Calibration of currentmeters in direction: results obtained on a stock of instruments with a new calibration platform

SHOM, 13 rue du Chatellier, CS 92803, 29228 Brest Cedex

^a Corresponding author: marc.lemenn@SHOM.fr

Abstract

In order to preserve their efficiency, submarine tidal turbines and other marine renewable energy systems need an accurate knowledge of sea currents directions. The speeds and directions of these currents can be measured in a cell of water with currentmeters (AQD) or on a water column with profilers called AQP or ADCP which stands for Acoustic Doppler Current Profiler. When these instruments are installed on carriers, mooring lines or mooring cages, it is necessary to correct the measured data in heading, roll and pitch, in order to interpret them in a terrestrial referential. In this way, they are equipped with a magnetic compass and tilt sensors. Magnetic environment influences compasses responses. There was no system to calibrate AQD and AQP in their using configurations. Following a method perfected in 2007 in order to correct angle errors and warranting a measurement uncertainty in the case of instruments fixed in mooring cages, a calibration platform for magnetic compasses and tilt sensors has been built. In order to show the interest and the corrections allowed by this equipment, this publication shows the effects of the instrumentation of a DORA mooring cage on the compass response of an AQD. The measurements underscore that the initial compass error of ± 4 ° peak-peak is between – 10 ° and + 5.7 ° after installation in the cage. This publication shows also the results obtained after the calibration of a stock of 30 AQD currentmeters and 22 AQP profilers. The manufacturer specifications of ± 2 ° in heading are respected only in 37 % of cases for AQD’s and 23 % for AQP’s. The heading and roll specifications of ± 0.2 ° are never respected but 86 % of profilers and 53 % of currentmeters show errors less than 2 °.