For calibration of the temperature and conductivity sensors, only the sensor modules need to be sent to Sea-Bird. It is not necessary to send the CTD main housing. See Shipping SBE 9plus, 25, and 25plus Temperature and Conductivity Sensors for details. It is usually not necessary to recalibrate the pressure sensor as frequently as the temperature and conductivity sensors. Experience has shown that the sensor’s sensitivity function almost never changes; only the offset drifts. The offset drift can easily be measured by reading deck pressure against a barometer. This small drift is easily corrected (Seasave V7 and SBE Data Processing provide an entry for the offset drift in the instrument .con or .xmlcon file). SBE 9plus and 25plus — If the pressure sensor does need to be calibrated, the entire CTD must be shipped to Sea-Bird. SBE 25 — If the pressure sensor does need to be calibrated, only the modular SBE 29 pressure sensor needs to be sent to Sea-Bird. It is not necessary to send the CTD main housing.