An Advanced Two Axis Acoustic Current Meter

Abstract

ABSTRACT A Current Meter for long "in situ" performance in the ocean environment is described. The problems solved in applying acoustic techniques to current measurements in open flow channels, the limitations of these techniques, and test results are presented. The meter has good accuracy, it has no moving parts, and it consumes very little power. Its short and long term stability are both excellent. These features together result in an instrument uniquely suited for long unattended use in the ocean environment. 1 INTRODUCTION The Oceanic Division of Westinghouse is a manufacturer of acoustic flowmeters. (1, 2) We also supplied the sensor system for the engineering experimental phase of the National Data Buoy Development Project. (3, 4) As a continuation of these programs, we have a development program on a new acoustic current meter. The purpose is to provide users with a reasonably accurate and reliable meter that is capable of long "in situ" operation. (5) We are using an extension of the principle used on our Westinghouse flowmeter, namely the difference between acoustic pulse travel times in opposite directions thru the same path. Flowmeters using this basic principle have been field proven to be both accurate and reliable. Systems are presently achieving approximately 0.1% accuracy. Our interest in current meters resulted from the difficulty we experienced with the water current sensors on the Engineering Experimental Phase of the National Data Buoy (NDB) program. As sensor system supplier, we had to identify sensors for the various measurands. We were able to identify sensors that offered a high probability of successful performance over the target one year deployment for all of the parameters except current. (4) The requirement for long deployments eliminated current meters with moving parts. Figure 1 shows one of the oceanographic sensors after 6 months dep10yment.which has been antifoulant coated with tetratributyl tin oxide prior to its deployment. The two types of meters that we concluded could function for a year, and had reasonable accuracy and power consumption were the electromagnetic and the doppler acoustic types. The problem with the electromagnetic sensor is that its response to currents other than in line with the probes is nonlinear. Cosine response is required for a sensor in order to output the two orthogonal components of the current. We saw no way to improve the EM meter and chose to use the acoustic doppler. Several of these sensors were procured and tested. We found them unacceptable in water where the concentration of particles for the sound to backscatter from was low. This problem was encountered in tests at the David Taylor Model Basin tow tank, and later in at-sea tests in the Gulf of Mexico. The Westinghouse flowmeter is specifically designed to measure volumetric flow in closed conduits such as pipes or rivers. It was included in the evaluation of transducers for the oceanographic sensors, but the development effort required to change it to a current sensor was not compatible with the NOB program schedule.