ABB 4623, Manual, Page: 29 / 32

27
0 20 40 60 80 100
Temperature (
°
C)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.8
0.055
0.706
Conductivity (
µ
S/cm)
'b'
Curve 'a' – Theoretical ultra-pure water conductivity
Curve 'b' – High purity water conductivity (ultra-pure
water with slight impurity)
'a'
25
A1 Automatic Temperature Compensation
At high purity water conductivity levels, the conductivity/
temperature relationship is made up of two components: the
first component, due to the impurities present, generally has a
temperature coefficient of approximately 0.02/
°
C; and the
second, which arises from the effect of the H
+
and OH
–
ions,
becomes predominant as the ultra-pure water level is
approached.
Consequently, to achieve full automatic temperature
compensation, the above two components must be
compensated for separately, according to the following
expression:
G
25
=
G
t
– G
upw
[1 +
∝
(t – 25)]
+ 0.055
Where: G
t
= conductivity at temperature t
°
C
G
upw
= ultra-pure water conductivity at
temperature t
°
C
∝
= impurity temperature coefficient
0.055 = conductivity in
µ
S/cm of ultra-pure
water at 25
°
C
The expression is simplified as follows:
G
25
=
G
imp
[1 +
∝
(t – 25)]
+ 0.055
Where: G
imp
= impurity conductivity at temperature t
°
C
The above expression was solved in earlier analog
instrumentation by using two temperature sensing elements
located in the conductivity measuring cell. However, models
4623 and 4628 now utilize the computational ability of a
microprocessor to achieve ultra-pure water temperature
compensation using only a single platinum resistance
thermometer and mathematically calculating the temperature
compensation required to give the correct conductivity at the
reference temperature.
APPENDICES
Fig. A1 Theoretical Ultra-pure Water Conductivity and
High Purity Water Conductivity v Temperature