YSI 63 - Manual (Page 32)

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Because the exact composition of a natural media is usually not known, it is best to report a
conductivity at a particular temperature, e.g. 20.2 mS/cm at 14 C. However, in many cases, it is also
useful to compensate for the temperature dependence in order to determine at a glance if gross
changes are occurring in the ionic content of the medium over time. For this reason, the Model 63
software also allows the user to output conductivity data in either raw or temperature compensated
form. If "Conductivity" is selected, values of conductivity which are NOT compensated for
temperature are output to the display. If "Specific Conductance" is selected, the Model 63 uses the
temperature and raw conductivity values associated with each determination to generate a specific
conductance value compensated to a user selected reference temperature (see Advanced Setup)
between 15 C and 25 C. Additionally the user can select any temperature coefficient from 0% to 4%
(see Advanced Setup). Using the Model 63 default reference temperature and temperature coefficient
(25 C and 1.91%), the calculation is carried out as in the equation below:
Specific Conductance (25°C) = Conductivity
1 + TC * (T - 25)
As noted above, unless the solution being measured consists of pure KCl in water, this temperature
compensated value will be somewhat inaccurate, but the equation with a value of TC = 0.0191 will
provide a close approximation for solutions of many common salts such as NaCl and NH
4
Cl and for
seawater.
9.3 Salinity
Salinity is determined automatically from the Model 63 conductivity and temperature readings
according to algorithms found in Standard Methods for the Examination of Water and
Wastewater (ed. 1995). The use of the Practical Salinity Scale 1978 results in values which are
unitless, since the measurements are carried out in reference to the conductivity of standard
seawater at 15°C. However, the unitless salinity values are very close to those determined by the
previously-used method where the mass of dissolved salts in a given mass of water (parts per
thousand) was reported. Hence, the designation "ppt" is reported by the instrument to provide a
more conventional output.
For further information on conductivity and the above standard information, refer to the ASTM
document, Standard Methods of Test for Electrical Conductivity of Water and Industrial
Wastewater, ASTM Designation D1125-82, and OIML Recommendation Number 56. ASTM
symbols for conductivity, cell constant, and path length differ from those preferred in the general
literature and also from those used in this manual.
9.4 Temperature
The YSI Model 63 system utilizes a thermistor which changes predictably in resistance with
temperature variation. The algorithm for conversion of resistance to temperature is built-in to the
Model 63 software, and accurate temperature readings, in degrees Celsius, are provided
automatically. No calibration or maintenance of the temperature sensor is required.