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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:
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)