ph sensors: know whether to calibrate the sensor, clean ......±0.2 ph as your tolerances for...

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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