Expendable bathythermograph

From Glossary of Meteorology

expendable bathythermograph

(Abbreviated XBT.) A device for obtaining a record of temperature as a function of depth to 1800 m from a ship with a speed as high as 15 m s-1.

Temperature is measured with a thermistor within an expendable casing. Depth is determined by a priori knowledge of the rate at which the casing sinks and the time of each recorded data value. A fine wire provides a data transfer line to the ship for shipboard recording. Airborne versions (AXBT) are also used; these use radio frequencies to transmit the data to the aircraft during deployment.
See conductivity–temperature–depth profiler.