Yokogawa EXAxt SC450 4-Wire Analyzer for Conductivity/Resistivity - Manual (Page 59)

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APPENDICES
Appendix 3
Calibration solutions for conductivity
Note!
This section should be read in conjunction with the calibration section (Chapter 6) and the mainte-
nance section (Chapter 7). The calibration (cell constant) of a sensor does not change unless the
sensor is damaged.
It can also appear to change because of coating of the electrodes, or partial blockage. Because
these changes should be handled as described in the maintenance section, it does not make
sense to regularly recalibrate the SC450G.
A calibration check, however, is another matter. When the objective is clearly defined as a diagnos-
tic exercise a regular check can bring an extra level of security and confidence to the measure-
ment.
Sensor damage, and/or coatings can be difficult to see and the calibration check can confirm their
presence, by a deviation from the known solution conductivity. The remedial action should be to
clean the sensor, and carefully check for blockage or damage (not simply to recalibrate).
Higher conductivity solutions should be used where possible. The lower the conductivity of the test
solution, the easier it is to contaminate. Carbon dioxide from the air can be quickly absorbed to
cause an error. All containers must be suitably clean, and all materials suitably pure. Outside of a
well-equipped laboratory these conditions are hard to meet.
Also note that the check must be performed with due regard to the cell constant of the sensor,
as it limits the effective working range. The documentation provided with the sensor should be
consulted to determine its maximum working value. By using a checking solution close to the
upper range limit of the sensor, the detection of contamination is better. A slightly contaminated
sensor may read perfectly at lower conductivity, but show significant errors at higher values. This
is caused by the early onset of polarization, the factor that limits the upper range of the sensor
in any case. Polarization is seen in contaminated sensors, and it is for this reason that there is a
sophisticated polarization check built into the SC450G. Details of this diagnostic tool are found in
the troubleshooting section (Chapter 8).
EXAxt SC450G is programmed with the following table of conductivity of Potassium Chloride (KCl)
solutions at 25°C. This is used in the Automatic Cell Constant setting calibration feature. (See
chapter 6 on calibration) The table is derived from the Standards laid down in “International Rec-
ommendation No. 56 of the Organisation Internationale de Métrologie Legale”.
Table 11-2. KCl values at 25 °C
mol/l