Appendix - testo 240 Bedienungsanleitung

Appendix

Principle o o f m m eas. o o f t t he c c onductivity m m easuring c c ells
It is up to the sensor to produce the connection
between the conductance or resistance and the
conductivity value. This is done by the fact that the
form of the sensor (length and cross-section) limits
the conduction of the current. The relation of the
length to the cross-section is described as a cell
constant. Alongside this geometric parameter, the
sensor is also marked by the polarisation behaviour
on its electrode surfaces (' the ions which separate
from the electrodes and the change in
concentration of the solution at the electrodes
cause an electromotive counterforce, which wea-
kens the passage of flow and lets the conductivity
of the solution seem smaller than it actually is).
Polarisation effects reduce with an increasing mea-
suring frequency and an increasing electrode surfa-
ce. The electrode material plays an additional
important role (also in relation to the response
time).
The sensors with the 4 electrode technology,
which we offer, have taken advantage of the fact
that polarisation only occurs on electrodes where
the current leaks into the electrolyte. This 4 electro-
de structure, however, requires a larger
volume of samples than the usual 2 electrode sen-
sors.
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