Expendable Probe

An expendable probe is a single-use ocean profiling sensor dropped from a moving ship and abandoned after one cast. A thin twin-wire conductor pays out from both probe and launcher so the ship keeps headway while the probe free-falls at a calibrated rate; depth is inferred from elapsed time, not pressure. The expendable bathythermograph (XBT) measures temperature to about 760 or 1830 m depending on probe type; the expendable CTD (XCTD) adds conductivity for salinity. When the wire runs out it snaps and the probe is lost, which is the design intent. Expendables let merchant ships of opportunity build temperature transects no research vessel could staff.