GB2029569A

GB2029569A - Fluid flowmeter

Info

Publication number: GB2029569A
Application number: GB7835900A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-09-07
Filing date: 1978-09-07
Publication date: 1980-03-19
Also published as: GB2029569B

Abstract

In a fluid flowmeter first and second light beams are transmitted across moving fluid. Transient turbulence effects in the fluid cause small changes in the intensity of the light beams, these changes being monitored by detectors D1, D2 one aligned with each beam. By measuring the time taken for a particular fluid eddy to cross the two light beams the fluid velocity is determined. Detector D1 is connected via a variable delay circuit, and detector D1 directly, to a correlation circuit, controlling the delay circuit. Either a differential mixer and peak detector, or a microprocessor, programmed to perform the correlation, may be used. <IMAGE>

Description

SPECIFICATION Fluid flowmeter This invention relates to fluid flowmeter and in particular to flowmeter of the optical type.

The accurate measurement of fluid flow involves numerous difficulties. In particular an instrument employed to effect such measurement must be insensitive to the presence of impurities, such as suspended solids, in the fluid and must also respond to discontinuous changes in flow rate. The instrument must also offer minimal restriction to the fluid flow and, where hazardous fluids are involved, must be electrically isolated from the fluids. Flowmeters which satisfy these requirements are generally costly and require complex electronic circuitry to process the measurement and to inhibit spurious error signals.

The object of the present invention is to minimize or to overcome these disadvantages.

According to one aspect of the invention there is provided an arrangement for measuring the velocity of a moving fluid, includng means for directing light across the fluid stream at a first upstream position and a second downstream position, means for detecting the intensity of the light transmitted through the fluid, from the first and second positions, and means for measuring the time taken for a transient light intensity fluctuation to pass from the first to the second positions, said time corresponding to the fluid velocity.

Accordng to another aspect of the invention there is provided a method of measuring the velocity of a moving fluid, including directing light across the fluid stream from a first upstream position and a second downstream position, measuring the intensity of light transmitted through the fluid from each position, and determing the time taken for transient light intensity fluctuations to travel between the first and second positions, said time corresponding to the fluid velocity.

We have found that, even at selectively low flow rates in clear fluids, transient turbulence effects cause variations in the intensity of a light beam transmitted through the fluid. Thus by measuring the intensity of light transmitted through the fluid at two positions, one upstream of the other, it is possible to measure the time taken for a transient intensity variation to travel from one position to the other thus giving a simple measure of the fluid velocity. By employing optical waveguides to couple light signals into and out of the fluid, the velocity measurement may be effected with minimal impedance of the fluid flow and without the need to provide electrical connection adjacent the fluid.

The term fluid as employed herein is understood to include both liquids and gases.

Embodiments of the invention will now be described with reference to the accompanying drawings in which Figure lisa schematic diagram of a fluid flowmeter arrangement; Figure 2 illustrates the fluid velocity measurement technique employed in the flowmeter of Figure 1; Figure 3 shows a correlator arrangement employed in the flowmeter of Figure 1; and Figure 4 shows an alternative correlator arrangement.

Referring to Figure 1, the flowmeter arrangement is coupled to a pipe or conduit 11, carrying a fluid whose flow into is to be measured, via optical waveguides 12 and 13. Advantageously these waveguides may comprise optical fibre bundles. The input waveguide 12 is of Y construction and couples a light source 14, e.g. a light emitting diode, to two transmission windows 15 spaced longitudinally along the pipe or conduit 11. Diametrically opposite each input window is an output window 16, each output window being coupled to a respective output waveguide 13. The waveguides 13 are coupled to first and second photo-detectors D1 and D2, which the detectors are coupled to a correlation circuit C1.

Light from the light source 14 is divided into two beams of substantially equal intensity by the Ywaveguide 12 which beams are then directed through the fluid to the output waveguides 13. The direction of transmission may be perpendicularto the flow direction of the fluid or, in some applications, may deviate from perpendicularity by an acute angle.

As the liquid flows along the pipe or conduit 11 minor transient turbulence effects cause diffraction of light signals passing through the fluid thus causing small changes in the intensity of light received by the output waveguides 13. If the two light beams are sufficiently close together, i.e. a few centimetres apart, a transient fluid eddy is substantially unchanged during its passage from one light beam to the other. Thus, a turbulence induced light intensity change received at one (upstream) output waveguide is mirrored by a subsequent intensity change at the other (downstream) output waveguide. This effect is illustrated in Figure 2 which demonstrates the correlation between the light signals received by the detectors D1 and D2. As can be seen the two received signals are similar, but one is time shifted with respect to the other.This time shift gives a measure of the fluid velocity, the necessary signal processing being effected by a correlation circuit C1.

A correlation circuit for processing the detector signals is shown in Figure 3. In this arrangement the detectors D1 and D2 are coupled, the upstream detector D1 via a variable delay circuit 21, to an analogue differential mixer circuit 22. The output of the differential mixer is coupled to a peak position detector circuit 23 which latter circuit controls the time delay circuit 21 via a feedback loop. The function of the peak position detector 23 is to so control the delay circuit 21 that the outputs of the two detectors D1 and D2 become time superimposed. When this is achieved the peak position detector output, which may be coupled to a display 24, gives a measure of the fluid velocity.

The peak position detector circuit receives the two mixed signals from the mixer circuit, this signal comprising the two random light fluctuations received by the detector. The circuit examines the mixed signal for pairs of signal peaks and produces an output signal corresponding to the degree of overlap of the peaks. This signal is then used to adjust the delay circuit to bring the signal peaks substantially into concidence.

An alternative correlation technique is shown in Figure 4. The technique is similar to that employed by the arrangement of Figure 3, in that the output of the upstream detector D1 is coupled to a variable time delay circuit 21, but the necessary correlation process is preformed by a suitably programmed microprocessor 31. As the microprocessor is a digital device the input signals are applied via analogue to digital converters 32 and 33 and the microprocessor output is fed through a digital to analogue converter 34.

Claims

1. An arrangement for measuring the velocity of a moving fluid, including means for directing light across the fluid stream at a first upstream position and a second downstream position, means for detecting the intensity of the light transmitted through the fluid, from the first and second position, and means for measuring the time taken for a transient light intensity fluctuation to pass from the first to the second position, said time corresponding to the fluid velocity.

2. An arrangement for measuring the flow velocity of a moving fluid, the arrangement including a light source, an optical waveguide for dividing the source output into two substantially equal intensity beams and arranged so as to direct the two beams across a moving fluid, one beam being upstream of the other, first and second detectors each coupled via a respective output waveguide to a respective beam, a variable delay circuit coupled to the detector receiving the upstream light beam, and correlation means for controlling the variable delay circuit such that the output of the downstream detector and the output of the upstream detector are time superimposed, and wherein the magnitude of the time shift applied via the delay circuit provides a measure of the fluid velocity.

3. An arrangement as claimed in claim 2, wherein the correlation means includes a signal peak detector circuit.

4. An arrangement as claimed in claim 2, wherein the correlation means includes a microprocessor.

5. An arrangement as claimed in claim 2, 3 or 4, and wherein a display is coupled to the correlation means

6. An arrangement as claimed in any one of claims 2 to 5, wherein the optical waveguides comprise optical fibre bundles.

7. A fluid flowmeter substantially as described herein with reference to Figures I to 3 orto Figures I, 2 and 4 of the accompanying drawings.

8. A method of measuring the velocity of a moving fluid, including directing light across the fluid stream from a first upstream position and a second downstream position, measuring the intensity of light transmitted through the fluid from each position, and determining the time taken fortran- sient light intensity fluctuations to travel between the first and second position, said time corresponding to the fluid velocity.

9. A method of measuring fluid flow substantially as described herein with reference to Figures 1 to 3 or to Figures 1,2 and 4 of the accompanying drawings