18 Glossary
Cell constant
The cell constant of a conductive conduc‐
tivity sensor is determined by the geom‐
etry (surface of the electrodes and their
distance) of the sensor.
The cell constant can deviate by up to
10% from the nominal value specified on
the sensor. The geometry can also
change through wear and tear or rough
attempts at cleaning it.
Calibrating the cell constant generally
compensates for both effects. Films on
the sensors, caused for example by dirt or
lime can also change the cell constant.
The cell constants are generally changed
gradually over a long period of time. In the
event of change, repeated monitoring or
cleaning of the sensor is imperative.
Temperature coefficients
If an aqueous solution is heated, it
increases its electrolytic conductivity,
although the concentration of,
for example, salts has not changed.
The temperature coefficient compensates
mathematically for this temperature influ‐
ence on the conductivity so that when the
temperature of the solution is changed,
the same display value always appears
(Attention: the temperature has to be
measured).
The temperature measurement of a con‐
ductivity sensor is sluggish, so that it
needs a few minutes until the temperature
of the liquid is displayed in a stable
manner. Wait until the value is stable or
slightly begins to change in opposite
directions. In the event of rapid tempera‐
ture changes, we would recommend an
external temperature sensor with a fast
response time.
The conductivity measurement is based
on a reference temperature, which is usu‐
ally 25°C. However the conductivity meas‐
urement can also be adapted to deviating
reference temperatures. A comparison of
electrolytic conductivity is also possible
with temperature changes to to automatic
temperature compensation. The tempera‐
ture coefficient α is given in %/°C or %/K.
With potable water, the temperature coef‐
ficient is approximately 2% /°C of the tem‐
perature change.
Glossary
111
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