11.
FUNDAMENTALS OF CONDUCTIVITY MEASUREMENT
Principle of measurement
Electrolytic conductivity measuring cells basically consist of two metal plates
arranged opposite each other which are immersed in the solution to be measured.
The conductivity of the solution is determined through the measuring voltage
and the resulting measuring current. The current between the metal plates depends
on their geometry (distance and area). The cell constant describes this dependence.
Subject to manufacturing tolerances, or because of dirt or wear, the real cell constant
of a measuring cell often deviates from its nominal value. This deviation is reflected
by the relative cell constant that is adjustable on the conductivity transmitter.
The conductivity of a solution depends on temperature (i.e. the conductivity of
a solution increases with rising temperature). The temperature coefficient of the
solution describes the dependence of conductivity and temperature. Since
conductivity is not always measured at the reference temperature, automatic
temperature compensation has been integrated. The transmitter uses the
temperature coefficient to calculate the conductivity that would be present at the
reference temperature from the current conductivity and the current temperature,
and then display this value. This process is called temperature compensation.
Modern transmitters offer various variants to carry out this temperature
compensation:
- Linear compensation (constant temperature coefficient)
This type of compensation can be used with normal water with an acceptable level of
accuracy. The temperature coefficient used is then about 2,2% /K.
- Natural water (DIN EN27888 or ISO 7888 as the case may be)
In this case, a so called non-linear temperature compensation is used. According to
the above standard, the corresponding type of compensation can be applied in the
case of natural ground water, mountain spring water and surface water. The
conductivity of the water is compensated in the range from 13 0°C to 36°C.
- Non-linear
Here, the actual graph of the temperature coefficient during a heating-up or cooling-
down process is determined by the transmitter.
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