Metex MS Series Conductivity Cells - Manual (Page 5)

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3.0 PRINCIPLE OF OPERATION
3.1 All solutions containing water conduct electricity to some extent. The ability of the solution to
conduct electricity is called “conductance” (the reciprocal of resistance). Addition of
electrolytes such as salts, acids, or bases to pure water will increase the ability of the liquid
to conduct electricity and hence, increase the solution’s conductance (decreases the
resistance).
3.2 An electrolytic conductivity system measures solution conductance by using an analyzer
interconnected with cable to a sensor immersed in the solution. The conductivity sensor is
composed of two or more electrodes and a temperature sensor. The electrodes are in
contact with the solution. The analyzer circuitry impresses an alternating voltage between
the electrodes and the magnitude of the resulting current is linearly related to the solution
conductivity.
3.3 When the temperature of a solution changes, its conductivity changes. The temperature
change is compensated with a thermistor (temperature sensitive resistor) in the conductivity
sensor to alter the gain of the measuring circuit. This is accomplished such that, regardless
of actual solution temperature, the instrument display indicates what the solution
conductivity would be if the solution temperature was 25°C (an internationally accepted
reference).