The expandable bathythermograph (XBT) provides a quick and inexpensive means of collecting temperature data for oceanographic and climate studies. The XBT contains a precision thermistor located in the nose of the probe. Changes in water temperature are recorded by changes in the resistance of the thermistor as the XBT falls through the water. The XBT is capable of temperature accuracies of +/-0.1°C. This XBT system has proved to be reliable in over 30 years of use.
A standard XBT system consists of an expendable probe, a data processing/recording system, and a launcher. The launcher is usually a simple hand held version, but more sophisticated automatic launchers are used when around the clock operation is required. An electrical connection between the probe and the processor/recorder is made when the canister containing the probe is placed within the launcher and the launcher breech door is closed. Following launch, the wire unwinds from the probe as it descends vertically through the water. Simultaneously, wire unwinds from a spool within the probe canister, compensating for any movement of the ship and allowing the probe to freefall from the sea surface unaffected by ship motion or sea state. The XBT system uses a sea water ground. As soon as an electrode within the nose of the expendable probe makes contact with the water, the circuit is complete and temperature data can be telemetered to the ship-board data processing equipment.
Data are recorded and displayed in real time as the probe falls. The nose of each expendable probe is precision weighted and the unit spin-stabilized to assure a predictable rate of descent. From this rate of descent, probe depth is determined to an accuracy of +/-2%. When the probe reaches its rated depth (a function of ship speed and the quantity of wire contained within the shipboard spool) the profile is completed, the wire breaks and the system is ready for another launch.