YOKOGAWA SC450G - Page 72

IM 12D08N05-01E
APP-6
<APPENDIXES>
Sensordamage,and/orcoatingscanbedifficulttoseeandthecalibrationcheckcan
conrm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signicant
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.