Conductivity exhibits the concentration and characteristics of a solution. Thus, by measuring the conductivity, the concentration can be determined. A particular use for these meters is the detection of NaCl, so they are also some times called salinometers.
Two metal plates are inserted into the liquid. These plates serve as electrodes. An alternating voltage of fixed amplitude is applied to the two electrodes, and the current flowing through the liquid is measured.
To explain the principle of conductivity measurement, arrange a setup as shown below. Two metal plates are inserted into the liquid. These plates serve as electrodes. An alternating voltage of fixed amplitude is applied to the two electrodes, and the current flowing through the liquid is measured.
J = proportionality constant determined by the shape of the electrodes
k = conductivity
E & J are fixed.
so by obtaining a value for i, conductivity can be measured
In actual practice, however, an electrical double layer forms on the surface of the electrodes, and ions in the solution receive the electrons and electrical charge from the electrodes (electrode reaction).
Platinum electrodes that are covered with platinum black and set facing each other in a parallel fashion is the optimum configuration.
A fixed alternating current voltage suited to the conductivity measurement range is applied between the electrodes. The frequency of this voltage is chosen so as to reduce impedance for the purpose of avoiding polarization from electrode reaction and so on. As for the materials used in the parts other than the electrodes, ethylene tetrafluoride and hard vinyl chloride are used as insulating materials, and stainless steel is often used for the outer tube.
As the conductivity of the liquid being measured has a temperature coefficient, some devices also have a temperature sensor built into the electrode, and perform a temperature compensation function.