5D-4 Reciprocal Operation of Ultrasonic Flow Meters: Criteria and Applications

When fluid flow approaches zero, the measurement uncertainty of ultrasonic flow meters (USMs) increases, unless specific precautions are made. At zero flow, the measured transit times of upstream and downstream propagation are not necessarily equal, 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 of the USM in this range. Possible precautions may be (a) "zero flow adjustment" or (b) exploitation of reciprocal operation of the USM. In the present paper, reciprocal operation of USMs is discussed on basis of general electro-acoustical principles. The paper extends earlier literature by (a) taking into account the finite electrical impedances of the electronics and transducers, (b) deriving design criteria for reciprocal operation of USMs, and (c) demonstrating reciprocal operation and validity of the theoretical results, also for transducers not being equal in performance. Laboratory measurements are made over a range of pressures, temperatures and signal excitation levels, for a USM custody transfer meter for natural gas. Consequences for USMs are significant, such as (a) simplified "zero flow adjustment" procedures, (b) cost reduction, and (c) improved accuracy, linearity and robustness at low flow velocities in relation to changes in temperature and pressure, ageing drift, etc. The results show the significance and potentials of utilizing reciprocity in USM technology, as well as improvements already achieved by realizing reciprocal operation in high precision flow meters for custody transfer metering of natural gas and petroleum liquids.