IM 12D08N05-01E
6-1
<6. CALIBRATION>
6. CALIBRATION
The nominal cell constant of a conductivity sensor is determined at the construction
stage, because it is a factor set by the size of the electrodes, and their distance apart.
A conductivity sensor does not change its cell constant during operation, as long as it
remains undamaged, and clean. It is therefore vital that in any calibration check the first
step should be to clean the sensor, or at least check its cleanliness. After cleaning ensure
that the sensor is carefully rinsed in distilled water to remove all traces of the cleaning
medium.
In the commissioning menu, the original sensor configuration will include the programming
of the cell constant defined for the sensor at manufacture. Follow the routing below to the
setup screen :
Commissioning >> Measurement setup >> Configure sensor
The Calibration menu of the SC450G is provided for fine tuning the sensor setup, and
checking and verification after a time in service.
Where 1st and 2nd compensations are referred to in this part of the menu, these provide
alternatives for the “wet” calibration, designed to give the user the greatest flexibility.
This does not mean that two cell constants can or should be calibrated, they are alternative
routes to the same end!
6.1 Cell constant manual
The intention of this calibration routine is to fine tune a sensor for which only the nominal
cell constant is known, or recalibrate a sensor that has been changed (or damaged) in
the course of operation. Choose 1st or 2nd compensation to suit the calibration solution
used. The solution should be prepared or purchased, meeting the highest standards of
precision available. Allow the sensor to reach stable readings for both temperature and
conductivity before adjusting to correspond to the calibration solution value. The setting of
a cell constant for a new (replacement) sensor is also possible in this routine. This avoids
the need for entry into the commissioning mode, which may have another authorization
(password) level.
Regarding how to enter cell constant of the SC21□G, SC8SG, and SC4AJ, see “Appendix
5.”
6.2 Cell constant automatic
This routine is built around the test method described in OIML (Organisation Internationale
de Metrologie Legale). International Recommendation No. 56. It allows the direct use
of the solutions prescribed in the test method, automatically selecting the appropriate
temperature compensation. The look up table is used to find the appropriate conductivity
reading for the measured temperature. See appendix 2 for OIML solutions
6.3 Air (zero) calibration
With the clean dry cell in open air, the reading should be zero. The Air cal compensates
for excess cable capacitance, and gives a better accuracy at low readings. This should be
done for all installations during commissioning. After some time in service a dirty sensor
may well show a high zero offset because of fouling. Clean the sensor and try again.
