Expendable bathythermograph

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The expendable bathythermograph, or XBT, is a device for obtaining a record of temperature as a function of depth from a moving ship.

Temperature is measured with a thermistor within an expendable, weighted 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 pair of fine copper wires which pay out from both a spool retained on the ship and one dropped with the instrument, provide a data transfer line to the ship for shipboard recording. Eventually, the wire runs out and breaks, and the XBT sinks to the ocean floor. Since the deployment of an XBT does not require the ship to slow down or otherwise interfere with normal operations, XBT's are often deployed from vessels of opportunity, such as cargo ships or ferries. Airborne versions (AXBT) are also used; these use radio frequencies to transmit the data to the aircraft during deployment.

The XBT was invented in the early-1960s by former The Sippican Corporation (TSC), today Lockheed Martin Sippican, which has manufactured over 5 million XBT's. An XBT which also measures conductivity, allowing salinity to be computed, is termed an XCTD.

[edit] Uses

Oceanography: to obtain information on the temperature structure of the ocean.
Submarine and Anti-submarine warfare: to determine the layer depth (thermocline) used by submarines to avoid active sonar search.