https://cdn.earthdata.nasa.gov/conduit/upload/5182/KeywordsCommunityGuide_Baseline_v1_SIGNED_FINAL.pdf 23.6 2026-03-19T10:41:57.720Z https://gcmd.earthdata.nasa.gov/KeywordViewer/scheme/instruments/92f66be9-30e6-4d0e-b4a8-631fe9341b25 BATHYTHERMOGRAPHS The most straightforward way to measure temperature versusdepth in the ocean is to lower a thermometer a known distance andtake the temperature. So-called 'protected reversing thermometers'have been developed for just that purpose, and are routinelyaccurate to 0.02 degrees C. One disadvantage of this method isthat temperatures can only obtained for a few depths. The great advantage of the 'bathythermograph' is that,although less accurate than the reversing thermometer, it gives acontinuous trace of temperature against depth. A liquid-in-metalthermometer causes a metal point to move in one direction over asmoked glass slide which is itself moved at right angles to thisdirection by a pressure sensitive bellows. The instrument islowered to its permitted depth (generally 60, 140 or 270 m) andthen brought back up. Since pressure is directly related to depth,the line scratched on the smoked glass forms a graph of temperatureagainst depth. It is read against a calibration grid to a typicalaccuracy of 0.2 degrees C and 2 meters. In wide use now is the expendable bathythermograph (XBT) whichuses a thermistor as temperature sensitive element. The thermistoris in a small streamlined weighted casing which is simply droppedover the ship's side. It is connected by a fine wire, on specialfree-unwinding spools, to a recorder on the ship which traces thetemperature of the water in a graphical plot against depth. Thedepth is not sensed directly but estimated from the time elapsedsince release, using the known rate of sink of the thermistorcasing. This casing is relatively inexpensive and is notrecovered. These XBTs are available for depth ranges from 200 to1800 meters, and can be used from ships underway or from circlingaircraft. They can also be dropped from aircraft in a small buoywhich contains a radio transmitter to send temperature/depthinformation to the aircraft while it continues its flight. The temperature range and depths of several commercialbathythermographs are given below:MANUFACTURER TEMPERATURE RANGE DEPTH degrees C mBelfort -1 to 30 60/135/250 -2 to 32 55/137/274GM -2 to 32 60/137/274Jules Richard -2 to 30 50/150/300Kahl -2 to 30 60/137/274Mashprib -2 to 30 200Wallace & Tiernan -1 to 30 60/135/270T.S.K. -2 to 32 75/150/270Taken from:Pickard, G.L, Descriptive Physical Oceanography, 3rd edition, Pergamon Press, Oxford, 1979. ISBN 0-08-023824-6Smith, F.G.W, (Editor), CRC Handbook of Marine Science, Volume I, CRC Press, Cleveland, 1974. ISBN 0-87819-388-X (Complete Set)