Hull Contact Sensor Tests

Two SBE 48 hull-contact temperature sensors have been tested in the Bureau of Meteorology sensor calibration lab and one installed on the RV Southern Surveyor for comparison tests with the SBE 3 calibrated thermistor installed in the thermosalinograph water intake.   The SBE 48 was attached using magnets to the exterior steel hull at a depth of approximately 3 m below the water line and approximately 20 m aft of the bow.  The SBE 48 was located approximately 3.5 m to port of the SBE 3 sensor and approximately 2 m higher up on the hull plating.  Thermal contact between the SBE 48 heat sink and the ship’s hull was achieved by the use of contact grease with a high thermal conductivity.  A two dimensional thermal analysis of the installation by CSIRO indicated that the ratio of the face area of the SBE 48 thermal sink in relation to the thickness of the hull affects the conduction of heat to the SBE 48 temperature sensor from the adjacent hull region.  It was proposed that the effect of the hull thickness (in this case 0.025 m) can be reduced by placing insulating material around the SBE 48 housing extending to a distance from the sensor element of at least 10 times the hull thickness.

The SBE 48 sensor housing and surrounding hull was insulated on 27 July 2008 at 0300 UTC using three layers of Bradford “Pink Batt” R2.5 ceiling insulation covering the sensor and surrounding hull to an approximate thickness of 270 mm and a minimum distance of 0.25 m from the sensor.  The results presented here are for the cruise commencing 24 July 2008 at 16.6°S, 145.8°E and finishing on 11 August 2008 at 23.8°S, 151.6°E.  Prior to insulation (for the period 24 to 27 July 2008), the SBE 48 temperature was on average 0.28°C warmer than the SBE 3 temperature, with a standard deviation of 0.14°C.  After insulation (for the period 27 July to 11 August 2008), the average offset was 0.19°C with a standard deviation of 0.12°C.  The majority of the error occurred during periods when the water mass exhibited sharp thermal gradients. In water masses with low thermal gradients the average offset was approximately 0.15°C.