ph sensors - know whether to calibrate the sensor
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pH Sensors: Know whether to calibrate the sensor,clean the sensor, perform a calibration check or ...?
Fred Kohlmann Analytical Product Business Manager
Executive SummaryThis paper will address
KnowingwhentodoapHsensorcalibrationversusacalibrat ioncheck HowtoproperlycleanapHsensor
HowtoperformapHsensorcalibration Adecisiontreeforstepbystepguidance
About the AuthorFredKohlmannisaProductBusinessManagerforAnalyt icalProductswithEndress+Hauser.Since1976,hehasbeeninvolvedinengineering,designservice,marketing,andsalesofonlineanalyticalwaterqualityandprocesscontrolinstrumentation.Mr.KohlmannhastaughtaccreditedcourseworkandauthorednumerousarticlesrelatingtopH,ORPandconductivitymeasurements.Pastpublications
includeWhatIspHandHowIsItMeasured?,aprimerontheuseofpHinstrumentation,andElectricalConductivityMeasurementsintheProcess/IndustrialInstrumentsandControlsHandbook,FourthEdition,byMcGrawHill,DouglasM.ConsidineEditor.
CP01202F/24/EN/13.12
US/INDDCS
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Thephraseintheabovetitleisactuallyincorrectinitssequenceofwording.AllpHreadingsaresupposedtobetakenandaccepted
onlywhenthepHsensorisclean.Afterall,acontaminatedpHsensormayyieldanincorrectreading.Soonemustmakesurethesensoriscleanbeforedoingacalibration.OnceapHsensoris
installedintheprocessandoperating,howdoyoudeterminewhenitistimetotakethesensoroutoftheprocessanddoacleaning,oracalibration?Doesoneperformbothacleaningandacalibration
orjustacleaning,orjustacalibration,ordoesonejustperformacalibrationcheckinbuffersor?
Thisissomethingthatcanbequiteconfusing,especiallywhen the operational practices and procedures documented
byyourcompanysQualityControlorEnvironmentalPractices
departmentmaynotbespecificenoughwhentheydescribetheprocedureorthetimingonwhentoconductthepHcalibration
andmaintenance.Inversely,theproceduresmaybetoospecific,detailing many more procedures and operations than are actuallyrequired.
Inpracticalterms,usersmustdeveloptheirownmaintenanceandcalibrationschedule.Thisscheduleisaccomplishedbytakingthe
pHsensoroutoftheprocessafterasetamountoftime,perhapsafteradayortwotoperformavisualinspectionofthesensor.Ifafterinspectionyoufindnodebrisorfoulingontheelectrode
andreferencesurfaceswiththenakedeye,rinsethesensoroffindistilledwaterandperformabuffercheck.
Toperformabuffercheck,placethesensorintothecalibration
buffersyoutypicallyuseandnotethereadings.Ifthereadingsare
with int hetolera ncesdef inedbyyourop erationalprocedures,itisnotnecessarytoperformacalibration.Forexample,letususe
0.2pHasyourtolerancesforpass/failofapHsensorreadinginacalibrationbuffer.Ifthesensorreadswithinthisvalue,intheoffset(7pH)andspanbuffers,(4pH),thesensorneedsnofurther
actionandcanbereinstalledintotheprocess.Acalibrationisnotnecessary.Repeatthisexerciseeveryfewdaysuntilyouseeachangeineitherthelevelofdebris/foulantontheelectrodeand
referencesurfaces,ormorethanthe0.2pHdeviationasshownintheexampleabove.
Toacertainextent,theaboveproceduresetsthebenchmarkfortimebetweencleaningandcalibration.Now,oneneedstodetermine whether the sensor needs just a cleaning or a cleaning
andre-calibration.Thisisdoneeasilybyfirstmakingsurethesensorisclean.(RefertothesectiononCleaningpHsensors).Itmaybeaseasyasrinsingthesensorinwaterorascomplicated
asusingacidorcausticsolutionstoremovetheparticularcontaminatebuildupthathasoccurred.
pH CALIBRATION CALIBRATION
CHECK CLEANING
Shouldtheabovestepsnotyieldresultsthatarewithinyourpass/failcriteria,itistimeforasensorcleaning.
About Cleaning the pH sensors:ForthepHsensortomaintainanaccuratereadingoftheprocesspH,thepHsensormustremainclean.Specifically,theglass
measuringelectrodecannotbecomecoatedandthereferenceelectrodeassemblymustsimilarlynotbecomecoated,pluggedorotherwisecontaminatedbytheprocesssolution.
IfthepHsensorhasaslightcoatingorscaling,thismightberemovedusingawaterjetfromafaucetorspraybottle.
Moreentrenchedcoatingsmayrequiretheuseofagentleacidbrushortoothbrushtocarefullyremovethecoating.
Dependingonthenatureofthescaleorcoating,youmayfinditnecessary to dip the sensor in a hot water solution containing
Dawndishwashingdetergentandthenlightlyscrubtheelectrode
forafewsecondsorsotofacilitatecleaning.
Foramoreaggressivecoatingofthesensorandoneinwhichthe
detergentcleaningdoesnotsuffice,youmayhavetodipthebrushina2%HCIacidsolutionandthenlightlyscrubtheelectrodeforafewsecondsorsotofacilitatecleaning.Alternatively,youmayhave
toallowthesensor(electrodes)tosoakinasimilarsolutionforafewminutestoreallyworkatattackingthecontaminant.Immediatelyaftercleaning,rinsethesensorinwaterandallowthe
pHsensortosoakintapwaterora7pHbuffersolutionforafewminutestoallowthepHsensortostabilize.
KNOWING WHEN TO DO A CALIBRATION, CALIBRATION CHECK,
A CLEANING, OR ALL...
DawnisaregisteredtrademarkofProcter&Gamble
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Contaminant Cleaning Solution
Al kal ineorScale 5%hydroch loricacidorvinegar
AcidicCoatin gs Weakcaustic(les stha n4%
NaOH)
Oil,Grease,Organic Detergentorifcoatingis
tenacious,anorganicsolventcompatiblewithsensor
material.
General Cleaning Procedure:
1. Keepsensorasreasonablycleanaspossible.
2. Removethebulkofcontami nantbycarefullyblotti ng/wiping
awaydebris.Becarefulnottorubtoovigorouslyasthismaycausestaticcharge.
3. Rinset hesensorinwarmtapwaterordistilledwater.
4. Prepareacleaningsolutioncontaini ngasoapandwatermixture.Usedishwashingdetergentandwarmwater.Useonlysoapsthatdonotcontainabrasivesorlanolin.
5. Soakthesensorinthi ssolutionforuptofiveminutesandthengentlyorwhilesoaking,useasoftbristlebrushtogentlyscrub
thebulbandreferenceareaofthesensor.
6. Rinset hepHsensorinwarmtapwaterandcheck/standardize
thesensorinbuffersolutions.
If the readings in buffers are still out of tolerance:
7. Soakthesensorin5%to10%HCIacidsolutionforafewlessthanfiveminutes.
CAUTION!
Do not use this procedure if the sensor has been
used in a solution containing cyanide as this may
produce poisonous cyanide gas.
8. Rinset hesensorinwarmtapwaterandthenplacethesensorintoamildsoapsolutionforaminuteortwotoneutralizeany
remainingacidandletthesensorcometoequilibrium.
9. Rinseinwar mtapwaterandcheck/standardizet hesensorin
buffersolutions.
Shouldtheaboveproceduresyieldresultsthatarewithinyour
operationaltolerances,thepHsensorisonceagainsuitableforuse.HoweveriftheaboveresultsdonotbringthereadingsofthepHsensorwithintolerance,itistimetoreplacethesensor.
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Start
Determine interval atwhich time to pull
sensor for inspectionsometime after initial
installation.
Rinsesensor inwater and
visuallyinspect
condition ofsensor?
Does the
sensorappear to be
clean?
No
Cleansensor in
proper
cleaningsolution.
Yes
Checkcalibrationin 7 and 4pH buffersand recordreadings.
Do the readingsfall within your
operationalguidelines +/-
x.xxpH?
No
End: Return thesensor to the
process or storeproperly for next
useage cycle.
Cleansensor in
propercleaning
solution oruse
elevatedlevel of
cleaning.
Yes
Do the readings
fall within youroperational
guidelines +/-x.xxpH?
Yes No
Soak thesensor
overnight in 4pH buffer
Perform a twopoint calibrationin 7 and 4 pH
buffers.
Do the readingsfall within your
operationalguidelines +/-
x.xxpH?
Yes No
Perform a twopoint calibrationin 7 and 4 pH
buffers.
Do the readings
fall within youroperational
guidelines +/-x.xxpH?
Yes
No
Discardsensor.
CP01202C/24/EN/13.12
US/INDDCS
DECISION TREE
Thedecisiontreeisaquickgraphicinterpretationoftheproceduresoutlined
intheprevioustext.
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