With fluid flow approaching zero, the USM method becomes increasingly dependent upon the reciprocity of the electro-acoustic measurement system, If reciprocity is not fulfilled, the measured transit times of upstream and downstream propagation will not be equal at zero flow, resulting in a zero flow timing offset, which becomes highly important in the low-velocity flow range, and provides false flow detection and degraded accuracy and linearity of the USM in this range. The AGA-9 report [1] and the API MPMS Ch. 5.8 standard [4] both prescribe need for "zero flow verification test (zero test)" or "zeroing the meter", for gas and liquid USMs, respectively. In the present work, criteria have been established for "sufficient reciprocal operation" of the USM. By such methods the USM manufacturer can - by design - eliminate (or reduce) the need for active (DELTA)t-correction, and achieve "auto-zeroing" of the USM. Possible changes of transducer properties with temperature, pressure, and time (ageing, drift) can be made less influent, so that these do not cause significant zero flow timing errors. This contributes to improve the accuracy and reliability of the USM. Moreover, the need for measuring the (DELTA)t-correction as part of the "dry calibration" may be eliminated (or reduced), which may reduce costs and in several ways is advantage both for the manufacturer and the user of the flow meter. In addition, criteria have been derived to establish requirements for the reproducibility of the transducer production, which can be used in transducer production QA.
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