GB2209216A - Ultrasonic flowmeter combined with a pressure sensor - Google Patents

Abstract

A flow-meter for sensing mass flow rate employs transducers 4, 5 for determining, from the time difference between the time of flight of ultrasonic signal upstream and downstream of a flow path, the velocity of flow and acoustic velocity, and a pressure sensor 6 for sensing pressure in the flow path, the pressure being related to the density. From these measurements the mass flow rate may be obtained. Both measurements of time difference and of pressure may be made in terms of frequency, enabling interfacing with digital electronics. The flow-meter may be used as a domestic gas meter. <IMAGE>

Description

Flow-Meter for sensing Mass Flow Rate of a Fluid This invention relates to flow-meters for sensing the mass flow rate of a fluid.

Conventional flow-meters of this type use vanes etc.

in the flow path which are deflected by the fluid to sense the mass flow rate.

However, this impedes the flow.

The invention provides a flow-meter for sensing the mass flow rate of a fluid, which comprises means defining a flow path for the fluid, transducer means for transmitting and receiving ultra-sonic signals along the flow path in both directions to enable the velocity of flow and acoustic velocity to be determined, and a pressure sensor to sense the pressure in the flow path.

The mass flow rate of the fluid can be calculated from a knowledge of the acoustic velocity, the flow velocity and the pressure, and it is not necessary for the transducer means or the pressure sensor to intrude into the flow path.

Thus, the flow velocity and acoustic velocity may be determined from any transducer means arranged to detect the difference in the time of flight of ultrasonic signals upstream and downstream (not necessarily parallel to the flow path axis). Preferably, however, one or more reflecting surfaces are arranged so that, over part of their paths, the ultrasonic signals travel along a region of the flow path substantially parallel to the axis of that region. The reflecting surfaces may be formed by walls of the flow path adjacent the region along which the signals travel substantially parallel to the axis.

Equally, any means may be used for detecting the pressure of the fluid, but preferably the pressure sensor comprises a flat tube closed at one end, the walls being of such a thickness that the spacing between the walls increases when the pressure inside the tube increases relative to that outside the tube, and means for vibrating the tube in a flexural mode at the respective resonant frequency.

A mass flow-meter for a gas constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a plan view of the flow-meter showing the shape of the flow path; Fig. 2 is a block diagram of an electric circuit for the velocity sensing transducers; Fig. 3 is a schematic view of the pressure sensor; Fig. 4 is a section at right angles to the axis of the pressure sensor; Fig. 5 shows a blank from which the pressure sensor can be machined; Fig. 6 shows the blank after machining; Fig. 7 is a block diagram of one form of electric circuit for the pressure sensor; and Fig. 8 is a block diagram of a second form of electric circuit for the pressure sensor.

Referring to figure 1, the flow-meter is formed in a metal block 1. A gas flow path 2 is defined by a slot 3 of rectangular cross-section which is machined into the top surface of the block and closed by a cover (not shown) which is bolted to theblock. Transducers 4,5 capable of emitting as well as receiving ultra-sonic vibrations are mounted in passages 6,7 which communicate with the gas flow path, and signals derived from these transducers give a measure of the velocity of gas flow along the path 2 as well as a measure of the velocity of the acoustic waves in the gas.

A knowledge of these two factors, together with a knowledge of the density of the gas, enables the mass flow rate of the gas flowing along the path 2 to be measured.

The pressure of the gas gives a measure of the density, and pressure sensor indicated generally by the reference numeral 8 in passage 9 is provided for this purpose.

The flow-meter may be a domestic gas meter, being inserted into the gas pipe between the point of entry into the premises and the pipe leading to the various appliances. The entire gas flow consumed thus passes along the flow path 2. However, it will be appreciated that the mass flow-meter may be used for sensing the mass flow rate of other fluids, including other gases and liquids.

The flow path 2 includes a region 2a along which the ultra-sonic waves travel axially. In the interests of keeping the overall length of the block 1 to a minimum, the passages 6,7 housing the velocity measuring transducers 4,5 are orientated at right angles to the region 2a, and the ultra-sonic waves are reflected by surfaces inclined at 45 between the region 2a and the passages 6,7.

The reflecting surfaces are formed by the walls of the regions 2b, 2c, the axes of which are inclined at 450 to the axis of the region 2a. The inclined regions 2b, 2c thus provide the necessary deflection of the ultra-sonic waves, but also do not provide any substantial impedence to gas flow. Short regions 2d,2e parallel to the region 2a form the inlet and outlet respectively of the flow meter.

The transducers 4,5, which are of identical form, emit and receive ultra-sonic waves in a cone-shaped region extending from the transducers. To prevent there being different possible path lengths between the transducers, a guide 10 for axial ultra-sonic waves only is positioned in the region 2a to suppress off-axis modes. The guide consists of an insert which has closely packed hexagonal channels extending throughout its length. The region 2a is lined with acoustic absorbing foam pads 11,12 at each end of the guide to suppress multiple reflections.

The transducers are supported in the passages 6,7 in plugs 13,14 of silicone rubber which block off the passages 6,7 and at the same time isolate the transducers from vibration.

The transducers 4,5 sense the velocity of gas flowing along the path 2 because a signal transmitted by one transducer and received by the other takes less time when it travels in the direction of gas flow than when it travels against the direction of gas flow, since the ultra-sonic signals are, of course propogated in the gas itself.

The time of flight between transmitter and receiver can be conveniently measured in frequency form by coupling the transmitter and receiver together in a "sing-around" system, where the oscillation frequency stabilises at two values which maintain a fixed phase shift between transmitted and received signals. Thus, the respective times of propogation from transducer 4 to transducer 5 and vice-versa are measured by varying for each direction of propogation the driving frequency until exactly the same number of wavelengths, not necessary an intregal number, exists between the transmitting transducer and the receiving transducer, and this is done by producing a predetermined phase difference, for example, zero, between the transmitted signal and the received signal. The difference between these two frequencies is proportional to the velocity of gas along the flow path and the sum of these two frequencies is proportional to the velocity of the ultra-sonic waves in the gas.

Thus, if one assumes that the component of path along the flow path axis between the transmitter and the receiver is L and the acoustic velocity in the gas is C, with a mean flow velocity V averaged over the flow path diameter, the time taken for sound at frequency fl to propogate between the transducers in the flow direction is tl, where

If the sing-arotind frequency is fl the phase shift along the acoustic path is proportional to the product fl tl which is maintained at a fixed value N by the singaround system.

Thus,

If the transducers are now interchanged so that sound propogates against flow, sing-around frequency f2 is given by

The difference in frequencies fl f f2 is therefore

The sum of the frequencies is

The difference in frequency therefore gives a measure of the flow velocity and the sum gives a measure of the acoustic velocity in the gas. The difference frequency therefore provides a measure of volume flow rate which is independent of the

of the gas.

The differencing technique allows a wide range of flow velocities to be covered without making impossible demands on the stability of operating conditions to achieve the necessary accuracy at low velocity at the low velocity end of the range.

Referring to figure 2, electronic analogue gate cross-over switch 15 switches over every i a second between states connecting contacts 15a, 15b and 1.6a, 16b together. In the first position, the transducer 5 is driven and the transducer 4 receives. The signal is fed to a phase detector 17 via an operational amplifier 18 which amplifies the signal until it is comparable with the signal fed to the transducer 5 which is directly fed back to the phase detector. The output of the phase detector 17 controls a voltage controlled oscillator 19 which varies the frequency of its output until there is a predetermined phase difference, for example, 00, between the two inputs to the phase detector.The same procedure happens when the contacts 16a,16b are connected together and a

frequency corresponding to the fixed phase difference for propogation with the gas flow is locked onto.

The frequency corresponding to zero flow is chosen in relation to the distance between the transmitter and receiver to ensure that the phase differences introduced when alternating between the two states are sufficiently small that the phase'locked loop locks onto the same

number of wavelengths between transmitter and receiver in each case and cannot lock onto a larger or smaller number of wavelengths.

In order to subtract one frequency from the other, a counter 20 counts up from zero during one L second period and then, during the second + second period, counts down from the number reached. The difference between the two counts is proportional to the difference between the frequencies. To obtain a more accurate count, the output frequency of the voltage controlled oscillator is multiplied by a factor of 16 for feeding to the counter, to achieve sufficient velocity resolution at low flow rates, and then divided by a factor of 16 for feeding to the transducers and the phase detector.

As an example of suitable dimensions and operating frequencies, the length of the section 2a may be approximately 80mm and the cross-section of the passage may be approximately 20mm by 20mm. The transducers 4,5 may each consist of an 8mm diameter piezo-electric ceramic disc with a small aluminium cone attached to the centre to act as a radiator. A few volts applied to the transducers at the resonance frequency generates sound pressure levels of 100dB a few centimetres from the transmitting aperture.

When used as receivers, the transducers generate output voltage at the order of 0.1 to 1 volt when placed a few centimetres away from the transmitter. The transducers have resonance bandwidths centred on 40kHz of approximately 2kHz. With this example, four complete wavelengths can be accommodated between transmitter and receiver in each direction of propogation of the ultrasonic waves. A flow rate of approximately 3 metres/second would on this basis produce a difference frequency of approximately 800Hz.

Variations are of course possible. Thus, the section 2a could be from 60mm to 100mm long, the flow path diameter could be from lOmm to 40mm, the centre frequency of the transducer 4,5 could be from 30kHz to 100kHz, and the response curve against frequency for amplitude and phase should be as flat as possible.

Equally, the regions 2b,2c could lie at different angles to region 2a, passages 6,7 being appropriately reorientated to receive the reflected signals. If desired, the passages 2b,2c could be at right angles to passage 2a, and the transducers could be recessed into the walls of 2b,2c, with the transducers aligned along the axis of 2a.

Alternatively, the transducers could be both arranged in the section 2a, the line joining them obliquely crossing the section 2a. Any other variation of time of flight ultrasonic velocity sensing is of course possible.

As an alternative to piezo-electric materials, multiple layer PVDF could be used, or bulk ceramic transducers using acoustic matching layers could be used.

It will be apparent that the difference frequency will be zero for zero gas flow down the flow path.

However, in principle, the same effect could be achieved by placing a powerful ultra-sonic source against the meter. This could lock the operating frequencies of the device to the external source frequency. The difference frequency output in the two directions would then be zero and the meter would read zero irrespective of actual flow rate.

To prevent a possibility of such fraud, a small frequency offset may be provided between the forward and reverse states so that a non-zero difference frequency is generated at zero flow. For example, a phase shift device 32 operated during one set of alternate measurement periods only would provide such a frequency offset. An indication of this, indicating tampering with the equipment, could be provided within the meter.

Alternatively, the transducers 4 and 5 could be arranged to detect any externally applied ultra-sonic source, for example, the alternate operation of the transducers described above could be interrupted periodically to sense for such signals.

The mass flow rate is obtained by combining the digital frequency output with a digital output of the pressure sensor in a digital processor. The power requirements of the transducers are low, enabling them to be powered by a battery.

The pressure P of a gas is related to its density P by the following relation.

Where C is the acoustic velocity and w is the ratio of the principal specific heats for the gas (which in the case of natural gas

for hydrogen = 1.41 and X for methane = 1.31 and is approximately independent of the mixture ratio).

Referring to figures 3 and 4, the pressure sensor 8 is a simple flexurally vibrating tube with a resonance frequency which can be changed by the pressure inside.

The short length of flat tube 21 is closed at one and mounted at the other end on a rigid support so that it can vibrate freely in transverse flexural modes. The pressure sensor consists of a brass tube 21 which has been flattened over a region 22 and closed at an end 23. The other end of the tube has a screw-threaded pipe fitting 24 secured to it by means of which it is secured in the passage 9 in figure 1. The tube 21 is secured into a capsule 24a, which is evacuated. Wires 25a,25b extend into the interior of the capsule through openings which are sealed by glass-to - metal plugs 26. The tube 21 is made in the following way. Referring to figures 5 and 6, about half the length of a brass tube of outside diameter of approximately 8mm and inside~diameter of approximately 5mm is machined until the wall thickness reduces to about O.lmm.The machined part of the tube is then crushed on a flat mandrel approximately 0.2mm thick so that the resulting thickness of the flattened tube is approximately 0.4mm. The end is crimped and soldered to make a

seal and the other end is brazed to a pipe fitting 24.

The tube is similat to a Bourdon tube, but does not deflect with changing pressure as a Bourdon tube does.

Small piezo-electric ceramic transducers 27,28 are attached to the flat surfaces of the tube on opposite sides in strain-free regions, and are connected by wires 25,26.

When the sensor 8 is fitted into passage 9 (figure 1), the interior of the pressure tube 21 communicates with the gas flow path 2, whereas the outside is maintained at a Vacuum in the capsule 24a.

The flattened section 22 of the tube can be excited in various modes of flexural vibration by driving one of the piezo electric transducers, and varying the frequency until the tube resonates in one mode. The resonance is detected by detecting the amplitude and phase of the vibration applied to the other transducer. The tube is maintained in oscillation by coupling the drive and sense transducers together through a broadly tuned amplifier with a frequency centred on the middle of the operating range.

Referring to figure 7, the output of the driven transducer is fed back via an operational amplifier 29 to a phase detector 30 which controls a voltage controlled oscillator 31 which drives the other transducer. The frequency is varied until the phase difference between the two inputs to the phase detector reaches the predetermined value corresponding to resonance.

The tube 22 is driven in the second flexural mode, that is, the mode with one node close to the free end.

In figure 4, the full line shape of the tube section 22 corresponds to the same pressure outside the tube and inside the tube. However, when the outside is evacuated and the inside is connected to the pressure in the flow path 2, the tube bows out into the dotted line position.

This has the effect of changing the resonance frequency, and the change h f is related to the pressure P in the following way.

If the tube has a uniform cross-section and a radius of gyration about the neutral bending axis K, the frequency of flexural vibration f is given by

where 1 is the tube length, Co is the velocity of extensional acoustic waves in the tube wall material, and M is an integer corresponding to the flexural mode number.

However, the radius of gyration K varies with the deflection y of the tube walls in the centre produced by the applied pressure K f B (l-b + 1.067 y) 2 B B where B is the overall thickness of the tube and b the wall thickness.

In turn y = PW4 32Eb where P is the applied pressure, E is Young's modulus of the tube wall material and W the width of the tube.

It can be shown from these relations that Af is -40 proportional to P where fo is the frequency at zero pressure difference across the tube andAf is the frequency shift produced in the resonant frequency when pressure p is applied across the tube walls.

A knowledge of fo, and of b f obtained by the measuring circuit of figure 7 enables the pressure to be calculated.

An alternate circuit for detecting the resonant velocity is the self-oscillating circuit of figure 8.

As an example of the size of the tube 22, the tube may be approximately 20mm in length, with a wall thickness of O.lmm and a spacing between the walls of 0.2mm, and a width of at least ten times its thickness of 0.4mm. The unstressed resonance frequency is approximately 4kHz, that is, with zero pressure difference across the tube wall.

Application of a pressure of iaOOm bar to the inside of the tube will cause a frequency increase of the order of hundreds of Hz.

Variations are of course possible. The unstressed spacing of the tube walls may lie in the range of from O.lmm to 0.4mm and the resonant frequency of unstressed the tube may lie between 2kHz and 6kHz. The wall thickness may lie between 0.05mm and 0.2mm. Instead of being formed by squashing on a mandrel, the tube could be electrochemically deposited on a mandrel, for example, the material could be nickel. Instead of piezo-electric means to set the tube into vibration, magnetic or optical means could be used instead.

As another alternative, the mass flow rate may be obtained from the velocity sensing transducers of figure 1 together with a pressure sensor of an entirely different type to that described.

Co-pending patent application no. 87 2034-2is directed to the arrangement of Figures 1 and 2, application

is directed to an anti-fraud device described with reference to Figure 2, and application

is directed to the arrangement of Figures 3 to 8.

Claims (10)

1. A flow-meter for sensing the mass flow rate of a fluid, comprising means defining a flow path for the gas, transducer means for ,transmitting and receiving ultrasonic signals along the flow path in both directions, to enable the velocity of flow and acoustic velocity to be determined, and a pressure sensor to sense pressure in the flow path.

2. A flow-meter as claimed in claim 1, in which one or more reflecting surfaces are arranged so that, over part of their paths, the ultrasonic signals travel along a region of the flow path substantially parallel to the axis of that region.

3. A flow-meter as claimed in claim 2, in which the transducers are mounted in passages which extend from the flow path.

4. A flow-meter as claimed in claim 3, in which the reflecting surfaces are formed by walls of the flow path adjacent to the region along which the signals travel substantially parallel to the axis.

5. A flow-meter as claimed in claim 4, in which the walls of the flow path which form the reflecting surfaces are each inclined at the same angle to the axis of the region of the flow path along which the signals travel substantially parallel to the axis.

S

6. A flow-meter as claimed in any one of claims 1 to 2, in which in use the transducers alternatively transmit and receive, and means is provided to vary the frequency between transmission in the direction of flow and counter to the direction of flow in such a way that the number of wavelengths and the phase between the transmitted and received signals remains the same in each case.

7. A flow-meter as claimed in any one of claims 1 to 6 in which the pressure sensor comprises a flat tube closed at one end, the walls being of such a thickness that the spacing between the walls increases when the pressure inside the tube increases relative to that outside the tube, and means for vibrating the tube in a flexural mode at the respective resonant frequency.

8. A flow-meter as claimed in claim 7, in which the tube is housed in an evacuated container.

9. A flow-meter as claimed in claim 8, in which the vibrating means includes a pair of piezo-electric transducers, one mounted on each side of the tube.

10. A flow-meter for sensing the mass flow rate of a fluid substantially as herein described with reference to the accompanying drawings.