98 AWT420 | UNIVERSAL 4-WIRE, DUAL-INPUT TRANSMITTER | OI/AWT420-EN REV. 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
the internationally accepted standard.
Most commonplace, weak aqueous solutions have temperature
conductivities of the solutions increase progressively by 2 %
coefficient tends to become less.
At low conductivity levels, approaching that of ultra-pure water,
dissociation of the HO 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.
-1
, the generally accepted
expression relating conductivity and temperature is:
Gt25
Where:
Gt
G25
-1
-1
,
temperature compensated measurements, a conductivity
analyzer must carry out the following computation to obtain G25:
G25
t
25 =
However, for ultra-pure water conductivity measurement, this
form of temperature compensation alone is unacceptable since
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:
[1 + ∞ (t – 25)]
tupw
25 =
Where:
Gt
Gupw
-1
of ultra-pure
The expression is simplified as follows:
G25
[1 + ∞ (t – 25)]
imp
25 =
Where:
Gimp
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