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Instruments > In Situ/Laboratory Instruments > Profilers/Sounders > BATHYTHERMOGRAPHS

Preferred term

BATHYTHERMOGRAPHS  

Definition

  • The most straightforward way to measure temperature versus depth in the ocean is to lower a thermometer a known distance and take the temperature. So-called 'protected reversing thermometers' have been developed for just that purpose, and are routinely accurate to 0.02 degrees C. One disadvantage of this method is that 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 a continuous trace of temperature against depth. A liquid-in-metal thermometer causes a metal point to move in one direction over a smoked glass slide which is itself moved at right angles to this direction by a pressure sensitive bellows. The instrument is lowered to its permitted depth (generally 60, 140 or 270 m) and then brought back up. Since pressure is directly related to depth, the line scratched on the smoked glass forms a graph of temperature against depth. It is read against a calibration grid to a typical accuracy of 0.2 degrees C and 2 meters. In wide use now is the expendable bathythermograph (XBT) which uses a thermistor as temperature sensitive element. The thermistor is in a small streamlined weighted casing which is simply dropped over the ship's side. It is connected by a fine wire, on special free-unwinding spools, to a recorder on the ship which traces the temperature of the water in a graphical plot against depth. The depth is not sensed directly but estimated from the time elapsed since release, using the known rate of sink of the thermistor casing. This casing is relatively inexpensive and is not recovered. These XBTs are available for depth ranges from 200 to 1800 meters, and can be used from ships underway or from circling aircraft. They can also be dropped from aircraft in a small buoy which contains a radio transmitter to send temperature/depth information to the aircraft while it continues its flight. The temperature range and depths of several commercial bathythermographs are given below: MANUFACTURER TEMPERATURE RANGE DEPTH degrees C m Belfort -1 to 30 60/135/250 -2 to 32 55/137/274 GM -2 to 32 60/137/274 Jules Richard -2 to 30 50/150/300 Kahl -2 to 30 60/137/274 Mashprib -2 to 30 200 Wallace & Tiernan -1 to 30 60/135/270 T.S.K. -2 to 32 75/150/270 Taken from: Pickard, G.L, Descriptive Physical Oceanography, 3rd edition, Pergamon Press, Oxford, 1979. ISBN 0-08-023824-6 Smith, F.G.W, (Editor), CRC Handbook of Marine Science, Volume I, CRC Press, Cleveland, 1974. ISBN 0-87819-388-X (Complete Set) (en)

Broader concept

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https://gcmd.earthdata.nasa.gov/kms/concept/92f66be9-30e6-4d0e-b4a8-631fe9341b25

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