Yokogawa EXA ISC202 2-wire Conductivity Transmitter/Analyzer - Manual (Page 51)

IM 12D06A03-01E
Parameter setting 5-9
5-2-3. Temperature compensation
Why temperature compensation?
The conductivity of a solution is very dependent on temperature. Typically for every 1 °C change in
temperature the solution conductivity will change by approximately 2 %. The effect of temperature varies
from one solution to another and is determined by several factors like solution composition, concentra-
tion and temperature range. A coefficient ( α) is introduced to express the amount of temperature influ-
ence in % change in conductivity per °C. In almost all applications this temperature influence must be
compensated before the conductivity reading can be interpreted as an accurate measure of concentra-
tion or purity.
1 Standard temperature compensation (NaCl)
From the factory the EXA is calibrated with a general temperature compensation function based on a
sodium chloride salt solution. This is suitable for many applications and is compatible with the compen-
sation functions of typical laboratory or portable instruments.
Table 5-1. NaCl-compensation according to IEC 60746-3 with Tref = 25 °C
T Kt α T Kt α T Kt α
0 0.54 1.8 60 1.76 2.2 130 3.34 2.2
10 0.72 1.9 70 1.99 2.2 140 3.56 2.2
20 0.90 2.0 80 2.22 2.2 150 3.79 2.2
25 1.0 --- 90 2.45 2.2 160 4.03 2.2
30 1.10 2.0 100 2.68 2.2 170 4.23 2.2
40 1.31 2.0 110 2.90 2.2 180 4.42 2.2
50 1.53 2.1 120 3.12 2.2 190 4.61 2.2
200 4.78 2.2
2-A. Calculation of Temperature Coefficient Factor (α)
(With known conductivity at reference temperature).
α = K t
- K
ref x 100%
T - T
ref
K
ref
α = Temperature compensation factor (in % / °C)
T = Measured temperature (°C)
K
t
= Conductivity at T
T
ref
= Reference temperature (°C)
K
ref
= Conductivity at T
ref
2-B. Calculation of Temperature Coefficient Factor (T.C.)
(with two known conductivity values at different temperatures)
Measure the conductivity of the liquid at two temperatures, one below the reference and above the
reference temperature with the temperature coefficient set to 0.00% per °C and use the following
equation to calculate a temperature coefficient ( α ).
K
t
K
ref
=
1+ α ( T - T
ref
)
K
2
- K
1 α = x 100%
K
1
( T
2
- T
ref
) - K
2
( T
1
- T
ref
)
K
1
K
2
K
ref
= =
1+ α ( T
1
- T
ref
) 1+ α ( T
2
- T
ref
)
K
1
(1+ α ( T
2
- T
ref
)) = K
2
( 1+ α ( T
1
- T
ref
))
K
1
α ( T
2
- T
ref
) - K
2
α ( T
1
- T
ref
) = K
2
- K
1
K
2
K
1
K
ref
T
2
T
1
T
ref
T [ºC]
K [
μS/cm]
T
1
, T
2
: liquid temperature (°C)
K
1
: conductivity at T
1
(°C)
K
2
: conductivity at T
2
(°C)