Ultrasonic gas flow meter with corrections for large dynamic metering range J. Delsing Department of Electrical Measurements, Lund Institute of Technology, PO Box 118, 221 00 Lund, Sweden Received 8 December 1988; revised 4 May 1989 Extreme accuracy is required in sing-around type gas flow meters in the sing-around period measurement. Thus the detection of the ultrasound is critical. Accurate detection of an ultrasound pulse transmitted through gas is not straightforward. Normally a zero crossing technique is applied, where a level trigger determines when to enable the zero crossing triggering. In a flowing gas, the ultrasound amplitude is modulated due to turbulence, humidity and changes in dynamic gas pressure. This introduces uncertainty as to the cycle in which the ultrasound pulse is detected. This in turn results in large errors in sing-around type gas flow meters. This paper discusses a new correction algorithm which will eliminate such trigger errors. To accomplish the verification of the new correction algorithm, a microprocessor-based sing-around gas flow meter, using 500 kHz ultrasound has been designed. The correction algorithm significantly increases the repeatability of this meter. Repeatability better than 0.5% over a dynamic meter range of 1 to 35 has been measured. Limitations of the sing-around method imposed by the new correction algorithm have been derived theoretically. Keywords: gas flow meter; sing-around; mathematical correction Ultrasound is finding its use in many applications that range from industrial cleaning to non-invasive measure- ments on foetuses. One established area of ultrasound use is flow measurements in ducts and pipes. This paper will discuss a new correction algorithm to be applied on the sing-around method for gas flow measurements. The correction algorithm enhances the performance of sing- around type gas flow meters, significantly increasing repeatability. The major area of application of gas flow metering is the natural gas industry. Large gas distribution systems require very high meter performance. These requirements are met only with a great deal of effort and cost. Other areas of application, such as heating of single family houses, put other requirements on gas metering devices. In a distribution network for single family houses, gas flow metering is used to divide the gas costs among a group of houses. In this application a flow meter of less accuracy but with a larger dynamic metering range is more useful compared to the extreme accuracy requirements set in the gas industry. For end user metering in distribution networks, consequent performance and high repeatability is more valuable than a high absolute accuracy. In spite of this, specifications for gas flow meters in local gas distribution networks often state accuracy of 1% or better, and a moderate meter range of about 1 to 10. Today a variety of methods exists for measuring gas flow. The most common devices used are of differential pressure type. Of these, the orifice plate meter and the Venturi meter are most frequently used in the gas zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0041-624X/89/060349-08 $03.00 @ 1989 Butterworth & Co (Publishers) Ltd industry. These types of meters are, if correctly installed, very accurate. Accuracies of a few tenths of a percent are often reported’. Unfortunately the accuracy of these devices is very dependent on the flow pattern in the meter. For example, swirl flow will totally ruin the accuracy of orifice plate meters’. This calls for the use of flow straighteners3 and long lengths of straight pipes both at meter inlet and outlet. All in all, these flow meters are accurate but very expensive when high accuracy is required. Furthermore, different types of turbine gas flow meters are in widespread use. The above mentioned measurement tasks can also be solved using other methods. The sing-around or transit time technique using ultrasound is the one we found worth considering. The technique has been known since the 1930s (Reference 4). Working meter devices did not appear before the 1960s when electronic devices of sufficient quality first became available. The fantastic development in electronics over the last decades now makes it possible to take full advantage of the powerful sing-around technique. In the 1980s several sing-around flow meter designs for gas flow metering have been proposed, for example for natural gas flow metering5 and for breathing dynamics6. This paper will discuss new ways to enhance the performance of sing-around type gas flow meters. A major problem in using ultrasound in gas is the difficulty of transmitting ultrasound into the gas. This is normally solved by means of clever transducer designs. The next problem occurs when turbulent flow or high degrees of humidity are present in the gas. The gas flow then consists Ultrasonics 1989 Vol 27 November 349