CMA ML57m User guide

CONDUCTIVITYSENSORML57M

USER’SGUIDE



CENTRE FOR MICROCOMPUTER APPLICATIONS

http://www.cma-science.nl

2|ML57mConductivitySensorUser’sGuide

Shortdescription

The Conductivity sensor ML57m measures conductivity of a solution in the range

between0to20,000S/cm.

The Conductivity sensor consists of a conductivity electrode and an amplifier. The

ConductivityelectrodeisasimpleABS‐bodied2‐cellgraphitetype.

The Conductivity sensor is an I2C digital sensor, which gives

calibrated values of the

measured quantity. This sensor can only be connected to special interfaces that

supportI2CdigitalsensorsliketheCMAMoLabinterface.Thesensorcableneededto

connect the sensor to MoLab is not supplied with the sensor, it is supplied with

MoLab.

Sensorspecifications

The Conductivity sensor ML57m is a digital sensor that converts the measured

conductivity value to a digital value via 16‐bit analog‐to‐digital conversion. The

maximalsamplingrateofthesensoris15Hz.

Howthesensorworks

TheConductivitysensormeasurestheabilitytoconductelectricityinwatersolutions.

When salts and other inorganic chemicals dissolve in water, they break  apart into

electrically charged ions. Ions increase the water’s ability to conduct electricity.

Common ions in water that conduct electrical current include sodium, chloride,

calcium, and magnesium.  Organic

 compounds, such as sugars, oils, and alcohols, do

notformions.

The principle by which the sensor measures conductivity is simple‐two graphite

plates(cells)areplacedinthesample,apotentialisappliedacrosstheplatesandthe

current is measured. The Conductivity probe actually measures the conductance of

the solution (the inverse of the resistivity R), which is determined from the voltage

andcurrentvaluesaccordingtoOhm'slaw(G=1/R=I/V)

The specific cell constant (K) of the conductivity electrode is used to determine the

conductivity (C). The conductivity is the cell conductance multiplied by the

 cell

constant, C=G*K.  The electrode separation distance divided by the electrode area

determinesthecellconstant.ThesuppliedelectrodehasanominalcellconstantKof

1.0cm

‐1

.

TheSIunitofconductanceisSiemens(S).SinceSisaverylargeunit,conductanceof

aqueoussamplesiscommonlymeasuredinS,andconductivityinS/cm.



In order to prevent complete ion migration to the two electrodes, the sensor uses

alternating current. With each cycle of alternative current the polarity of the

electrodes are reversed which in turn reverses the direction of ion flow. This will

preventelectrolysisandpolarization.

ML57mConductivitySensorUser’sGuide |3

Calibration

The Conductivity sensor issupplied witha factory calibration in S/cm. The Coach 6

program allows shifting the pre‐defined calibration or creating a new 3‐point

calibration if needed. The user calibration is stored in non‐volatile user sensor

memory.

For more accurate measurements sensor can be calibrated.  To

be sure that the

electrodes are clean soak the tip of the conductivity electrode in distilled water for

about 10 minutes.Ifthis isnot possible, rinse thetip thoroughlywith distilledwater

beforeuse.Wipetheouterpartof theelectrodebodywithacleanpapertowel.Shake

vigorouslyto

removeanydropletsfromthecellchamber.

 Zerocalibrationpoint

Simpleperformthisc alibrationpointwiththeelectrodeoutofanysolution(e.g.in

air).Callthisvalue0S/cm.

 Standardsolutioncalibrationpoint

Place the sensor into a standard solution (solution of kno wn concentration), for

exampleSodium

ChlorideCalibrationStandardwhichisequivalentto1000S/cm.

Be sure the entire elongated hole with electrode surfaces is submerged in the

solution.Waitforthedisplayedvoltagetostabilize.Enterthevalueofthestandard

solutione.g.1000S/cm.



For even better results, a three‐point calibration can be performed using three

standardsolutionsthatbrackettheexpectedrangeofconductivityvalues.



Having accuratestandard solutions is essentialfor performinggood calibrations. You

canprepareyourownstandardsolutionsasshowninthetablebelow.



Amount of NaCl to make 1 liter of solution Equivalent Conductivity values

0.0474 g (47.4 mg/l)

100



S/cm

0.491 g (491 mg/l)

1000



S/cm

1.005 g (1005 mg/l)

2000



S/cm

5.566 g (5566 mg/l)

10000



S/cm



Collectingdata

This Conductivity sensor works only with specific interfaces. The sensor will be

automaticallydetectedwhenconnectedtosuchaninterface.Fordetailedinformation

aboutmeasurementswithsensorsconsulttheUserManualsoftheinterfaceandthe

Coach6software.

Totakemeasurements:

 SoakthetipoftheConductivityelectrodein

distilledwaterforabout10minutes.If

4|ML57mConductivitySensorUser’sGuide

thisisno tpossible,rinsethetipthoroughlywithdistilledwater.

 Wipe the outer part of the electrode body with a clean paper towel. Shake

vigorouslytoremoveanydropletsfromthecellchamber.

 PlacetheConductivityelectrodeinthesampletobetested.Thesamplemustbeat



least3cmdeeptoensurethecellchamberisfullysubmerged.

 Stirthesolutiongentlytogetridofanyairbubblesthatcouldbetrappedinthecell

chamber.Waitfor10secondstoallowthereadingstostabilize.

 Ifyouaretakingreadingsina

solutionthathasatemperaturebelow10

Corabove

C,allowmoretimeforthereadingstostabilize.

 Cleanthoroughlywhenthemeasurementiscompleteto  avoidanycontamination

fortheelectrode’snextuse.



Warning: Do not place the electrode in viscous, organic liquids, such as heavy oils,

glycerin(glycerol) orethyleneglycol.Do notplacetheprobe  inacetoneornon‐polar

solvents,suchaspentaneorhexane.

Automatictemperaturecompensation

Temperaturehasalargeeffectonconductivity.TheConductivityelectrodehasabuilt‐

in temperaturesensor thatis usedto compensate forchangesin theconductivity of

solutions with a temperature between 5 and 35°C.Readings are automatically

referenced to a conductivity value at 25°C – therefore the sensor will give

 the same

conductivityinasolutionthatisat15°Casitwould ifthesamesolutionwerewarmed

to 25°C. This means that one calibration can be used for measurements in water

samples of different temperatures. Without temperature compensation the

conductivity readings change with temperature, even though the actual

ion

concentrationdidnot.

Practicalinformation

 TheConductivityelectrode needstobe  keptcleanandfreeof deposits andother

typesofbuild‐up.Whenfinishingusingthesensor,simplyrinseitoffwithdistilled

waterandblotitdryusingapapertowel.

 Avoid scratching the inside electrode surfaces of the probe. The probe

 can be

storeddry.

 If the probecell surface is contaminated (alsoif used in solutions witha high ion

concentration), soakthe electrodecell portion in waterwith a milddetergent for

15minutes. Thensoakitinadilutedacidsolutionsuchas0.1Mhydrochloricacid

or 0.5 M acetic acid for another 15 minutes. Then rinse thoroughly with distilled

water.

 Themostcommonreasonfor  inaccuratemeasurementsiscrosscontamination of

samples. Take care not to transfer droplets of one sample to another. Clean the

ML57mConductivitySensorUser’sGuide |5

electrode with distilled water between different  samples and shake vigorously to

removedroplets.Ideallyair‐dryandthenplacethecellinasampleofthesolution

tobemeasured.

 Be sure that samples are capped to prevent evaporation. It is best to fill sample

bottlestothebrimto

preventagassuchascarbondioxidedissolvingin thewater

sample.

 Do not use the sensor in a situation that could result in  damage to  the graphite

platesinthecellchamber.Donotattempttoblotorwipetheinsideofthecell.

 The automatic temperature

compensation for this electrode operates over the

range10

Cto35

C,butitcanbeplacedinsolutionswithinatemperaturerangeof

0to80

C.

 The conductivity electrode not only measures conductivity between the graphite

platesbutalso,toalesserextent,inafieldtothesidestheelectrode.Inanarrow

vessel,the wallsmay  interfere withthis field.If theelectrodeis held tooclose to

the top of the liquid

 level or other objects (e.g. the bottom of a  beaker) an

incorrectreadingmayresult.

 Thereisnoexactrelationshipbetween ConductivityinµS/cmand  totalsolidsTDS

inppm(partspermillion).Ithasbeendiscoveredexperimentallythatforparticular

types of water there is  an approximate relationship.

In water with a  higher 

proportion of sodium c hloride, to get to ppm  just multiply the µS/cm reading by

0.5.Formostotherwatersolutionsuseafactorof0.67instead.

UsingtheConductivitySensorwithothersensors

ItisveryimportanttoknowthattheConductivitySensor willinteractwithsomeother

sensors,if theyare placedin thesame  solutionand theyareco nnected tothe same

interface(e.g.,thesameMoLab).Thissituationarisesbecausetheconductivitysensor

outputs a signal in the solution, and this

 signal can affect the reading of another

sensor.Thefollowingsensorscannot beconnected tothesameinterfaceand placed

inthesamesolutions:

 dissolvedoxygensensor,

 pHsensor,

 salinitysensor,and

More sensors can be connectedat thesame time to the interface but only one

at a

timecanbeplacedinsidethesolutiontotakereadings.

Suggestedexperiments

Conductivity is one of the easiest environmental tests  for aquatic samples. Even

thoughitdoesnottellyouspecificionsthatarepresent,itdoesquicklydeterminethe

total concentration of ions in a  sample. It can be used to perform a wide variety of

6|ML57mConductivitySensorUser’sGuide

experimentstodeterminethechangesinorlevelsoftotaldissolvedionsorsalinity:

 Confirmationof thedirect relationbetweenconductivityandionconcentrationin

aqueoussolutions.Concentrationofunknownsamplescanbedetermined.

 Measurement of changesin conductivity resultingfrom photosynthesis inaquatic 

plants, with the resulting decrease

 in bicarbonate‐ion concentration from the

carbondioxideconsumption.

 Monitoringtherateofreaction in achemicalreactioninwhichdissolvedionsand

solution conductivity varies with time due to ionic species being consumed or

produced.

 Performingaconductivitytitration todeterminewhenstoichiometricquantitiesof 

twosubstances

havebeencombined.

 Findingtherateatwhichionicspeciesdiffusethroughamembranesuchasdialysis

tubing.

 Monitoring changes in conductivity or total dissolved solids in an aquarium

containing aquatic plants and animals. These cha nges could be due to

photosynthesisorrespiration.



ML57mConductivitySensorUser’sGuide |7



TechnicalSpecifications



Sensorkind Digital:16‐bitsresolution

(on‐sensordigitalconversion)

communicationviaI2C

Measuringrange

0..20,000S/cm

Resolution

0.3S/cm

Accuracy

±2%aftercal.at25

Responsetime

98%offull‐scalereadingin5s,100%in15s.

Temperature

‐compensation

‐range



automaticbetween10°Cand35°C.

between2°Cand80°C

Cellconstant

1.0cm‐1

Sensordimensions

Housing:60 x 18 x 16 mm

Connection

5‐pinsminijackplug



8|ML57mConductivitySensorUser’sGuide



Warranty:

TheConductivitysensorML57miswarrantedtobefreefromdefectsinmaterialsand

workmanshipfor a periodof12monthsfromthedateofpurchaseprovidedthatithas

been used under normal laboratory conditions. This warranty does not a pply if the

sensorhasbeendamagedbyaccidentormisuse.





Note:Thispro ductistobeusedforeducationalpurposesonly.Itisnotappropriatefor 

industrial,medical,research,orcommercialapplications.    

Rev. 10/06/2012

CMA ML57m User guide

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