YSI ADV6600 - Manual (Page 115)

Section 9. Principles of Operation
ADV6600
Y S I
Environmental
Page 105
mS/cm, which have been prepared in accordance with recommendation 56-1981 of the Organization
International De Metrologie Legale (OIML), are available from YSI. The instrument output is in
mS/cm or µS/cm for both conductivity and specific conductance. The multiplication of cell constant
times conductance is carried out automatically by the software.
9-6.1. Effect of Temperature
The conductivity of solutions of ionic species is highly dependent on temperature, varying as much
as 3% for each change of one degree Celsius (temperature coefficient = 3%/°C). In addition, the
temperature coefficient itself varies with the nature of the ionic species present.
Because the exact composition of a natural media is usually not known, it is best to report
conductivity at a particular temperature, e.g. 20.2 mS/cm at 14
o 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 sonde
software also allows the user to output conductivity data in either raw or temperature compensated
form. If Conductivity is selected, values of conductivity that are NOT compensated for temperature
are output to the report. If Specific Conductance is selected, the sonde uses the temperature and raw
conductivity values associated with each determination to generate a specific conductance value
compensated to 25°C. The calculation is carried out as in equation (1) below, using a temperature
coefficient of 1.91%/°C (TC = 0.0191):
Specific Conductance (25
o
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 seawater and for solutions of many common salts such as NaCl
and NH
4
Cl.
9-7. Salinity
Procedure from Standard Methods for the Examination of Water and Wastewater: 2520
Salinity is determined automatically from the sonde conductivity and temperature readings
according to algorithms found in Standard Methods for the Examination of Water and Wastewater
(ed. 1989). The use of the Practical Salinity Scale results in values that 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.