XCTD

An XCTD (expendable conductivity-temperature-depth probe) is a single-use sensor that profiles temperature and conductivity, and thus salinity, as it free-falls through the water column, launched from a moving ship without stopping. Unlike the temperature-only XBT, the XCTD carries a conductivity cell, so it returns a salinity profile; depth comes from a calibrated fall-rate equation rather than a pressure sensor. Data transmit up the thin wire to the shipboard recorder until the wire parts. Common Sippican and Tsurumi-Seiki models reach about 1000 to 1850 m depending on probe type. XCTDs fill salinity coverage between full CTD stations and across underway transects, with depth accuracy near 2 percent.