In Situ Calibration of the SeaBird 9plus CTD Thermometer

Abstract

A Sea-Bird Electronics (SBE 35) deep ocean reference thermometer is used with the SBE 9plus CTD system to calibrate the SBE 3 ocean thermometers of the CTD. The SBE 35 is standardized in water-triple-point and gallium-melting-point cells. The SBE 3 is calibrated with the SBE 35 under the assumption that discrepancies between SBE 3 and SBE 35 data are due to pressure sensitivity, the viscous heating effect, and time drift of the SBE 3. Based on the results of an in situ calibration, the pressure sensitivity and the viscous heating effect were evaluated for 11 SBE 3 thermometers. Three SBE 3s showed little pressure sensitivity, and eight had pressure sensitivities of 1?2 mK at 6000 dbar. The average viscous heating effect on the standard SBE 3 measurements was 0.5 mK. Both the accuracy and precision of the in situ calibrated SBE 3 data at depths greater than 2000 dbar were 0.4 mK relative to the SBE 35 reference.