ph sensors: know whether to calibrate the sensor, clean ......±0.2 ph as your tolerances for...
TRANSCRIPT
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 • KnowingwhentodoapHsensorcalibrationversusacalibrationcheck• HowtoproperlycleanapHsensor• HowtoperformapHsensorcalibration• Adecisiontreeforstepbystepguidance
About the AuthorFredKohlmannisaProductBusinessManagerforAnalyticalProductswithEndress+Hauser.Since1976,hehasbeeninvolvedinengineering,designservice,marketing,andsalesofonlineanalyticalwaterqualityandprocesscontrolinstrumentation.Mr.KohlmannhastaughtaccreditedcourseworkandauthorednumerousarticlesrelatingtopH,ORPandconductivitymeasurements.Pastpublicationsinclude“WhatIspHandHowIsItMeasured?”,aprimerontheuseofpHinstrumentation,and“ElectricalConductivityMeasurements”inthe“Process/IndustrialInstrumentsandControlsHandbook”,FourthEdition,byMcGrawHill,DouglasM.ConsidineEditor.
CP01202F/24/EN/13.12US/INDDCS
2
Thephraseintheabovetitleisactuallyincorrectinitssequenceofwording.AllpHreadingsaresupposedtobetakenandacceptedonlywhenthepHsensorisclean.Afterall,acontaminatedpHsensormayyieldanincorrectreading.Soonemustmakesurethesensoriscleanbeforedoingacalibration.OnceapHsensorisinstalledintheprocessandoperating,howdoyoudeterminewhenitistimetotakethesensoroutoftheprocessanddoacleaning,oracalibration?Doesoneperformbothacleaningandacalibrationorjustacleaning,orjustacalibration,ordoesonejustperformacalibrationcheckinbuffersor…?
Thisissomethingthatcanbequiteconfusing,especiallywhen the operational practices and procedures documented byyourcompany’sQualityControlorEnvironmentalPracticesdepartmentmaynotbespecificenoughwhentheydescribetheprocedureorthetimingonwhentoconductthepHcalibrationandmaintenance.Inversely,theproceduresmaybetoospecific,detailing many more procedures and operations than are actually required.
Inpracticalterms,usersmustdeveloptheirownmaintenanceandcalibrationschedule.ThisscheduleisaccomplishedbytakingthepHsensoroutoftheprocessafterasetamountoftime,perhapsafteradayortwotoperformavisualinspectionofthesensor.Ifafterinspectionyoufindnodebrisorfoulingontheelectrodeandreferencesurfaceswiththenakedeye,rinsethesensoroffindistilledwaterandperformabuffercheck.
Toperformabuffercheck,placethesensorintothecalibrationbuffersyoutypicallyuseandnotethereadings.Ifthereadingsarewithinthetolerancesdefinedbyyouroperationalprocedures,itisnotnecessarytoperformacalibration.Forexample,letususe±0.2pHasyourtolerancesforpass/failofapHsensorreadinginacalibrationbuffer.Ifthesensorreadswithinthisvalue,intheoffset(7pH)andspanbuffers,(4pH),thesensorneedsnofurtheractionandcanbereinstalledintotheprocess.Acalibrationisnotnecessary.Repeatthisexerciseeveryfewdaysuntilyouseeachangeineitherthelevelofdebris/foulantontheelectrodeandreferencesurfaces,ormorethanthe±0.2pHdeviationasshownintheexampleabove.
Toacertainextent,theaboveproceduresetsthebenchmarkfortimebetweencleaningandcalibration.Now,oneneedstodetermine whether the sensor needs just a cleaning or a cleaning andre-calibration.Thisisdoneeasilybyfirstmakingsurethesensorisclean.(RefertothesectiononCleaningpHsensors).Itmaybeaseasyasrinsingthesensorinwaterorascomplicatedasusingacidorcausticsolutionstoremovetheparticularcontaminatebuildupthathasoccurred.
pH CALIBRATION – CALIBRATION CHECK – CLEANINGShouldtheabovestepsnotyieldresultsthatarewithinyourpass/failcriteria,itistimeforasensorcleaning.
About Cleaning the pH sensors:ForthepHsensortomaintainanaccuratereadingoftheprocesspH,thepHsensormustremainclean.Specifically,theglassmeasuringelectrodecannotbecomecoatedandthereferenceelectrodeassemblymustsimilarlynotbecomecoated,pluggedorotherwisecontaminatedbytheprocesssolution.
IfthepHsensorhasaslightcoatingorscaling,thismightberemovedusingawaterjetfromafaucetorspraybottle. Moreentrenchedcoatingsmayrequiretheuseofagentleacidbrushortoothbrushtocarefullyremovethecoating.Dependingonthenatureofthescaleorcoating,youmayfinditnecessary to dip the sensor in a hot water solution containing Dawn®dishwashingdetergentandthenlightlyscrubtheelectrodeforafewsecondsorsotofacilitatecleaning.
Foramoreaggressivecoatingofthesensorandoneinwhichthedetergentcleaningdoesnotsuffice,youmayhavetodipthebrushina2%HCIacidsolutionandthenlightlyscrubtheelectrodeforafewsecondsorsotofacilitatecleaning.Alternatively,youmayhavetoallowthesensor(electrodes)tosoakinasimilarsolutionforafewminutestoreallyworkatattackingthecontaminant.Immediatelyaftercleaning,rinsethesensorinwaterandallowthepHsensortosoakintapwaterora7pHbuffersolutionforafewminutestoallowthepHsensortostabilize.
KNOWING WHEN TO DO A CALIBRATION, CALIBRATION CHECK, A CLEANING, OR ALL...
Dawn®isaregisteredtrademarkofProcter&Gamble
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Contaminant Cleaning Solution
AlkalineorScale 5%hydrochloricacidorvinegar
AcidicCoatings Weakcaustic(lessthan4%NaOH)
Oil,Grease,Organic Detergentorifcoatingistenacious,anorganicsolventcompatiblewithsensormaterial.
General Cleaning Procedure:
1. Keepsensorasreasonablycleanaspossible.
2. Removethebulkofcontaminantbycarefullyblotting/wipingawaydebris.Becarefulnottorubtoovigorouslyasthismaycausestaticcharge.
3. Rinsethesensorinwarmtapwaterordistilledwater.
4. Prepareacleaningsolutioncontainingasoapandwatermixture.Usedishwashingdetergentandwarmwater.Useonlysoapsthatdonotcontainabrasivesorlanolin.
5. Soakthesensorinthissolutionforuptofiveminutesandthengentlyorwhilesoaking,useasoftbristlebrushtogentlyscrubthebulbandreferenceareaofthesensor.
6. RinsethepHsensorinwarmtapwaterandcheck/standardizethesensorinbuffersolutions.
If the readings in buffers are still out of tolerance:
7. Soakthesensorin5%to10%HCIacidsolutionforafew–lessthanfive–minutes.
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. Rinsethesensorinwarmtapwaterandthenplacethesensorintoamildsoapsolutionforaminuteortwotoneutralizeanyremainingacidandletthesensorcometoequilibrium.
9. Rinseinwarmtapwaterandcheck/standardizethesensorinbuffersolutions.
Shouldtheaboveproceduresyieldresultsthatarewithinyouroperationaltolerances,thepHsensorisonceagainsuitableforuse.HoweveriftheaboveresultsdonotbringthereadingsofthepHsensorwithintolerance,itistimetoreplacethesensor.
Start
Determine interval at which time to pull
sensor for inspection sometime after initial
installation.
Rinse sensor in water and
visually inspect
condition of sensor?
Does the sensor
appear to be clean?
No
Clean sensor in
proper cleaning solution.
Yes
Check calibration in 7 and 4 pH buffers and record readings.
Do the readings fall within your
operational guidelines +/-
x.xxpH?
No
End: Return the sensor to the
process or store properly for next
useage cycle.
Clean sensor in
proper cleaning
solution or use
elevated level of
cleaning.
Yes
Do the readings fall within your
operational guidelines +/-
x.xxpH?
Yes No
Soak the sensor
overnight in 4 pH buffer
Perform a two point calibration in 7 and 4 pH
buffers.
Do the readings fall within your
operational guidelines +/-
x.xxpH?
Yes No
Perform a two point calibration in 7 and 4 pH
buffers.
Do the readings fall within your
operational guidelines +/-
x.xxpH?
Yes
No
Discard sensor.
CP01202C/24/EN/13.12US/INDDCS
DECISION TREEThedecisiontreeisaquickgraphicinterpretationoftheproceduresoutlinedintheprevioustext.
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