testo 252 Bedienungsanleitung Seite 123

The measuring cell sets up the correlation between
the ionic conductance or resistance and the
conductivity value:
- the measuring cell limits the conduction of
the current on account of its geometrical
form (length and cross-section). The
relation of the length to the cross-section is
described as a cell constant.
- the polarisation on the electrode surfaces
of the measuring cell can be described as
follows: the ions which separate from the
electrodes and the change in concentration
of the solution at the electrodes cause an
electromotive counterforce, which weakens
the passage of flow and lets the
conductivity of the solution seem smaller
than it actually is. Polarisation effects
reduce with an increasing measuring
frequency and an increasing electrode
surface.
The electrode material also plays and
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
electrode structure, however, requires a larger
volume of samples than the usual 2 electrode
sensors.
Non Testo products
It is also possible to connect the conductivity
measuring cells from other firms. In the
configuration menu (see page 10) you have to
differentiate between conductivity measuring cells
of type A and type B. 2 and 4 pin measuring cells
can be connected in both modes. With type B
there is an increased resolution available in the
range
< 1µS/cm. Please order additional connection
allocations as required.
Description of probes
Conductivity sensors
51