YSI 6000UPG - Manual (Page 101)

6-1
6. PRINCIPLES OF OPERATION
6.1 CONDUCTIVITY
The 6000 UPG utilizes 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 milliSiemens per cm (mS/cm), the
conductance is multiplied by the cell constant which has units of reciprocal cm (cm
-1 ). The cell
constant for the 6000 UPG 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; see Section 3.2. 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 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 6000
UPG
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 report. If Specific Conductance is selected, the 6000
UPG 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 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.