2. Measuring system
2.1 Measuring principle
The test medium is a liquid conductor
coupling the magnetic field of two
magnetically isolated induction coils. The
AC voltage U1applied to coil T1is transmitted
to the second coil T2proportional to the
conductance G of the test medium. This
yields a linear relationship between the
conductance G and the output voltage U2
which is converted and rectified into a voltage
proportional to the conductance.
This measuring principle offers following
advantages:
•no electrodes, therefore no polarization
•faultless measurement in media or solutions
with a tendency to form deposits
A Pt 100 temperature sensor is fitted inside
the sensor to measure the temperature.
2.2 Sensor
The sensor is injection-moulded of chemically,
mechanically and thermally resistant PEEK
(polyether ether ketone), ensuring that it is
free of crevices and joints, and making it
biologically safe.
The sensor shaft contains the two induction
coils and a temperature sensor Pt 100 which
is in direct contact with the test medium.
The use of special elements and materials
makes the sensor suitable for continuous
operation at temperatures of up to + 100 °C,
and short-term operation at up to + 130 °C.
Therefore, Smartec can be sterilized.
The sensor is available in different mounting
variants with integrated or separate
measuring transmitter.
2.3 Measuring transmitter
The measuring transmitter
•determines the conductivity of the liquid,
and indicates the conductivity value based
on reference temperature 25 °C and
•supplies this measured value as current
(0 or 4 ... 20 mA) at the signal output.
The instrument requires power supply (see
cover label).
The actual measured value and the set
temperature coefficient can be read externally
through a viewglass in the housing cover.
Four basic measuring ranges can be freely
selected:
•0 ... 2000 µS/cm
•0 ... 20 mS/cm
•0 ... 200 mS/cm
•0 ... 1000 mS/cm
The zero point can be displaced within the
selected measuring range, increasing the
sensitivity, and thus spreading the range,
considerably. This permits precise
measurement even in media with slight
conductivity changes.
Medium
Chemical attack Resistance
Medium Concentr.
(%) Temp.
(°C) PVC-C PVDF PEEK
Nitric acid HNO35
up to 40
20
60
20
60
+
+
+
+
+
+
+
+
+
+
+
–
Phosphoric acid H3PO4up to 10 20
60 +
++
++
+
Caustic soda solution
NaOH 320
50
90
+
+
+
+
0
–
+
+
+
sodium hypochlorite
Labarraque's solution
NaOCl
diluted
aqueous 20
50
90
+
+
0
0
0
–
+
+
+
Fig. 2.2 Excerpt from resistance
tables for PVDF, PEEK
and PVC-C
Legend:
+ resistant
0 conditionally resistant
– not resistant
Fig. 2.1: The inductive conductivity
measuring principle
2. Measuring system Smartec CLD 130
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