Digitization and Calibration of the Expendable Bathythermograph

Roemmich, D, Cornuelle B.  1987.  Digitization and Calibration of the Expendable Bathythermograph. Deep-Sea Research Part a-Oceanographic Research Papers. 34:299-307.

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A study was undertaken of signal digitization and temperature calibration in expendable bathythermographs (XBT's) to learn how to minimize temperature errors in that system. An XBT digitizer was built into a PC-type microcomputer and used to calibrate 24 XBT probes at 5 temperature points, and later, to calibrate 72 probes at a single temperature. Twenty of the first set of probes were fastened rigidly in pairs and dropped in the ocean as a field test of the calibrations. Calibration of individual probes reduced the standard deviation of temperature calibration errors from around 0.05°C to <0.01°C. The calibration procedure is simple and nondestructive, so the probes can be used normally after calibration. Errors in the temperature digitizer can be held to about 0.01°C by periodic adjustment. An advantage of the PC-based digitizer is the ease with which the calibrations are accomplished and applied to the ocean tracers. Two substantial sources of transient systematic error in XBT temperatures are mentioned: an electronic transient lasting about 0.1 s occurs on entry of the probe into seawater, and a longer transient is due to the thermal mass of the XBT nose.