XBT

The expendable bathythermograph (XBT) is a single-use probe dropped from a moving ship that telemeters temperature versus depth down a thin wire, deriving depth from a fall-rate equation rather than measuring pressure. XBTs gave the bulk of upper-ocean temperature data along repeated ship tracks for decades; a known fall-rate bias required corrections in ocean heat-content reconstructions. The XCTD variant adds a conductivity sensor for salinity.