Pressure dependence of reference deep-ocean thermometers

In this work, the pressure dependence of the temperature readings of reference deep-ocean thermometers (i.e. SBE 35, Sea-Bird Electronics) was investigated in a thermostatically controlled and pressurized environment. To this end, a deep-ocean environment emulator was constructed to reproduce the extremely stable temperature and high-pressure nature of the deep ocean; it consisted of a pressure vessel, a pressure controller and a thermostatic water bath. With the aid of this emulator, the pressure effect on three SBE 35 thermometers was investigated at various hydrostatic pressures from atmospheric pressure to 68 MPa, equivalent to the pressure at a depth of 6,800 m. In addition, the effect of external thermal disturbances was corrected by measuring the real-time temperature variation around the SBE 35 thermometer in order to achieve the highest possible accuracy. From extensive testing of the pressure dependence of the temperature readings of the three SBE 35 thermometers, it was found that two of the three thermometers exhibited clear and repeatable pressure dependences, although the respective tendencies were different from each other. The third thermometer showed no repeatable pressure dependence. All the measured pressure dependences of the three thermometers amounted to changes which were within +/- 1 mK. The results indicated that the pressure dependence of these reference deep-ocean thermometers is significant, and the pressure dependence is not the same for all SBE 35 devices but specific to each individual device. It was concluded that the pressure dependence of the individual reference deep-ocean thermometers needs to be accounted for during calibration.