GB1595973A - Flow sensor - Google Patents

Description

(54) FLOW SENSOR (71) We, NATIONAL RESEARCH DEVEL OPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 6974 Victoria Street, London, S.W.l, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particuarly described in and by the following statement:- This invention relates to a method and apparatus for sensing flow using beams of wave energy.

In previous arrangements for sensing the flow of a meterial through a pipe, such as that described in the Complete Specification of U.K. Patent No. 1,359,151, four ultrasonic transducers are provided arranged as two pairs each comprising a transmitting and a receiving transducer on opposite sides of the pipe, with the pairs spaced axially along the pipe. Energy emitted by each transmitter is received, after passage through the flowing material, by the associated receiver. Fluctuations in the received sound, due to fluctuations in the acoustic properties of the flowing material, are crosscorrelated to provide the transit time of the material between the pairs of transducers.

Similar arrangements of transducers emitting and receiving other types of radiation are also known.

However, disadvantages of the arrangements are that not all materials transmit suitable radiation; that the axial spacing of the pairs of transducer is sometimes difficult to determine precisely; and that access to the pipe from diametrically opposite directions is required.

According to the invention a method of of sensing the flow of a material comprises launching a beam of wave energy into the material substantially perpendicular to the general direction of flow from each of two positions spaced in the direction of flow; sensing at each of the two positions any fluctuations in the impedance to the energy of the flowing material; and determining by cross correlation of the sensed fluctuations whether there exists a transit time between the two positions of irregularities in the flow which cause said fluctuations.

It is believed that it has not previously been realised that it is not necessary to transmit a beam of wave energy through the' flowing material in order to provide signals suitabIe for flow measurement by cross correlation, but that the impedance of the material to the wave energy may fluctuate and can be sensed to provide the required signals.

If the velocity of flow is required, there may also be included the. additional steps of measuring the transit time and computing the velocity from that time and the spacing of the two positions.

Also according to the invention, apparatus for sensing the flow of a material comprises two transmitter/receivers of wave energy spaced in the direction of flow; sensing means for sensing fluctuations in a property of each transmitter/receiver dependent on the impedance to the wave energy of the flowing material; and circuit means responsive to outputs of said sensing means to determine by cross correlation of the sensed fluctuations whether there exists a transit time between the transmitter/receivers or irregularities in the flow which cause said fluctuations.

Each transducer must be capable of both transmitting and receiving radiation. The radiation may be acoustic radiation, when the transducers will usually be piezoelectric transducers operating at ultrasonic frequencies, but may alternatively be electromagnetic, electrostatic, or magnetostrictive transducers, or the raditiaion may be electromagnetic energy, for example at microwave frequencies.

When the launched radiation is acoustic radiation, fluctuations in the acoustic impedance of the flowing material may be sensed by exciting each transducer to emit acoustic radiation at a constant frequency and sensing fluctuations in an electrical variable of each transducer which is dependent on the acoustic impedance, for example electrical resistance or electrical reactance. Alternatively, each transducer may be excited at a frequency which is allowed to vary as the acoustic impedace varies, and fluctuations in the frequency may be sensed.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 indicates schematically one physical arrangement of acoustic apparatus according to the invention: Figure 2 shows a typical cross correlation function; Figure 3 shows the electrical part of the apparatus associated with one ultrasonic transmitter/receiver; and Figure 4 shows an alternative arrangement of the acoustic apparatus.

In Figure 1, a fluid 10 flows through a pipe 12, shown in longitudinal section.

Two continuous wave ultrasonic transmitter/ receivers 14, 16 each comprising a piezoelectric crystal are attached to the pipe wall and are spaced in the direction of fluid flow indicated by the arrow. Both of the ultrasonic transducers are supplied by an oscillator 18, through respective resistors 19, 21 and each is connected through a respective detector circuit 20, 22 to a cross correlator 24, such as those described in the Complete Specifications of U.K. Patents Nos. 1,401,099 and 1,477,833.

In operation, the oscillator causes the transducers 14, 16 each to transmit a beam of ultrasonic radiation into the fluid 10 transverse to the direction of flow.

If the fluid 10 does not severely attenuate the ultrasonic radiation, standing wave patterns are set up by multiple reflections within the pipe. Radiation reflected back towards the transducers alters the acoustic impedance of the fluid into which the transducers are radiating, which in turn affects the electrical impedance of each transducer.

When the standing wave patterns are altered by fluctuations in the properties of the flowing fluid, the electrical impedance of the transducers fluctuates accordingly. Similarly, the presence of a discontinuous phase causes considerable distortion of the standing wave patterns and large fluctuations in the electrical impedance of each transducer. If the fluid 10 is such as to attenuate the ultrasonic radiation strongly, standing waves will not form, but the transducers will respond to fluctuations in the specific accoustic impedance of the material adjacent to their radiating surfaces.

The electrical impedance of the crystals are sensed by the respective detection circuits 20, 22 and supplied to the cross corre lator 24.

Since the fluctuations in acoustic properties are transported by the flow, there is a time delay between sensing each fluctuation at the two transducers due to the distance between them, and this transit time may be determined by computation of the cross correlation function:

where the a.c. components of the outputs of the detector circuits 20, 22 are x1(t) and x2(t) respectively and T is the integration period. If the function is computed for several possible values of the delay parameters T, the value of 7 for which the function has a maximum value can be determined, giving the transit time. A typical function is shown in Figure 2.

If the transit time is measured and the spacing of the transducers is known, then the flow velocity can be determined.

The detector circuit 20 associated with the transducer 14 is shown in Figure 2. The oscillator 18 is connected to the transducer 14 through a series resistor 19. The transducer 14 is connected through a capacitor 26 to a diode 28, and the diode output is connected through a filter circuit consisting of a series resistor 30 and a capacitor 32 to output terminals 34 which can be connected to the cross correlator.

In operation, the oscillator 18 gives a constant voltage output; when the electrical impedance of the transducer changes, the voltage supplied to the diode also changes.

The filter circuit removes the high frequency components from the signal passed by the diode and supplies to the cross correlator fluctuations only at lower frequencies.

The detector circuit 22 connected to the transducer 16 is identical to the circuit 20.

Advantages of apparatus according to the invention are that only two transducers are required in comparison with four in previously used arrangements; access is required from only one side of the pipe; axial displacement of the transducers can be measured more easily than when transducers on opposite sides of the pipe are used; and the system can be used to sense the flow of fluids through which radiation, such as ultrasonic radiation, cannot be readily propagated.

In the electrical arrangement described above, the piezoelectric transducers have been driven at a fixed frequency, and the electrical impedance of each crystal has been measured. In an alternative arrangement, shown in Figure 4, a separate oscillator 17, 18 is provided for each crystal 14, 16, the crystal frequency is allowed to vary with fluctuations in the acoustic properties of the fluid. The variations in frequency are sensed by frequency sensors 34, 36 and cross correlated by circuit 24 as before.

Apparatus according to the nvention may be used to sense or measure the flow of a gas or a liquid, or the flow of a second discontinuous phase may be sensed, that is, solid particles or liquid droplets entrained in a gas flow, gas bubbles or solid particles entrained in a liquid flow etc.

It is to be understood that the method and apparatus according to the invention are not restricted to sensing flow through a pipe; flow along an open channel etc., may also be sensed.

WHAT WE CLAIM IS: 1. A method of sensing the flow of a material comprising launching a beam of wave energy into the material substantially perpendicular to the general direction of flow from each of two positions spaced in the direction of flow; sensing at each of the two positions any fluctuations in the impedance to the energy of the flowing material; and determining by cross correlation of the sensed fluctuations whether there exists a transit time between the two positions of irregularities in the flow which cause said fluctuations.

2. Apparatus for sensing the flow of a material comprising two transmitter/receivers of wave energy spaced in the direction of flow; sensing means for sensing fluctuations in a property of each transmitter/receiver dependent on the impedance to the wave energy of the flowing material; and circuit means responsive to outputs of said sensing means to determine by cross correlation of the sensed fluctuations whether there exists a transit time between the transmitter/receivers of irregularities in the flow which cause said fluctuations.

3. Apparatus for sensing the flow of a material comprising two ultrasonic transmitting/receiving transducers spaced in the direction of flow; sensing means for sensing fluctuations in a property of each ultrasonic transducer dependent on the acoustic impedance of the flowing material; and circuit means responsive to outputs of said sensing means to determine by cross correlation of the sensed fluctuations whether there exists a transit time between the transducers of irregularities in the flow which cause said fluctuations.

4. Apparatus according to Claim 3 comprising means to excite both ultrasonic transducers at a constant frequency; and sensing means to sense fluctuations in an electrical property of each transducer.

5. Apparatus according to Claim 4 in which said electrical property is the electrical resistance.

6. Apparatus according to Claim 3 com prising means to excite each ultrasonic transducer independently at a frequency which can vary in accordance with fluctuations in the acoustic impedance of the flowing material; and sensing means to sense the fluctuations in the operating frequency of each transducer.

7. Apparatus according to any one of Claims 2 to 6 further comprising means to compute the value of its transit time and to compute the flow velocity of the material from the transit time and the spacing of the two transmitter/receivers.

8. Apparatus for sensing the flow of a material substantially as hereinbefore described with reference either to Figures 1 and 3 or to Figure 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. Apparatus according to the nvention may be used to sense or measure the flow of a gas or a liquid, or the flow of a second discontinuous phase may be sensed, that is, solid particles or liquid droplets entrained in a gas flow, gas bubbles or solid particles entrained in a liquid flow etc. It is to be understood that the method and apparatus according to the invention are not restricted to sensing flow through a pipe; flow along an open channel etc., may also be sensed. WHAT WE CLAIM IS:

1. A method of sensing the flow of a material comprising launching a beam of wave energy into the material substantially perpendicular to the general direction of flow from each of two positions spaced in the direction of flow; sensing at each of the two positions any fluctuations in the impedance to the energy of the flowing material; and determining by cross correlation of the sensed fluctuations whether there exists a transit time between the two positions of irregularities in the flow which cause said fluctuations.

2. Apparatus for sensing the flow of a material comprising two transmitter/receivers of wave energy spaced in the direction of flow; sensing means for sensing fluctuations in a property of each transmitter/receiver dependent on the impedance to the wave energy of the flowing material; and circuit means responsive to outputs of said sensing means to determine by cross correlation of the sensed fluctuations whether there exists a transit time between the transmitter/receivers of irregularities in the flow which cause said fluctuations.

3. Apparatus for sensing the flow of a material comprising two ultrasonic transmitting/receiving transducers spaced in the direction of flow; sensing means for sensing fluctuations in a property of each ultrasonic transducer dependent on the acoustic impedance of the flowing material; and circuit means responsive to outputs of said sensing means to determine by cross correlation of the sensed fluctuations whether there exists a transit time between the transducers of irregularities in the flow which cause said fluctuations.

4. Apparatus according to Claim 3 comprising means to excite both ultrasonic transducers at a constant frequency; and sensing means to sense fluctuations in an electrical property of each transducer.

5. Apparatus according to Claim 4 in which said electrical property is the electrical resistance.

6. Apparatus according to Claim 3 com prising means to excite each ultrasonic transducer independently at a frequency which can vary in accordance with fluctuations in the acoustic impedance of the flowing material; and sensing means to sense the fluctuations in the operating frequency of each transducer.

7. Apparatus according to any one of Claims 2 to 6 further comprising means to compute the value of its transit time and to compute the flow velocity of the material from the transit time and the spacing of the two transmitter/receivers.

8. Apparatus for sensing the flow of a material substantially as hereinbefore described with reference either to Figures 1 and 3 or to Figure 4 of the accompanying drawings.