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ULTRASONIC DOMESTIC GAS METERS-A REVIEW

N. Bignell
National Measurement Laboratory
CSIRO Telecommunications and Industrial Physics
Lindfield, NSW, Australia
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Abstract: There are many designs of ultrasonic domestic gas meters but all the main ones use the measurement of the transit time of an ultrasonic signal through the flowing gas to estimate its velocity. The shape of the duct and devices to control the waveform of the signal passing through it are significant parts of the design. The transducers used to produce the signal, reciprocal operation and the main techniques to time the transit of the signal to several nanoseconds are discussed. The acceptance of these meters has been restricted and their possible future is discussed. Keywords: ultrasonic, omestic, gas, meter, flow, transducer, transit time, reciprocity

1 INTRODUCTION
xxxxThe purpose of the domestic gas meter is to charge the customer for heat used and this is done by assuming that this is proportional to the quantity of gas used. The best measurement of quantity is mass, but as mass is difficult to measure for gases, the volume has been the traditional quantity metered. Sometimes temperature corrections are made but not pressure, except in an average way. The usual meter uses a bellows that fills and empties to achieve a measurement of flow rates from a pilot flow of 0.015 m3/h to a maximum of 6 m3/h. With a turndown ratio of 400, operation from (at least) -10°C to 50°C, an uncertainty over most of the range of 1.5% and a production cost of about $20, this meter is the culmination of a century of development. However, for at least some systems, a large fraction of the gas consumed is due to pilot flows that have been difficult to measure with the bellows meter. Further development of it seems unlikely to improve its performance.
Due to market resistance to this rather bulky and unattractive traditional diaphragm meter, a
competition was set up by British Gas in the 1980s for the development of a more compact gas meter. In the end, the successful designs were mostly ultrasonic. During the last decade several ultrasonic domestic gas meters have been developed to the commercial stage and there has been much activity in the patent literature [1, 2, 3, 4, 5, 6].
xxxxAll of the meters seriously developed have used the transit-time principle of operation. A pulse of ultrasound is transmitted through the flowing gas and its passage timed over a length L. It will have a different time in the direction of flow Td to that in the opposite direction Tu and the equation

xxxxxxxxxxxx(1)

allows the velocity v to be calculated. From this velocity the flow may be obtained and, by integration, the volume passed in a given time.
xxxxFor a meter that is about the size of a common house brick, the path length for the ultrasound is about 175 mm. This gives a transit time of 500 ms, varying of course with temperature and the nature of the gas. The stream velocity cannot be too high or the pressure drop will exceed the normal specification. For a velocity of 22 mm/s that might be typical of pilot flow, the difference between the upstream and downstream times is 57 ns. Thus a resolution of a few nanoseconds is needed for reasonable uncertainty. In the laboratory context this is not a difficult task using a high speed timer. For low power battery operation that is required for independence from the electricity mains, this solution is not feasible as high speed oscillators and timers use high power. Various solutions to this problem have been found and will be discussed.
xxxxThough the times measured allow a velocity to be measured this is not necessarily proportional to the flow. The correct determination of the flow from the velocity is a problem for all ultrasonic flow meters. In the large ultrasonic meters, based on the work of British Gas, multiple beams are used to allow the flow profile to be measured and hence the flow to be estimated. This technique is not currently feasible in a small battery operated device costing $20 but other techniques are used and these are described.
xxxxUltrasound propagation in a flowing gas is not straightforward. Transducers normally used in
ultrasonic work are made from piezo-ceramic materials that have a high acoustic impedance. The acoustic impedance of gases is low and this impedance mismatch makes it difficult for ceramic transducers to put energy into gases, or at least the process is inefficient. The characteristics of the received signal also depend on whether the direction of travel of the beam is with the flow or against it. Various meters handle these problems in different ways.

2 TRANSMISSION AND RECEPTION OF SIGNALS USING TRANSDUCERS
xxxxThe traditional approach [1] uses a very low density composite material on the surface of the
transducer, which is usually a piezo-ceramic material, to provide a matching of the different impedances. This layer may have stability and construction problems but the coupling can be considerably improved using this technique. To reduce the ringing of the transducer, that is to reduce the Q, an absorptive backing may be added. These transducers usually have a frequency range of from 140 kHz to 180 kHz.
xxxxPolyvinylidene fluoride (PVDF) film has a naturally lower acoustic impedance and can be made to have piezo-electric properties by stretching and poling it. One transducer developed using it [7] consists of a strip of metal-coated PVDF film 25μ thick, held in a smooth "M" shape. The curvature assists some of the modes of vibration of the film when it is excited by a signal applied across the thickness of the film. The result is a transducer of low Q and with a requency of about 115 kHz that operates with low voltage excitation, and can be used either as a transmitter or as a receiver, in a reciprocal manner. A disadvantage is that the sensitivity depends on the temperature and so the gain of the electronics must be varied to allow for this.
xxxxA commercially available transducer that operates at the low end of the ultrasonic range, at about 40 kHz, has also been used in gas meters [2]. It uses a piezo-ceramic element and coupling to the gas is enhanced by a small speaker cone attached to it. This transducer has a large Q and so it rings a lot in use. A technique mentioned later is used to achieve the precise timing required.
xxxxOne of the operational difficulties that the transducers must face is that of dust. In all reticulation systems there is some dust but in old systems there can occur what are known as dust storms. The dust is composed mainly of iron oxides and silica and gets picked up by the flowing gas when there is a change in the distribution pattern or some other disturbance. It can be very upsetting for the operation of the transducers for mechanical reasons, for reasons of abrasion and for loss of linearity in performance.
xxxxLinearity in transducers is important for the accuracy of the meter at low flows. Non-linear behaviour can cause the timing to be different in the two directions even with no flow present. As a result there will appear to be a flow when really there is not. The need for good linearity occurs because when transmitting the amplitude of vibration of the transducer is orders of magnitude greater than when it is receiving. Dust on the surface of an otherwise linear transducer can adhere differently at different amplitudes of vibration of the transducer giving non-linear performance.
xxxxSome gases at some frequencies absorb the ultrasound energy much more than others. Gases with a simple molecular structure such as argon and diatomic molecular gases such as nitrogen and oxygen have low absorption. Gases such as methane and carbon dioxide as well as mixtures of these and water vapour with simpler gases can cause much higher absorption. A mixture of methane with 6% carbon dioxide is often used as a test gas because it is especially absorptive. The signal loss will depend on the frequency of operation as well as the path length but can be around 30 db.

 
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