The XBT (eXpendable BathyThermograph) allows us to take the temperature of the water column quickly when we do not have time to stop the ship to do a proper CTD "dip" or when deploying the CTD will be too risky, like inside canyons.
The XBT is housed in a black plastic tube which is mounted onto a white plastic launcher. The cable coming from this launcher connects to a computer which records the data from the unit. Within the tube is a length of very fine copper wire, a torpedo-shaped probe with a lead nose cone to help it sink quickly, and inside the hollow nose of the torpedo, a thermistor. The thermistor is an electronic component which varies its resistance to electrical current depending on temperature. So, if we have a calibrated thermistor (one for which the relationship between temperature and resistance is known), we can use the resistance of the circuit to tell us the temperature of the water. The probe sinks at a known rate, so simply by timing how long since the launch happened, we can tell how deep the XBT is, and thus relate the temperature to the depth without requiring a depth sensor within the XBT which would massively increase its cost.
XBTs can even be deployed from low-flying airplanes, making them very useful for rapidly surveying the temperature structure of large areas of ocean. The XBTs we use are made by Sippican.
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