshold circuit the total particle concentration above a predetermined threshold particle size can be measured.

For a particle containing fluid flowing past the detector arrangement the mean pulse frequency dN dt is given by the expression: dN/dt=g TDV Where T is the distance between the light source and the detector and V is the velocity of the fluid. From this expression the particle concentration g is obtained directly and no calibration is required.

The arrangement described herein may be used in a variety of applications. In particular it can be used for monitoring and measuring trace solid contaminants in liquids. Thus it can be used e.g. to monitor water supplies for semiconductor processing or for monitoring contaminants, such as rust particles, in boiler feed water or nuclear reactor cooling water. Other applications include the measurement of suspended solids in effluent water streams.

Where the arrangement is used to monitor contaminants in purified water supplies the output of the pulse counter 1 5 may be coupled via an integrator 16 to switching means 17. The integrator and switching means are arranged so that, when the rate at which pulses are counted by the counter 1 5 exceeds a predetermined rate the switching means operates an alarm device (not shown) and/or shuts off the water supply.

The term 'light' as employed herein is understood to include not only the visible region of the spectrum but also the infra-red and ultraviolet regions.

Claims

1. An arrangement for detecting and measuring solid or liquid particles suspended in a moving fluid, the arrangement including a light source for generating a substantially parallel beam, a photodetector disposed so as to receive light from said source through the moving fluid, the source and detector being so arranged that the light path therebetween is not co-linear with the fluid velocity vector, and means for detecting the magnitude and means frequency for attenuation signals produced by the detector in response to the passage of said particles through the light beam.

2. A water supply monitoring arrangement adapted to respond to contaminant particles suspended in a water stream the arrangement including a light source for generating a substantially parallel beam, a photodetector disposed so as to receive at least a portion of the parallel beam via the water, the source and detector being so arranged that the light path therebetween is not co-linear with the fluid velocity vector, a threshold circuit coupled to the detector output and arranged to produce an output pulse when the detector output falls to a predetermined value corresponding to the presence of a said particle in the light path between the source and the detector, and means for counting said pulses so as to provide an indication of the number of said particles per unit volume of water.

3. An arrangement as claimed in claim 2, wherein the pulse counter is coupled to switching means for disabling the water flow when the pulse rate from the threshold circuit exceeds a predetermined value.

4. An arrangement as claimed in claim 1, 2 or, 3, wherein the light source comprises a semiconductor laser.

5. A fluid monitoring arrangement substantially as described herein with reference to the accompanying drawings.

6. A semiconductor processing plant including a water monitoring arrangement as claimed in any one of claims 1 to 5.

7. Water supplied via a monitoring arrangement as claimed in any one of claims 1 to 5.

8. A method of measuring the concentration of suspended solid particles in a moving fluid, the method including directing a substantially parallel light beam through the fluid towards a photodetector whereby solid particles passing through the light beam cause a transient reduction or negative pulse in the photodetector output, and rejecting the negative pulses from the detector having an amplitude less than a predetermined minimum value, and counting and time averging the remaining pulses so as to provide measure of the number of particles per unit volume of fluid.

9. A method as claimed in claim 8, wherein said light beam is derived from a semiconductor laser.

10. A method as claimed in claim 8 or 9 wherein the fluid is water.

11. A method of measuring the concentration of suspended particles in a fluid which method is substantially as described herein with reference to the accompanying drawings.