Acoustic current meter zero offset drift

Acoustic travel-time velocity sensors have a linear response through zero velocity. The benefit this provides in freedom from deadband and hysteresis comes at a cost of zero point uncertainty. One determines the zero flow offsets in still water or in carageenan gel for subsequent subtraction from the travel-time differences that are measured. Salt water immersion, pressure, and temperature affect the capacitance of the transducers and their connections to the electronics and thus the phase of the received signal. Two modes of zero-point shift have been observed: one in which the zero moves upon initial exposure to high pressure salt water after which the new zero is stable, and one in which small offsets appear only while the sensor is subjected to pressure. Details of the urethane bonds within the sensor under hydrostatic compression and their resulting distortion appear to cause these two modes of offset behavior. Temporary offsets are typically 0.3 cm/s while permanent offsets are typically 4.5 cm/s. Compatibility between urethanes to be bonded and flexible sensor tube construction are solutions to the permanent offset shift while matching of the cable capacitance promises hope of reducing pressure related offsets