Publication Abstract

New advice on speed sensor calibration from tow-tank tests of anderaa RCM8 instruments

J.W. Read and K. Medler

As part of its general investigation of instrumental data quality, the EC-VEINS project workshop in November 1998 recommended the joint processing by CEFAS of all German, Finnish, and British Aanderaa RCM8 current meter records from the VEINS array off SE Greenland. These instruments all have the "paddle wheel" type of speed sensor, but the joint processing of their records revealed significant differences in derived flow speeds due to differences in the value assumed for rotor pitch in the speed equation. The Institut für Meereskunde, University of Hamburg, employs the rotor pitch value of 46.5 supplied by the manufacturer, whereas CEFAS apply their own long-established standard value of 42 both to their own instruments and to those that they process on behalf of the Finnish Institute for Marine Research, Helsinki. These different assumed values result in a difference of around 10% in the calculated speeds, the CEFAS value being the lower. Once revealed, the discrepancy was investigated in a series of tank tests at the hydraulics research laboratory HR Wallingford, in which the rotor pitch was measured directly. Pending the outcome, the CEFAS equation was applied to the processing of all instruments. In four sessions (March 1999, June 1999, July 2000, and March 2001), a total of 45 speed sensor calibrations were conducted on 29 different instruments at tow rates of up to 200cm s^-1. The results show a mean pitch of 41.8 ±0.4 cm rev^-1 confirming the CEFAS assumption. In effect this means that the same value for rotor pitch is appropriate for the "paddle wheel" type of speed sensor as had previously been applied to the Savonius rotor type of sensor on Aanderaa RCM 4’s and 5’s. Repeat calibrations of the same instrument appear to show little change in the rotor pitch value before and after long deployments at sea, or with and without the presence of environmental sensors on the instrument’s top plate, though this result is still under investigation. The results of these trials were communicated to the manufacturer. As a result of these and other new data, Aanderaa Instruments have confirmed that a revised rotor pitch value of 43 will be recommended for the processing of this type of instrument and sensor.

Reference:

J.W. Read and K. Medler, 2001. ANew advice on speed sensor calibration from tow-tank tests of anderaa RCM8 instruments. ICES CM 2001/W:13. 12pp.