Single and Dual Input Analyzers for Low Level Conductivity
AX410, AX411, AX413, AX416, AX418, AX450, AX455 & AX456
Appendix A
74
IM/AX4CO Issue 11
A.1.1 Calculation of Temperature Coefficient
The temperature coefficient of a solution can be obtained
experimentally by taking non-temperature compensated
conductivity measurements at two temperatures and applying
the following expression:
Where: G
t2
= conductivity measurement at a temperature of
t
2
ºC
G
t1
= conductivity measurement at a temperature of
t
1
ºC
One of these measurements could be made at the ambient
temperature and the other obtained by heating the sample.
Temperature coefficient (%/ºC) =
∝
x 100.
For ultra pure water applications the temperature compensation
equation becomes,
Where: G
imp1
= G
t1
- G
upw1
G
imp2
= G
t2
- G
upw2
A.2 Relationship Between Conductivity and Total
Dissolved Solids (TDS) Measurement
The TDS factor (i.e. the relationship between conductivity [
μ
S
cm
-1
] and TDS in p.p.m.) is totally dependent on the properties
of the solution being measured.
In simple solutions where only one electrolyte is present, the
conductivity/TDS ratio can be ascertained easily, e.g. 0.5 in the
case of sodium chloride. However, in complex solutions where
more than one electrolyte is present, the ratio is not calculated
easily and can be reliably determined only by laboratory testing,
e.g. precipitation and weighing. The ratio in these cases varies
between approximately 0.4 and 0.8, depending on the chemical
constituents, (e.g. the ratio for sea water is about 0.6) and is
constant only when the chemical ratios remain constant
throughout a particular process.
In cases where the TDS factor cannot be determined easily,
refer to the supplier of the particular chemical treatment being
used.
Fig. A.1 Ultra-pure Water Temperature Compensation
Temperature ºC
0
10
20
30
40
50
60
70
80
90
100
micr
oSiemens/cm
0.1
0.0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
μ
S cm
–1
A
B
Curve 'A' – Theoretical ultra pure water conductivity
Curve 'B' – High purity water conductivity
(ultra pure water with slight impurity)
G
t2
– G
t1
G
t1
(t
2
– 25) – G
t2
(t
1
– 25)
∝
=
G
imp1
– G
imp2
[G
imp2
(t
1
– 25) – G
imp1
(t
2
– 25)]
∝
=
