Current profilers and current meters: compass and tilt sensors errors and calibration

Abstract

Current profilers and current meters have a magnetic compass and tilt sensors for relating measurements to a terrestrial reference frame. As compasses are sensitive to their magnetic environment, they must be calibrated in the configuration in which they will be used. A calibration platform for magnetic compasses and tilt sensors was built, based on a method developed in 2007, to correct angular errors and guarantee a measurement uncertainty for instruments mounted in mooring cages. As mooring cages can weigh up to 800 kg, it was necessary to find a suitable place to set up this platform, map the magnetic fields in this area and dimension the platform to withstand these loads. It was calibrated using a GPS positioning technique. The platform has a table that can be tilted to calibrate the tilt sensors. The measurement uncertainty of the system was evaluated. Sinusoidal corrections based on the anomalies created by soft and hard magnetic materials were tested, as well as manufacturers’ calibration methods.

Perfecting of a Calibration Bed for Current Profilers

“…Ils sont équipés de compas du type 'flux-gate' pour retrouver l'amplitude des composantes (U, V, W) des courants par rapport au Nord magné tique (angle ), puis, connaissant la dé clinaison magné tique, par rapport au Nord gé ographique (2). Ils sont, de plus, corrigé s en inclinaison à l'aide de capteurs de 'tilt' qui mesurent les angles  et  de roulis et tangage.…”

“…Ces dernières années, l'étalonnage ou simplement le test de ces instruments, est resté un problè me non traité dans les organismes océ anographiques. Au Shom, une plateforme a d'abord été construite et mise en service en 2012, pour étalonner, dans leur configuration instrumentale d'utilisation, les compas et capteurs d'inclinaison dont ils sont dotés (voir [1][2][3][4]). Ces compas servent à retrouver la direction des instruments par rapport au Nord magné tique, les trois transducteurs dont ils sont é quipé s permettant de calculer la direction des courants dans le référentiel de l'instrument.…”

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

“…The figure 3 shows the magnetic anomalies obtained after correcting for the temporal and secular variations of the terrestrial field. The conception follows feasibility experiences realized in 2007 [2] and it was the subject of a second publication in 2014 [3]. In the area with the lower gradients, a flat concrete block was constructed with a measured maximum slope of 3 mm m -1 or 0.18 ° and an average roughness equivalent to an uncertainty of 0.15 °.…”