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XBT

Abbreviation for expendable bathythermograph. The accuracy of modern XBT probes is about 0.05$^\circ$ C and most used are rated to 760 m depth.

XCP

Abbreviation for Expendable Current Profiler, an instrument developed at the APL. These use electric field measurements to estimate horizontal velocity relative to an unknown offset. They typically cost 40 times more than an XBT.

XCTD

Abbreviation for eXpendable Conductivity, Temperature and Depth instrument. The accuracy of XCTD probes is 0.02$^\circ$ C in temperature and 0.03 psu in salinity, and they are rated to about 1000 m depth. They typically cost 20 times more than an XBT.