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Instrument: STD : Salinity, Temperature, Depth
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Vertical profiles of salinity and temperature in the ocean
have been obtained by a variety of instruments, referred to here
generically as STDs or (when the salinity is obtained by way of the
water's conductivity) CTDs. Data described in the Master Directory
have in some cases been obtained over considerable periods of time
and by a variety of investigators. Hence, the specifics of the
STDs vary considerably - sometimes within the same data set.
For descriptions of the instrumental sources and
characteristics of a particular data set, investigators will have
to contact the appropriate archive. The brief sketch of salinity,
STDs and CTDs given below is taken from a standard and well-
accepted text in descriptive physical oceanography. It is intended
for those not already familiar with the field.
Temperature and depth are physical parameters common to all
science - indeed common to ordinary language; salinity is much more
specialized. The total amount of dissolved material in sea-water
is termed the 'salinity' and is defined as 'the total amount of
solid materials in grams contained in one kilogram of sea-water
when all the carbonate has been converted to oxide, the bromine and
iodine replaced by chlorine and all organic matter completely
oxidized'. For example, the average salinity of ocean water is
about 35 gm/km of sea water, usually stated as 'thirty-five parts
per thousand'. One of the more remarkable features of sea-water is
that while the total concentration of dissolved salts varies from
place to place, the ratios of the more abundant components remain
almost constant.
The direct determination of salinity by evaporating sea-water
to dryness is too difficult to carry out as a routine process.
Instead, the classical (Knudsen) method of measurement exploits the
observed invariance of component ratios. One first determines the
chlorinity by titration with standard silver nitrate solution and
then scales up to the salinity by a relation based on the measured
ratio of chloride ion to total dissolved substances:
Salinity = 1.80655 X Chlorinity
In routine use, an accuracy of 0.02 parts per thousand is
considered reasonable for this laboratory technique. More advanced
lab procedures easily yield accuracies of 0.003 parts per thousand.
In the 1960s inductive salinometers were developed for in situ
use and a number of such instruments are now available. Because
they measure Conductivity, Temperature and Dept, they are generally
referred to as CTD instruments. Accuracies of 0.005 parts per
thousand, 0.005 degrees C and 0.15% of full scale depth are claimed
(Pickard, p.93). The table below (from the CRC Handbook of Marine
Science, p.550) gives more modest values for several commercially
available in situ instruments.
parts per degrees C* meters*
Beckman RS6 0-40 (0.2) 0-30 (0.2) 0-130 (2.5)
Geodyne 30-40 (0.02) -2-35 (0.05) 0-9000 (0.25%)
Kjeler 30-40 (0.3) -2-35 (0.02) 0-2500
T.S.K. 29-36 (0.03) -2-32 (0.2) 0-100 (3%)
* Accuracies are given in parentheses, in the appropriate units
unless percent is indicated.
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)

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