User guide

IM 12D08N05-01E

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