Xylem 6-Series Multiparameter - Manual (Page 239)

Principles of Operation Section 5
YSI Incorporated Environmental Monitoring Systems Manual 5-1
SECTION 5 PRINCIPLES OF OPERATION
5.1 CONDUCTIVITY
The sondes utilize a cell with four pure nickel electrodes for the measurement of solution conductance.
Two of the electrodes are current driven, and two are used to measure the voltage drop. The measured
voltage drop is then converted into a conductance value in milli-Siemens (millimhos). To convert this value
to a conductivity value in milli-Siemens per cm (mS/cm), the conductance is multiplied by the cell constant
that has units of reciprocal cm (cm -1 ). The cell constant for the sonde conductivity cell is approximately
5.0/cm. For most applications, the cell constant is automatically determined (or confirmed) with each
deployment of the system when the calibration procedure is followed. Solutions with conductivities of
1.00, 10.0, 50.0, and 100.0 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 uS/cm for both conductivity and specific conductance. The multiplication of cell
constant times conductance is carried out automatically by the software.
CALIBRATION AND 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 a 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.
MEASUREMENT AND CALIBRATION PRECAUTIONS
1 - When filling the calibration vessel prior to performing the calibration procedure, make certain that the
level of calibrant standard is high enough in the calibration cup or beaker to cover the entire conductivity
cell.
2 - After placing the sonde in the calibration solution, agitate the sonde to remove any bubbles in the
conductivity cell.
3 - During calibration, allow the sensors time to stabilize with regard to temperature (approximately 60
seconds) before proceeding with the calibration protocol. The readings after calibration are only as good as
the calibration itself.