IM 12D08N05-01E
APP-6
<APPENDIXES>
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).
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 Recommendation No. 56 of the Organisation Internationale de Métrologie
Legale”.
Table 2
KCl values at 25 °C
mol/l
mg KCl/kg
Conductivity
0.001
74.66
0.1469 mS/cm
0.002
149.32
0.2916 mS/cm
0.005
373.29
0.7182 mS/cm
0.01
745.263
1.4083 mS/cm
0.1
7419.13
12.852 mS/cm
1.0
71135.2
111.31 mS/cm
If it is more convenient, the user may make solutions from Sodium Chloride (NaCl) with the
help of the following relationship table. This table is derived from the IEC norm 60746-3.
