Flow Meas. Instrum. Vol 2 October 1991 205 The zero-flow performance of a sing-around ultrasonic flowmeter J. DELSING* Zero-flow performance was tested on a new sing-around flowmeter. Design considerations were made to correct for the influences of speed of sound and use was made of reciprocity. The meter was tested from 1000 to 20 000 sing-around loops, the loop period being approximately 64 l~s. To investigate averaging phenomena, 1-100 primary velocity values were averaged to form one value. The temperature range tested was 2-45°C, with measurements at 5°C intervals. The measurements show a zero-flow stability of better than +__ 0.6 rams -1 (_+ 1 I h -1 for 25 mnm diameter pipe) over the 'entire temperature range. The standard deviation at each temperature tested was less than 0.45 mm s- I. This implies that flow velocities as low as 6 cms -1 (100 I h -I, 25 mm diameter pipe) could be measured with an accuracy of better than +__1%. The tests were made with water, but equivalent results are expected for other liquids as well as gases. Keywords: sing-around flowmeter; zero flow performance; flow velocity Introduction The accuracy of flowmeters is of great importance because the fluids that they measure can be very expensive. For many applications the accuracy and reliability of commonly used flowmetering techniques, such as turbine meters and orifice-plate meters, are insufficient. So a search for better flowmeters is necess- ary. Modern electronics have made it possible to devise new flowmetering technologies, such as electromagnetic, Coriolis and ultrasonic. It is hoped that these technologies can fulfil the requirements for accuracy and reliability called for by advanced users in industry. New technologies give rise to possible new errors. Conventional measuring techniques, such as turbine meters, do not have problems with measuring zero flow. However, the modern flowmetering technologies mentioned do have problems with zero-flow instabili- ties. This necessitates that accuracy is stated both in terms of measured value relative to actual flow and in terms of stability relative to zero flow. This paper will present zero-flow measurements for an ultrasonic sing- around flowmeter developed at Lund Institute of Tech- nology. sound pulse is transmitted from, say, the transducer positioned upstream. This pulse is received by the transducer downstream. The pulse is fed to the sing- around electronics, which will detect it and immed- iately start the transmission of the next ultrasound pulse in the same direction, thus establishing a 'sing- around' loop. This will continue for a number of loops. The same procedure is subsequently repeated in the upstream direction. The sing-around loop will oscillate with a certain period, t, called the sing- around period. The sing-around period depends on the speed of sound, c, of the fluid between the transduc- ers, the transducer distance, L, and the fluid velocity, v. Thus the downstream and upstream sing-around periods tl and t2 can be written as: L L tl - t2 - (1) c + vcosc~ c- vcoso~ From the sing-around periods, tl and t2, the fluid velocity, v, is easily found to be v- 2cosol Ultrasonic transducer I The sing-around flowmeter A brief review of the sing-around method is presented for the following discussion. Assume a configuration as shown in Figure 1. A sing-around loop is started when a short ultra- *Department of Heat and Power Engineering, Lund Institute of Technology, PC)Box 118,5-221O0Lund, Sweden 2s rnrn [ ~'~ Ultrasonic " transducer 2 Figure 1 Sing-around flowmeter body 0955-5986/92/040205-04 © 1992 Butterworth-Heinemann Ltd