horiba U-51, Инструкция по эксплуатации

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6 Reference
110
6.5 Conductivity (COND) measurement
6.5.1 Four-AC-electrode method
Conductivity is an index of the flow of electrical current in a substance.
Salts dissolved in water are separated into cations and anions. Such solution is called
electrolytic solution.
Electrolytic solution has the property of allowing the flow of current according to Ohm’s law.
This property is referred
to as ionic conductivity, since current flow is caused by ion movement in electrolytic solution.
Metals, on the other hand, allow the flow of current by means of electrons. This property is
called electronic conductivity,
which is distinguished from ionic conductivity.
A cube with 1 m on each side, as shown in Fig. 3, is used to demonstrate an electrolytic
solution. Two electrode plates are placed on opposite sides, and the cube is filled with
solution. If the resistance between these two electrode plates is represented by r(

), the
conductivity of the solution L(S

m
-1
) is represented as L=1/r. S stands for Siemens, a unit of
measurement of conductance.
Fig. 3 Definition of conductivity
The most general method for measuring conductivity is based on the above principle, and is
called the 2-electrode method.
In the 2-electrode method the influence of polarization cannot be ignored for solutions with
high conductivity and conductivity cannot be measure accurately. In addition, contamination
on the surface of the electrode increases apparent resistance, resulting in inaccurate
measurement of conductivity.
The U-50 series has adopted the 4-electrode method to overcome these disadvantages of
the 2-electrode method.
As shown in Fig. 4, the U-50 series uses two voltage-detecting electrodes and two voltage-
applying electrodes, for a total of four electrodes. The voltage-detecting electrodes are for
detecting AC voltage, and the voltage-applying electrodes are for applying AC voltage.
Fig. 4 Principle of the 4-electrode method
Electrode plate
1 m
Solution
1 m
1m
Voltage-applying electrodes
Voltage-detecting electrodes