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.
Although the RV Southern Surveyor has a particularly thick steel hull of 25 mm, and the positioning of the SBE 48 surrounded by black water pipes and hull ribs was far from ideal, this study indicates that the SBE 48 is capable of providing ship SST observations of sufficiently accurate for satellite SST validation and possible calibration. If the SBE 48 has good thermal contact with the hull, is positioned well below the water line away from on-ship heat sources, and the sensor and surrounding hull is sufficiently insulated from the interior ship’s atmosphere, the hull-contact sensor should provide a bulk sea surface temperature measurement of comparable accuracy to thermosalinograph water intake temperatures, albeit possibly biased slightly warm. Further comparison tests are planned for the SBE 48 sensor on vessels with thinner hulls and wider spaced hull ribs.