Accurate Evaluation of Thermometers for Deep-Ocean Measurements

Abstract Climate change research must ensure international data comparability over long periods, and deep-ocean temperature measurements require accurate evaluation of thermometers because temperature changes there are small. We calibrated three reference thermometers [Sea-Bird Electronics (SBE) 35] at the triple point of water and the gallium melting point, as defined by the International Temperature Scale of 1990, and confirmed that they were extremely stable over a 20-yr period; the temporal drift for two of the three was within ±0.2 mK decade −1 . We evaluated the pressure sensitivities of two conductivity–temperature–depth (CTD) thermometers in a laboratory up to 68 MPa; the results agreed with values estimated using SBE 35s in the deep ocean. We evaluated temperature hysteresis of the three SBE 35s in the laboratory: one showed no hysteresis and the other two exhibited hysteresis of 0.3–0.5 mK. Pressure hysteresis was examined in the deep ocean. Of 22 CTD thermometers, more than half showed estimated pressure hysteresis of 0.5–1 mK. The overall expanded uncertainty of the deep-ocean temperature measurement (depths greater than 20 MPa) by the CTD thermometer with small hysteresis calibrated in reference to the SBE 35 is estimated to be 0.8 mK. To eliminate the influence of systematic errors due to hysteresis, we strongly recommend aligning the temperature data 0.3 s ahead of the pressure data to account for the temperature data delay due to the sensor’s response time and applying the in situ calibration coefficients obtained from upcast data to the continuous upcast profile, thereby modifying both continuous profile data and data from discrete water sampling depths from the upcast.