YOKOGAWA SC150, Instruction Manual

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IM 12D7B4-E-H
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7. CALIBRATION
7-1 General
The calibration (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
Commissioning >> Measurement
setup >> Configure sensor setup >> Configure sensor
The Calibration menu of the SC150 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!
7-2. 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 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.
7-3. 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
7-4. 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.
7-5. 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 completed. The
laboratory data can then be entered
regardless of the current process value,
without the need for calculations.