Yokogawa 2-Wire Dual Channel Transmitter/Analyzer FLXA21 - Manual (Page 110)

<8. CALIBRATION OF SC (Conductivity)>
8-2
IM 12A01A02-01E 5th Edition : Oct. 31, 2013-00
NOTE
When a sensor is exchanged or replaced, sensor wellness data should be reset.
When a sensor is replaced, the replacement can be recorded manually into a logbook. (Refer to
the figure 6.9.)
8.1 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 to recalibrate a sensor that has been changed (or damaged) during
operation. Choose the 1st or 2nd temperature compensation to suit the calibration solution used.
The solution with an appropriate precision should be prepared or purchased. Allow the sensor to
reach stable readings for both temperature and conductivity before adjusting to the value of the
corresponding calibration solution.
Calibration here is performed by measuring a solution with known conductivity at the set
reference temperature. The calibration solution of an accurate concentration should be prepared
by dissolving a certain amount of solute in water. Allow the conductivity to become stable at the
set reference temperature (initial setting of 25°C) of the instrument and obtain the conductivity
from literature data, etc. Appendix 2 shows the conductivity values of sodium chloride (NaCl) as a
typical example.
8.2 Cell constant (automatic)
This routine is built around the test method described in International Recommendation No.
56 of OIML (Organisation Internationale de Metrologie Legale). 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 an appropriate conductivity reading for the
measured temperature. See Appendix 2 for OIML solutions.
8.3 Air calibration
It is not necessary to conduct general air calibration unless the cable length is long and a low
conductivity needs to be measured.
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
when a sensor is installed or replaced. After some time in service, a dirty sensor may well show
a high zero offset because of fouling. Clean the sensor and try again. Moreover, air calibration
should be made in an environment free of electromagnetic interference.
NOTE
The temperature compensation is not performed around zero. In this case, a warning may be
issued. See “● For all temperature compensation” on App. 2-8.