Chemical network monitoring sensor for pH Technology Journal of Applied Researchand

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Availableonlineatwww.sciencedirect.com

Journal of Applied Research and Technology

www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology14(2016)1–8

Original

Chemical sensor network for pH monitoring

Claudia Manjarrés

^a

, David Garizado

^a

, Maria Obregon

^a

, Natalia Socarras

^a

, Maria Calle

^a^,∗

, Cecilia Jimenez-Jorquera

^b

aUniversidaddelNorte,Barranquilla,Atlántico,Colombia

bInstitutodeMicroelectrónicadeBarcelona(IMB-CNM),CSIC,CampusUAB,08193Bellaterra,Spain Received23July2015;accepted25January2016

Availableonline20February2016

Abstract

Monitoringofwatersourcesisamajorconcernworldwide.Wirelesssensornetworks(WSN)maybeusedforthismonitoring.However,current systemsemploymainlyphysicalsensorsforvariablessuchastemperature,pressure,humidityandlight.Wirelesschemicalsensorsnetworks (WCSNs)forenvironmentalmonitoringarescarceduetothelackofautonomyofconventionalsensors.ThispaperpresentsresultsofaWCSNfor monitoringpHbasedonionselectivefieldeffecttransistors(ISFETs).Sensingnodesemployahumaninterfacerequiredforinsitucalibrationof chemicalsensors.Unlikemoststudies,ourworkevaluatesthenetworkemployingchemicalmeasurementsandwirelessnetworkmetrics.Results showzeropacketlossesbyusingatimedivisionmultipleaccess(TDMA)protocol.Thenetworkallowswirelesscommunicationwithin300m includingattenuationfrombuildingsandtrees.Therefore,thesystempresentedinthispaperissuitableforlongrangeapplicationswithunobstructed lineofsight.pHmeasurementspresentastandarddeviationbelow1%,showinghighrepeatability.WhencomparedtoacommercialpHmeter, differenceinmeasurementsisbelow5%.Asaconsequence,accuracyisadequatefortheapplication.Measurementsalsopresentedhighstability during3hofcontinuousmeasurement.

Keywords:Waterresources;Wirelesssensornetwork;TDMA;pH-ISFET;Pseudo-referenceelectrode

1. Introduction

Waterqualityisamajorconcernworldwideduetoincreased demand andpollution.Regulations such as The Clean Water Act(UnitedStates Congress, 1972) inthe UnitedStates and TheWaterFrameworkDirective(EuropeanParliament,2000) inEurope,provideguidelines formonitoringaquaticenviron- ments.Therearedifferentparametersrelatedtowaterquality, suchaspH,dissolvedoxygenandelectricalconductivity(Ding, Cai,Sun,&Chen,2014).Sensortechnologyisrapidlyevolving andtheseparameterscanalreadybemeasuredinfield.However, mostwateranalysesarecarriedoutincentralizedlaboratories basedontheapplicationofconventionalmonitoringtechniques tofield-collectedsamples.

∗Correspondingauthor.

E-mailaddress:[email protected](M.Calle).

PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.

Recent advances in micro-nano-systems, microprocessors andlow-powerradiotechnologieshavecreatedlow-cost,low- power,miniaturesensordevices.Thedevicescanbeconnected toform awirelesssensor network(WSN) (Chong&Kumar, 2003),akeyenabling technologytoobservechangesinphys- icalphenomena. The technology islikely toreplace portable instrumentation and data-logging devices requiring a wired connection.Thesedevicesprovidereal-timeinteractionofmon- itoringsystems, especiallyinremote areas. WSNshave been alreadyproposedforapplicationsrelatedtowastewater,surface waterandpipelinemonitoringandrepresentaformidablecan- didateforfulfillingtherequirementsofthoseapplications(Hart

&Martinez,2006).

Currently, the practical application of WSN to chemical analysis for environmental monitoring is scarce (Bonastre, Capella,Ors,&Peris,2012).Mainexamplesemployphysical sensors,suchastemperature,pressure,humidityandlightsen- sors (Crowley,Frisby, Murphy,Roantree,& Diamond, 2005;

Martinez,Hart, &Ong,2004).However,the useof chemical

http://dx.doi.org/10.1016/j.jart.2016.01.003

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sensorsforinfieldmeasurementsischallengingduetotheinher- entdifficultiesassociatedwithmeasuringchemicalparameters inliquids for longperiods of timeandwithout sensor main- tenance.Maindifficultiesaresensorautonomy,durabilityand responsevariabilityduetotemperatureandotherexternalfac- tors.Therefore,theuseofISFETbasedsensorsispromising, since they exhibit characteristics such as robustness, minia- turization, seamless integration in microsystems, low power consumptionandrapidresponse.

Additionally,wirelesscommunicationsmaybechallenging depending on application and distance between nodes. One examplepresentsatemperaturemonitoringsystemforshellfish catches.Thesystememploys 433MHzradioswithmaximum distanceof10mbetweentransmitterandreceivernodes.How- ever, communication is interrupted when nodes do not have direct line of sight (Crowley et al., 2005). Another example presentsaWSNformeasuringtemperature,humidity,leafwet- ness,rain,solarradiation,windspeedanddirection.Thesystem uses868MHzradioswithlinklenghtsupto7.64km(Mari˜no, Fontan, Dominguez, & Otero, 2010). Both of these systems employproprietarycommunicationprotocols.

In the other hand, there are several examples of WSNs employingstandardcommunicationtechnologies.Kim,Evans, andIversen(2008)presentedasystemformeasuringsoilelec- tricalconductivity, withradio ranges upto 700m employing Bluetooth.Therefore,maximum numberof nodesin thenet- workisseven(Bluetooth-SIG,2004).Anothersystememploys ZigbeeProradiomodulestosendsoilmoistureandtemperature data(Gutierrez,Villa-Medina,Nieto-Garibay,&Porta-Gandara, 2014).However,thereisnoinformationondistancescovered withthenetwork.

All systems mentioned employ communication protocols withfewoptions(ifany)forcustomization.Additionally,allof thememployphysicalsensorsanddonotprovideinformation regarding measurement precisionor wireless network packet reception.

RegardingtheapplicationofWSNtochemicalsensing,asys- temwithopticalsensorsbasedonLEDsandpolymercoatings hasbeen describedfor chemicalplumesdetection(Shepherd, Beirne,Lau,Corcoran,&Diamond,2007).Thestudyshows5 nodeswithinachamberofmaximum2mwide.Otherexamples includesystemsbasedonvoltamperometricsiliconmicroelec- trodesfor cadmium(Yunetal.,2004)andmagnetoelasticpH sensors(Yangetal.,2002),with3mmaximumdistancebetween nodes.Anotherworkdescribestheuseofionselectiveelectrodes (ISE)fornutrientmonitoring(Capella,Bonastre,Ors,&Peris, 2010).Thissystememploys fourteennodesandresults com- pared withstandardtechniques aregood.However, thestudy does not mention the distance betwee nodes. None of these papersprovidequantitativeresultsregardingwirelessnetwork performance.

Thereareseveralexamplesintheliteratureaboutthespecific monitoringapplicationofpHandotherparameters.Thestudy presentedin(López,Go ´mez,Sabater,&Herms,2010)describes pHdetectionanddatatransmissionusingIEEE802.15.4.How- ever,the studydoes notincludeinformationregardingpacket lossoraccuracyofpHmeasurements.Anotherstudydescribes

awirelesssensorsystemformeasuringchlorine,pHandtemper- ature.Thenetworkincludesrepeaters,sensorsandmainnodes, using the 2.4GHz frequency band (Chung, Chen, & Chen, 2011).However,thepaperdoesnotreportonnetworkperfor- mance variablesandaccuracy onchemicalvariable readings.

The work presented in (Cheng, Chou, Sun, Hsiung, &Kao, 2012) uses foursensors (pH, K⁺,Na⁺ and Cl⁻) inthe same node andBluetoothcommunication.The study employsonly onesensornodewithmaximumtransmissiondistanceof10m with99%reliability.AccuracyinpHreadingswas1.3%.The workdescribedinFayetal.(2011)usesionselectiveelectrode (ISE) for pHmeasurements. Thenetworkincludesonetrans- mitterandonereceiver,using amplitudemodulation(AM)at 400MHz frequency band. However, AM has an energy effi- ciencyof33%(Haykin&Moher,2005),consideredverysmall inwirelesscommunications.pHaccuracyinthestudyisbetter than2.6%,butthepaperdoesnotreportresultsonthewireless system.Tothebestofourknowledge,theliteratureemploying Xtendmodemsisscarce.Modemshavebeenusedtotestwireless networkswithnolineofsightlinks(Berdugo,Buchelly,Calle,

&Velez,2012).Thetestincludedamulti-hopnetworkwith86m maximumdistancebetweennodesand1mWtransmitpower.

The purpose of this work is to present a wireless sensor networkdesignedfor afuturepHmonitoringsysteminwater environments.Therefore,thesystemneedstotransmitinforma- tionthroughlongdistances,anditmustprovideawayforinsitu sensorcalibration.TheWCSNincludesthreeIonSensitiveField Effect Transistors (ISFETs) (Hayes, Beirne, King-Tong Lau,

&Diamond,2008;Jimenez-Jorquera,Orozco,&Baldi,2009) devicesandemploysXtendradiomodems(MaxStream,2007) totransmitinformationthroughlongdistances.Communication usesFrequencyShiftKeying(FSK)in900MHzfrequencyband, 1Wmaximumtransmissionpower,andtimedivisionmultiple access (TDMA)protocol.Becauseof thelongrangecommu- nication, allsensingnodes arewithin range of thesink.This paperpresentstheresultsofpHmeasurementandwirelessnet- workperformanceregardingpacketlossesandmaximumradio range,thusprovidinginformationontheactualusabilityofthe application.

2. Systemdescription 2.1. Systemoverview

Thesystemincludesthreesensingnodesdirectlytransmitting toonesink,formingastartopology.Thesystemalsoincludes asoftware interfacefordatavisualization. Fig.1 showsmain systemcomponents.

2.2. Transducers

ISFET sensors were built using conventional microelec- tronic Aluminum gate NMOS technology (Jimenez-Jorquera etal.,2009).Thedevicesareproducedonp-typesiliconwafers withborondopingof1.5×10¹⁶cm⁻³.Thechiphasasizeof 3×3mm².Thedrainandsourceareasareoftendesignedlong enoughtoavoidmetalcontactsclosetothegatearea,whichis

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Sensor

node Sink

Transducer

Sensor node

Transducer

Sensor node

Transducer Software interface

Fig.1.Wirelesschemicalsensornetwork.Startopology.

Fig.2.EncapsulatedISFETinaPCBanddetailofthechip.

exposedtothesolution(Jimenez,Bratov,Abramova,&Baldi, 2006).Fig.2showsthefinalencapsulatedchip.

ISFETsrequireareferenceelectrodetoclosethecircuit.Ini- tialISFETtestsemployedoneAg/AgCldoublejunctionOrion electrode.However, longdistance measurements employed a pseudo-reference electrode based on a gold microelectrode with an electrodeposited film of Ag and a film of AgCl formedbychlorinization(Almeida,AndradesFontes,Jimenez,

&Burdallo,2008).

ISFETresponseisbasedontwocharacteristics:itsbehavior asanelectronicfieldeffectdeviceandthephenomenaoccur- ringatlimitbetweentheelectrolyteandthe insulator.Ionsin thesolutioncreateagatevoltage(VGS)shiftandadraincurrent shift(ID).Thecurrentflowsbetweenthedrainandthesource ofthetransistor.GatevoltagecanmodulateIDtoamplifyvolt- agebetweendrainandsource(VDS)(Artigasetal.,2001).This situationismodeledbyEq.(1)(Bergveld,1986):

ID=μ¯nCoxW L

(VGS−Vth)VDS−VDS²

2

(1)

whereμ¯nisthemobilityofelectronsattheinversionlayer,Cox

isthegateinsulatorcapacitanceperunitarea,Wisthewidthof thechannelandListhelengthofthesamechannel.

ISFETs employed in this work have a linear response of 45–54mV/pHinarangeof 2–10pHvalues.Calibrationpro- cedureemploystwostandardsolutions,selectingfrom4,7and 9pHvalues.TheprocessplacesoneISFETinthefirstselected

Signals from transducers

Conditioning Control

Radio Human interface Power

supply

Fig.3.Generalsensornodearchitecture.Whitearrowsshowinformationflow.

pHsolution,thenwashesitwithdistillatedwater.Finally, the processinsertstheISFETinadifferentpHsolution.Eachnode collectsvoltagesignalsforeachofthetwopHvaluesandcal- culates the slope for a linear fit. After calibration, the node computesanyotherpHvaluebymeasuringthecorresponding voltagesignalandinterpolatingwiththelinearfitparameters.

2.3. Sensornodes

WCSNhaveimportantdifferencescomparedtoWSN.One ofthemiscalibrationrequiredforchemicalsensors.Theprocess isexecutedwithaparticularfrequencytoassuredataprecision forlongtermperiods.Thisworkestablishedamanualcalibra- tion protocol,as afirst approachfor WCSN implementation.

Therefore,eachnoderequiresahumaninterfaceforanoperator toperformcalibrationproceduresandverificationsinsitu.Each nodeisimplementedaccordingtothesensornodearchitecture presentedinFig.3.

Powersupplyemploysa12VDC,4A-hbatteryforthewhole nodeincludingtransducersandtheradio.Voltagelevelforthe nodeisveryimportantsincesignalsgeneratedbyISFETsvary accordingto50mVforpHunit.Therefore,voltagelevelsshould beaspreciseaspossible.Thenodeusesa±5VDCpowersupply.

ISFETsalsorequire±0.5VDC.Thesevalueswereimplemented usingaWilsoncurrentmirrorandadiode.Additionally,ISFETs aresensitivetoelectrostaticdischarges.Inordertoavoiddam- ages,thenodedisconnectsallpowersourceswhentheISFET movesfromonesolutiontoanotherduringcalibrationorsample measurements.

TheconditioningsectioninFig.3includescircuitsforISFET biasingwithconstantcurrentof100␮AasinValdes-Perezgasga (1990),andforcreatingaDC-leveloffsetof+1.0VtoISFET outputvoltage.Fig.4showsallconditioningelementsrequired forvoltagelevelsbeforeconnectiontotheControlsection.

Control section in Fig. 3 uses aMicrochip PIC 18F4553 microcontroller, with an embedded 12-bit Analog to Digital Converter(ADC).Themicrocontrollerreceivesvoltagesignals accordingtopHvalues.ThesevaluesaretransformedtoASCII charactersrepresentingpHvaluesthatwillbetransmittedevery 10sfromeachnode.Themicrocontrolleralsoimplementsthe communication protocol,by sending informationtothe radio throughRS232interface.Thecommunicationprotocolcanbe modifiedbychangingmicrocontrollerprogramming.Therefore, nodesintheWCSNallowfortestingdifferentprotocolswithout hardwaremodification.

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ISFET

R35

3 2

4 5

6 2

Out 5 6

1 1K

1K

+ V19 5 VDC

b a

R34

1K 3

U13 7

V+

V- 1K R32 V18 5 VDC

+

R33

1K Pseudoref.

Electrode

4 – +

2 Out

5 6

1 3

U14

7 TL071/301/Ti V+

V- 4 – + Out

1 V+

7 R31

U15 +0.5 VDC

–0.5 VDC +

+ V20

V21

1K 10K

V- – +

Fig.4.Conditioningsection.Part(a)showsconstantcurrentbiasingcircuit(Valdes-Perezgasga,1990).Part(b)provides+1.0Voffsettothesignal.

Fig.5.OnenodemeasuringpH7.Buttonsallowto:entercalibrationroutine (CAL),selectpHvalues(UP,DOWN)andenterthedesiredvalue(SEL).

Fig.5showshumaninterfacesection,withoneliquidcrystal display(LCD)andfourbuttons. Themicrocontrollerpresents pH values using the LCD, so a user can verify pH mea- surementsinsitu.Additionally,themicrocontrollerguidesthe user while executing calibration procedures by pressing the buttons.

Eachnode has oneswitchfor manually disconnectingthe LCD.Thus,aftercalibration,thenodesavesenergybynotpre- sentinginformationlocally.Nonetheless,allmeasurementsare sentusingtheradio.

Radiosection inFig.3 employs oneXtendradiomodem, bymanufacturerDigi.Theradioworksin900MHzband,FSK modulation,115,200bitpersecond (bps)datarateandmaxi- mum1Wtransmitpower(MaxStream,2007).Therefore,Xtend modemsallow for longrange transmission requiredin water sourcemonitoring.Themodemwasconfiguredwiththe most basicsetup.Hence,therewerenoacknowledgementsorretrans- missions.Themodemdoes,however,detecterrorsviaa16-bit

cyclic redundancycode(CRC).Asaconsequence,Xtenddis- cardsallframesarrivingwitherrors.

Fig.6showsthefinalhardwareimplementation.

2.4. Networkconﬁguration

Multi-hop networks require routing algorithms such as Ghaffari(2014).However,theWCSNimplementsastartopol- ogy whereallsensornodesdirectlytransmittothemainnode (sink). Therefore,all nodes are onehop away fromthe final destination, andthe networkdoes not require aroutingalgo- rithm. EventhoughallXtendmodemsallowfortransmission andreception,sensor nodesare programmedfortransmission andthesinkisprogrammedonlyforreception.

The networkimplementsaprotocolbasedintimedivision multipleaccess(TDMA)(Haykin&Moher,2005).Theprotocol assigns eachnodeaparticulartimetosend information(time slot). Every node can onlytransmit during its own timeslot andmustremainsilentinothertimeslots.Thisbehavioravoids collisionscreatedbyseveralnodescontendingduringonetime slot. Alltimeslots formaFrame,whichrepeats periodically.

Fig.7showsatimediagramfortheparticularversionofTDMA implemented.Eachframeis10slong.

2.5. Softwareinterface

The team developed a software interface inJava for data visualization.TheinterfaceshowspHmeasurements,generates alarmsif valuesareoutside rangesandshows historicaldata.

Fig.8presentsthemainscreenandevolutionofpHvaluesin time.

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ISFET power supply cutoff system

Processing unit

RS232 conversion

Signal conditioning +/– 0.5V power

supply +/– 5V power

supply

Fig.6.HardwareimplementationofeachnodeintheWCSN.

10

0 20 30

1

Node ID

2 3

40 t (seg)

Fig.7.Blackrectanglesshowtransmissionsfromeachnode,repeatingevery 10s.Emptyspacesmeaneachnodeisidleandcannottransmit.

3. Resultsanddiscussion

The WCSN was evaluatedconsidering threeaspects: sen- sorprecisionandaccuracyandwirelessnetworkperformance.

ResultsinthissectionusestandardpH4and7solutionsasan example.Calibrationresultsemployingothercombinationsof pHvaluesweresimilar.Table1showsresponsecharacteristics forthethreenodes.Therepeatabilityofcalibrationdata(slope andintercept)washighasindicatedbythevaluesofthestandard deviation.Table1summarizesresultsfor10repetitions.

Thesevalueswereprogrammedbydefaultineachnode.Dur- ingcalibration,the slopemustbebetween43and55mV/pH.

Ifthecomputedvaluefallsoutsidethisrange,theLCDshows anerrormessageandthenodeusesdefaultcalibrationvalues.

Otherwise,thenodestoresthemostrecentslopevalue.

3.1. RepeatabilityandaccuracyofpHmeasurements

Tests measured three pH solutions (4, 7 and 9) for each node,inordertoestablishtheprecisionandaccuracyofISFET sensorsandthemeasurementsystem.Resultswerecompared withthosefromapHcommercialmeterWTWMulti3420.Test

Table1

LineparametersofeachpHsensingnode.Standarddeviationinparenthesis.

Parameter Node1(S.D.) Node2(S.D.) Node3(S.D.) Slope(mV/pH) 42.6(1.2) 48.5(1.7) 46.0(1.8) Intercept(mV) 1490.4(7.8) 1145.1(9.6) 660.4(12.1)

employedtenrepetitionsforeachpHvalue.Measurementswere doneatroomtemperatureusingthesamesolutionsandcondi- tionsforboththepHmeterandthenodes.Table2summarizes resultsofthistest.Asshown,therepeatabilityofpHdatarep- resentedbythestandarddeviationagreeswiththecommercial pHmeter.

NoteallISFETvaluesinTable2wereextrapolatedemploy- inglinearfitparametersobtainedduringthecalibrationprocess.

Additionally,theaccuracywascalculatedastherelativediffer- encebetweenthepHvalueobtainedwiththeISFETnodeand withthepHmeter.Therefore,accuracyisbelow5%,whichis acceptablefordifferentinstrumentsofmeasurement.

3.2. Wirelessnetworktests

Thesecondtestmeasuredthenumberofpacketslostwhen thenumberoftransmittingnodesincreased from1 to3.Each nodetransmitted100packetsandexperimentswererepeatedfive times.WhenusingtheprotocolexplainedinFig.7,whereeach nodesendsanewpacketevery10s,therewerenopacketlosses.

ThemeasurementofpHinsurfacewatersmaytoleratehigher samplingperiods.Therefore,ifincreasingthesamplingperiod, newnodescanjointhenetworkwithoutcreatingcollisions.The threenodesweretestedusing1mWtransmissionpower,asin study(Berdugoetal.,2012).

Oneadditionaltestwasperformedinordertoverifythemax- imumradiorange.OnenodewasmeasuringpHvaluesinsidea laboratory.Transmissionpoweremployedwas1W.Thereceiver node wasplaced indifferentpositionsoutside the laboratory.

Fig. 9 shows different locations employed during tests. The square isthe transmitter. Circles show the smallestreception levels.Thetriangleshowslocationwiththebestreceptionlevel andthediamondshowsmediumlevel.

Fig.9 showsmaximumdistancewas474mbetweentrans- mitterandreceiver,butthesignallevelwassmall.Bestreception occurred inside the laboratory building, at 30m through metal doors and concrete walls. Every Xtend modem in the WCSNemploysoneomnidirectionalantenna;hence,theyare

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Fig.8.(a)Mainscreenofsoftwareinterface.ISFETinnode3wasdisconnectedtogenerateanalarmforout-ofrangevalue.(b)EvolutionofpHvalueswithtime.

Table2

pHvalueandstandarddeviationforeachnodeforthreedifferentbuffersolutions.

pH 4 7 9

Node pHmeter ISFET pHmeter ISFET pHmeter ISFET

1 4.15 0.02 4.13 0.04 7.02 0.00 7.06 0.06 8.84 0.01 8.82 0.06

2 4.13 0.02 4.15 0.04 7.00 0.03 7.02 0.02 8.85 0.01 9.12 0.06

3 4.12 0.01 4.01 0.04 7.02 0.01 7.30 0.08 8.86 0.00 8.83 0.05

transmittinginformationinalldirections.Note receptionwas possible when testing in parking lots and different roads.

Nonetheless,becauseoftheobstacles,thesystemwasworking inthegrayareaofcommunicationsystems,wherepacketlosses arebetween10%and90%(Aguirreetal.,2014).Additionally,

tests verifiedlocations insidedifferent buildings,going south fromthetransmitter.However,therewasnopacketreceptionin theselocations.Theresultsareexpectedsincesignalhadtoprop- agatethroughconcrete,glass,metalandtrees,thussignificantly reducingradiorange.

Fig.9.Longrangetestwithobstaclesusing1Wtransmissionpower.Transmitteristhesquare,circlesshowthesmallestreceptionlevels,trianglethebestand diamondshowsmediumlevel.

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8.80 8.85 8.90 8.95 9.00 9.05 9.10

14:38:24 13:26:24

12:14:24 11:02:24

9:50:24

pH values

Time

Fig.10.pHvaluesmeasuredwhiledoinglongrangetests.Gapcorrespondsto thetimewhenreceivercouldnotobtaininformationfromthetransmitter.

Additionally,the testprovidedinsightson stabilityperfor- manceofthenode.Fig.10showsmeasurementsforonenode takenfrom10AMto14:10PM.FigureshowstypicalISFET behavior,withadriftduringthefirsttwohoursandastabilized driftafterthattime.Notethereisagapindatafrom10:50AM to12:54PM.Duringthistime,receiverpositiondidnotallow datareception.

4. Conclusions

This paper demonstrates the performance of a chemical sensor network for pH monitoring. The pH values and the repeatabilityare comparable witha commercial sensor. This resultcorroboratesthefeasibilityof ISFETtechnologyas pH sensorsandtheviabilityofthemeasurementcircuit.Themajor partofsystems employedintheliteraturereporttransmission distances smaller than 30m. The Xtend radio modems used inthisworkallowtransmissionupto474mthroughobstacles suchasbuildingsandtrees.Therefore,theapplicationcouldbe implementedinlongerdistances,withunobstructedlineofsight.

Usinga TDMA algorithm andsending information every 10s,the networkpresented nopacket losses.Since pH mon- itoringinwatercanbe performedthrough largerperiods,the systemcan increase the numberof nodeswithout increasing packetlosses.

Humaninterfacewasdesignedandimplementedinorderto fulfillcalibrationrequiredbychemicalsensors.Thisfunction- alityalso allowseachnodetoworkas stand-alonepHmeter, independentofthewirelessnetwork.

Conﬂictofinterest

Theauthorshavenoconflictsofinteresttodeclare.

Acknowledgements

ThisresearchwasfundedinpartbyDirectionofResearch, DevelopmentandInnovation(DIDI)atUniversidaddelNorte through project 2011-1154, by CYTED Project number P509AC0376andbyColciencias-CSICExchangeGrant2011.

WewouldliketothankAntonioRamosandÁngelaSierrafor

theirhelpinusingChemistryandWaterLabsatUniversidaddel Norte.

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