IM 12D03D02-01E
7-2
7. Measuring Principles of this Instrument
7.2 Principles of Operation
AC power supply Ammeter
Figure 7.2 Operational Schematic
Dip two metallic plates (used as sensor electrodes) in a solution and apply a certain
voltage to ow current (I):
I = (1)
E
Rc
where Rc = solution resistance between two sensors
The relationship between Rc and conductivity K is expressed by:
Rc = J • (2)
1
K
where J = constant (also called cell constant) — determined by sensor and distance
between two sensors
From equations (1) and (2) above, we obtain:
K = • I (3)
J
E
E and J are constant, so the conductivity (K) can be obtained by measuring the
current (I).
7.3 Temperature Compensation and Finding
Temperature Compensation Coecient
• Temperature Compensation
Table 7.1 shows the conductivity ratio at each liquid temperature when the
conductivity of a NaCl solution at 25°C is 1.
Table 7.1 Conductivity Ratios at Dierent Temperatures in NaCl solutions
0°C 25°C 50°C 75°C 100°C
0.542 1 1.531 2.103 2.677
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