YSI 30 - Manual (Page 25)

YSI, Incorporated Model 30/30M 21
SECTION 7 PRINCIPLES OF OPERATION
The Model 30 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 milliSiemens (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
Model 30 conductivity cell is 5.0/cm
±4%. 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 6.2. 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 Organisation Internationale
de Métrologie Légale (OIML), are available from YSI. The instrument output is in
µS/cm or mS/cm
for both conductivity and specific conductance. The multiplication of cell constant times
conductance is carried out automatically by the software.
7.1 TEMPERATURE EFFECT
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 Model 30 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 30 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 section 6, Advanced Setup) between 15°C
and 25°C. Additionally the user can select any temperature coefficient from 0% to 4% (see Section
6, Advanced Setup). Using the Model 30 default reference temperature and temperature coefficient
(25°C and 1.91%), the calculation is carried out as in equation (1) 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.