H83.0.01.6C-6
Operating manual GMH 5450
page 9 of 20
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6.7 Electrodes / measuring cells
6.7.1 Assignment bayonet-connector
device pin assignment
1: electrode I+
2: electrode U+
3: electrode U-
4: electrode I-
5: temperature sensor
6: temperature sensor
7:
not connected
6.7.2 Design and selection
Basically there are two types of measuring cells: 2-pole and 4-pole cells. The operation is done similarly; the
4-pole measuring cells can compensate polarization effects and
– up to some degree – soiling due to its
complex measuring method.
2-pole measuring cell
4-pole measuring cell
The selection of a suitable electrode depends on the desired application.
The
widest range of application
is guaranteed by high-quality 4-pole graphite measuring cells
(
LF 400 or LF 425
, all the above applications and: seawaters, titration and sewage).
For
low conductivities
(<10 0µS/cm)
stainless steel measuring cells offer advantages (
LF 200 RW
,
pure and ultrapure water, boiler water, osmosis, filter technology).
2-pole platinum electrodes with glass shaft are good solution for used in
petrol, diesel, etc. with low
conductivities (< 1000 µS/cm)
(
LF 210
)
6.8 Temperature compensation
The conductivity of aqueous solutions depends on its temperature. The temperature dependency is strongly
dependent on the type of solution. The temperature compensation recalcula
tes solutions’ conductivity to a
consistent reference temperature. The most common reference temperature is 25 °C.
6.8.1
Temperature compensation “nLF” according to EN 27888
For most applications (e.g. in the area of fish farming, surface or drinking water measurements, etc.) the non-
linear temperature compensation for natural water (“nLF”, according to EN 27888) is sufficiently accurate.
The common reference temperature is 25 °C.
Recommended application range of nLF-compensation: between 60 µS/cm and 1000 µS/cm.
6.8.2 Linear temperature compensation and determination of temperature coefficient
“t.Lin“
If the actual function needed for exact
temperature compensation is not known, “linear temperature
compensation” is normally selected (Menu, t.Cor = Lin, t.Lin corresponds
), i.e. one assumes that the
actual temperature dependency at the considered concentration range is approximately equal:
Temperature coefficient of about 2.0 %/K are most common.
A temperature coefficient can be determined for example by measuring a solution with deactivated
temperature compensation at two different temperatures (T1 and T2).
