Current measurement technology development progress in the '90s-a review

In the last six years, measurement of water current in the ocean has progressed from predominantly rotor and vane or propeller type instruments to acoustic Doppler type instruments. Long term measurements of the type of flow important to understanding the general circulation still use the older mechanical current meters but these are being equipped with modern data loggers. The ADCP, initially used as a profiler on ships, has developed a life as a moored instrument, protected in a cage from trawling for bottom mounted deployments. In addition, the acoustic Doppler and acoustic travel time techniques have spawned precise point measurements, useful for turbulence and dissipation measurements, wave and boundary layer measurements, and sediment transport studies. Radar backscatter at HF and VHF from surface waves have provided surface current maps. Broadband ADCPs provide finer resolution in space and velocity but require greater care in their use to avoid biases from fish among other things. Miniature current sensors are becoming available for inclusion in instrument suites. Vorticity and heat flux are possible in addition to vector flow for wave reduction when shear is wanted and mixing rates are to be estimated. Electromagnetic sensing is still useful but development has been slower. Laser Doppler velocimetry has also developed only slowly, principally being useful for dissipation studies. Current meter development is a vigorous field with the number of ADCPs on the market doubling every three years. Intercomparison between current meters is an essential task that is being steadily if slowly pursued. Use of current meters is increasing in coastal studies, environmental work, and in conjunction with other measurements but is still not ubiquitous, a situation that should be corrected. Thus there is still a challenge to those people in the water flow measurement business to make current measurements easier, more accurate, and less expensive.