Yokogawa EXAxt SC450 4-Wire Analyzer for Conductivity/Resistivity - Manual (Page 44)

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IM 12D08N05-01E
6-1. General
The nominal cell constant of a conductiv-
ity 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 cleanli-
ness. 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 alter-
natives 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!
6-2. 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 solu-
tion 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 ad-
justing 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.”
6-3. Cell constant automatic
This routine is built around the test method
described in OIML (Organisation Internationale
de Metrologie Legale). International Recom-
mendation No. 56. It allows the direct use of
the solutions prescribed in the test method, au-
tomatically selecting the appropriate tempera-
ture compensation. The look up table is used
to find the appropriate conductivity reading for
the measured temperature. See appendix 2 for
OIML solutions
6-4. 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.
6-5. Sample calibration
With the sensor in situ, a sample can be taken
for laboratory analysis. Sample calibration
records the time and reading, and holds these
in memory until the analysis has been com-
pleted. The laboratory data can then be entered
regardless of the current process value, without
the need for calculations.
6-6. Temperature coefficient calibration
Simply input the solution conductivity at refer-
ence temperature (T
R
), after the sensor is
allowed to stabilize at elevated temperature.
EXAxt SC450G will calculate the temperature
coefficient for you. The ideal temperature for
this calibration, is the normal process value
(T
P
). For good calibrations, the minimum span
(T
P
T
R
) should be at least 2ºC.
Note that the Temperature Compensation
should be set to TC first.
6. CALIBRATION