Appendix B 2-Electrode; Conductivity Calculations; Automatic Temperature Compensation - ABB AWT420 Operating Instruction

98 AW T4 2 0 | U N I V E R S A L 4 - W I R E , D U A L- I N P U T T R A N S M I T T E R | O I/A W T4 2 0 - E N R E V. B
Appendix B 2-electrode conductivity calculations

Automatic temperature compensation

The conductivities of electrolytic solutions are influenced
considerably by temperature variations. Thus, when significant
temperature fluctuations occur, it is general practice to correct
automatically the measured, prevailing conductivity to the
value that would apply if the solution temperature were 25 °C,
the internationally accepted standard.
Most commonplace, weak aqueous solutions have temperature
coefficients of conductance of the order of 2 % per °C (i.e. the
conductivities of the solutions increase progressively by 2 %
per °C rise in temperature). At higher concentrations the
coefficient tends to become less.
At low conductivity levels, approaching that of ultra-pure water,
dissociation of the H2O molecule takes place and it separates
into the ions H+ and OH-. Since conduction occurs only in the
presence of ions, there is a theoretical conductivity level for
ultra-pure water which can be calculated mathematically. In
practice, correlation between the calculated and actual
measured conductivity of ultra-pure water is very good.
Figure 34, page 99 shows the relationship between the
theoretical conductivity for ultra-pure water and that of high
purity water (ultra-pure water with a slight impurity), when
plotted against temperature. The figure also illustrates how a
small temperature variation considerably changes the
conductivity. Subsequently, it is essential that this temperature
effect is eliminated at conductivities approaching that of ultra-
pure water, in order to ascertain whether a conductivity
variation is due to a change in impurity level or in temperature.
For conductivity levels above 1 µS cm
expression relating conductivity and temperature is:
G = G [1 + ∝ (t – 25)]
t 25
Where:
G = conductivity at temperature t °C
t
G = conductivity at the standard
25
temperature (25 °C)
∝ = impurity temperature coefficient
∝ = temperature coefficient per °C
, the generally accepted
-1
At conductivities between 1 µS cm
∝ lies generally between 0.015/°C and 0.025/°C. When making
temperature compensated measurements, a conductivity
analyzer must carry out the following computation to obtain G
G
t
G = G =
25
25
[1 + ∞ (t – 25)]
However, for ultra-pure water conductivity measurement, this
form of temperature compensation alone is unacceptable since
considerable errors exist at temperatures other than 25 °C.
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 – G
t upw
G25 = G =
25
[1 + ∞ (t – 25)]
Where:
G = conductivity at temperature t °C
t
G = ultra-pure water conductivity at
upw
temperature t°C
∝ = impurity temperature coefficient
0.055 = conductivity in µS cm
water at 25 °C
The expression is simplified as follows:
G
imp
G =
G =
25
25
[1 + ∞ (t – 25)]
Where:
G = impurity conductivity at temperature t °C
imp
-1 and 1,000 µS cm -1 ,
+ 0.055
of ultra-pure
-1
+ 0.055
:
25