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Title: Reciprocity And Its Utilization In Ultrasonic Flow Meters
Author: Per Lunde, Magne Vestrheim, Reidar B, Skule Smrgrav, Atle K. Abrahamsen
Source: 2005 North Sea Flow Measurement Workshop
Year Published: 2005
Abstract: In ultrasonic transit time flow meters for gas and liquid (USMs), the flow direction, the flow velocity and the sound velocity are estimated from the measured up- and downstream transit times. At no-flow conditions, the up- and downstream transit times of such meters should ideally be the same, or the difference should be negligible. This may not be the case unless special precautions are made. In order to reduce the possibility of the meter to detect a false flow at no-flow conditions, USMs are typically dry calibrated before being installed in the field. Dry calibration (which may be made in different ways), in general involves measurement of (a) the time delays due to electronics, cables and transducers, (b) the socalled ?t-correction (for each acoustic path, also denoted zero flow offset factor), and (c) geometrical parameters. Various ?t-correction approaches may be used by different manufacturers, but these are basically similar and have the same purpose: to reduce the false flow detection and improve the accuracy at low and no-flow conditions (zero flow adjustment), without significantly affecting the accuracy at the high velocity measurements. The AGA-9 report and the API MPMS Ch. 5.8 standard both prescribe need for zero flow verification test (zero test) or zeroing the meter, for gas and liquid USMs, respectively. Advances in USM technology based on the electroacoustic reciprocity principle have provided methods for reduction or even neglection of the need for ?t-correction of USMs. That means, if the USM measurement system is reciprocal, and operated in a sufficiently reciprocal way, the ?t-correction may be negligibly small over the operational range of pressure and temperature, and irrespective of whether the transducers are equal or not. Thus, dry calibration may be simplified, since reciprocal operation may provide possibilities for auto-zeroing of the USM. However, reciprocal operation is not an obvious property of an USM. Even though the USM measurement system consisting of two transducers, electronics, etc. (e.g. an acoustic path), may be reciprocal, it may not necessarily be reciprocally operated. Control and careful design is essential to realize reciprocal operation at no-flow conditions in an acoustical measurement system such as a USM.

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