Comparison of five current meters in a tidally dominated flow

Velocity measurements from five current meters of four different types were compared: Two single-bin acoustic backscatter sensors from Nortek and Sontek, one profiling acoustic backscatter sensor from RD Instruments (RDI) one acoustic phase-shift sensor from Falmouth Scientific Inc. (FSI), and a Vector Measuring Current Meter (VMCM). two-axis propeller sensor. The instruments were deployed in Buzzards Bay, Massachusetts, between 8 Feb and 16 May 2000. The water depth was about 12 m. The longest record available was 96 days and the shortest was 9.5 days. Tidal decomposition of the 96 day record showed that the flow was dominated by the semi-diurnal tide. The comparisons were done using hourly average data during the 9.5 day interval when all five instruments were operating. All five sensors agreed on the speed to within 1.5 cm/s, or about 20% of the mean flow speed of 8 cm/s. Regression results showed that the RDI reported higher speeds than the Sontek, Nortek and VMCM. This discrepancy increased with increasing speed. The VMCM showed a low bias relative to the Sontek and RDI for all speeds. A consistent explanation of these results is that the RDI overspeeds by about 10% and the VMCM has a low bias of about 1 cm/s. Directional errors, as measured by the standard deviation of the difference from the (fixed) RDI direction, had magnitudes of 6-10° and were generally consistent with expectations from laboratory compass tests. The exception was the FSI, which showed directional errors in the field that were notably larger than in the laboratory.