CMA ML75m User guide

Brand: CMA

Model: ML75m

Type: User guide

CMA ML75m is a digital Dissolved Oxygen sensor that accurately measures the concentration of dissolved oxygen in water samples, making it ideal for various applications such as monitoring water quality, conducting scientific experiments, and educational purposes. With its automatic temperature compensation, the sensor delivers precise readings even in fluctuating temperature conditions. The user-friendly interface allows for easy calibration and data collection, while the durable design ensures long-lasting performance.

DISSOLVEDOXYGENSENSORML75M

USER’SGUIDE



CENTRE FOR MICROCOMPUTER APPLICATIONS

http://www.cma-science.nl

2|ML75mDissolvedOxygenSensorUser’sGuide

Shortdescription

TheDissolvedOxygensensorML75mmeasurestheconcentrationofdissolvedoxygen

in water samples. The sensor consists of an oxygen electrode, which is a Clark‐type

polarographic electrode that senses the oxygen concentration in waterand aqueous

solutions. A platinum cathode and a silver/silver chloride reference anode in KCl

electrolyteare

separatedfromthesamplebyagas‐permeableplasticmembrane.



A fixed voltage is applied to  the platinum electrode. As oxygen diffuses through the

membranetothecathode,itisreduced,½O

+H

O+2e‐>2OH

‐

.

Theoxidationtakingplaceatthereferenceelectrode(anode)isAg+Cl

‐

‐>AgCl+e

‐



Accordingly,acurrentwillflowthatisproportionaltotherateofdiffusionofoxygen,

andinturntotheconcentrationofdissolved oxygeninthesample.



TheDissolvedOxygensensorisdeliveredwith:

 replacementmembranecap,

 SodiumSulfiteCalibrationStandard(2MNa

)

 oxygenprobeelectrodefilingsolution,

 calibrationbottle(empty,lidwithhole),and

 fillingpipette.



The Dissolved Oxygen sensor is automatically  temperature compensated. The

temperatureoutputofthissensorisusedto automaticallycompensateforchangesin

permeabilityofthemembranewithchangingtemperature.

TheDissolvedOxygen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.

Sensorspecifications

The Dissolved Oxygen sensor ML75m is a digital sensor that converts the measured

oxygen level to a digital value via 16‐bit analog‐to‐digital conversion. The maximal

samplingrateofthesensoris1Hz.

ML75mDissolvedOxygenSensorUser’sGuide |3

Collectingdata

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

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

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

Coach6software.

PreparingtheDissolvedOxygensensorforuse

1. Unscrewthemembranecapfromthetipoftheelectrode.

2. Using a pipette, fill the membrane cap

with 1 mL of oxygen probe electrolyte.

Mindthat therearenoairbubbles inthe

electrolyte.

3. Carefully thread the membrane cap back onto the electrode being careful not to

over

‐tightenthecap.Rinsetheelectrodewithdistilledwater.

4. Placethesensorintoabeakerfilledwithabout100‐mlofdistilledwater.

RemovemembranecapAddelectrodefillingsolution Placethemembraneback.

5. Connectthesensortotheinterface.

6. Itisnecessarytowarmupthesensorfor10minutes.Towarmupthesensor,leave

itinthewaterandcollectthedatafor10minutes.Thesensormuststayconnected

at all times to keep it warmed up. If

disconnected for a few minutes, it will be

necessarytowarmupthesensoragain.

7. Thesensorisreadyformeasurement.



4|ML75mDissolvedOxygenSensorUser’sGuide

Calibration

TheDissolvedOxygensensorissuppliedwithafactorycalibrationfordissolvedoxygen

level in mg/L. The Coach 6 program  allows shifting the pre‐defined calibration if

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



The user calibration can be done for two calibration points (prior  to calibration the

sensormustbewarmedupabout10minutes).

 Zero‐OxygenCalibrationPoint

Use the provided Sodium Sulfite Calibration  Solution. Sodium sulfite is a good 

reducingagentthatreadilyremovesdissolvedoxygenfromasolution.Ifthe

sulfite

solution has been stored brim fully in its storage bottle, you can assume it to be

oxygenfree.

Removethesensorfromthe waterandplacethetipofthesensorintotheSodium

SulfiteCalibrationSolution.

Important:Bewarethatnoairbubbleswillbetrappedbelowthetip

ofthesensor

or the sensor will sense an inaccurate dissolved oxygen level. If the voltage does

not rapidly decrease, tap the side of the bottle with the sensor to dislodge the

bubble.

Whenthevoltagestabilizes(~1minute)enter“0”astheknownvalueinmg/l.

 SaturatedDOCalibration

Point

- Rinsethesensorwithdistilledwaterandgentlyblotdry.

- Unscrewthelidofthecalibrationbottleprovidedwiththesensor.

- Slidethelidandthegrommetabout1cmontothesensorbody.

- Add water to the bottle to a depth of about 1/2‐cm and

screw the bottle into

thecap.Important:Donottouchthemembraneorgetitwetduringthisstep.

- Keepthesensorinthispositionforaboutaminute.Whenthereadingstabilizes

enter the correct saturate d dissolved‐oxygen value (in mg/l) from Table 1 (for

example, “8.66”) using the

current barometric pressure and air temperature

values.Ifyoudonothavethecurrentairpressure,useTable2toestimatethe

airpressureatyouraltitude.

Collectingdata

Youarenowreadytocollectdissolvedoxygenconcentrationdata.Placethetipofthe

sensorintothewaterbeingtested(s ubmerge4‐6cm). Donotcompletelysubmerge.

Thehandleisnotwaterproof.Gentlystirtheprobeinthewatersample.Monitorthe

dissolvedoxygenconcentration.



Note: It is important to keep stirring the probe in the water sample. There must

alwaysbewaterflowingpasttheprobetipwhenyouaretakingmeasurements.Asthe

probe measures the concentration of dissolved oxygen, it removes oxygen from the

ML75mDissolvedOxygenSensorUser’sGuide |5

wateratthejunction of the probemembrane.Iftheprobe is left stillincalm water,

reporteddissolvedoxygenreadingswillappeartobedropping.



Table1.Dissolvedoxygenlevels(inmg/l)inair‐saturateddistilledwater

(atvarioustemperatureandpressurevalues).

6|ML75mDissolvedOxygenSensorUser’sGuide



Table1.(continuation)

ML75mDissolvedOxygenSensorUser’sGuide |7

ElevationBarometricPressureTable

If you do not have a barometer available to read barometric pressure, you can

estimatethebarometricpressurereadingatyourelevationinfeet(1feet=30.48cm)

fromTable2.



Table2.Thevaluesarecalculatedbasedon

abarometricairpressurereadingof760mmHgatsealevel.

Suggestedexperiments

Since dissolved oxygen is one of the primary indicators of the quality of an aquatic 

environment, you  can use this sensor to perform a wide variety of tests or planned

experimentstodeterminethechangesindissolvedoxygenlevels:

 Monitor dissolved oxygen in an aquarium containing different  combinations of

plant

andanimalspecies.

 Measurechangesindissolvedoxygenconcentrationresultingfromphotosynthesis

andrespirationinaquaticplants.

 Usethissensorforanaccurateon‐sitetestofdissolvedoxygenconcentrationina

stream or lake survey,in order to evaluatethe capability of the water to support

different

typesofplantandanimallife.

 Measure Biological Oxygen Demand (B.O.D.) in water samples containing  organic

matterthatconsumesoxygenasitdecays.

 Determine the relationship between the dissolved oxygen concentration and the

temperatureofawatersample.



8|ML75mDissolvedOxygenSensorUser’sGuide

StorageandMaintenanceoftheDissolvedOxygenSensor

Followingtheprocedureoutlinedbelowwillenhancethelifetimeofyoursensor.

- Long‐term storage (more than 24 hours): Remov e  the membrane cap and rinse

the inside and outside of the cap with distilled water. Shake the membrane cap

dry. Also rinse a tip of the sensor. Reinstall the membrane  cap loosely  onto the

sensorbodyforstorage.Donotscrewit

ontightly.

- Short‐termstorage(lessthan24hours):Storethesensorwiththemembraneend

submergedinabout2cmofdistilledwater.

Important:Storingthesensorinthismannerforlongerthan24oursmayresultin

damagetothemembrane.

- Polishing the metal electrodes: If the cathode

 (the small metal contact in the

centeroftheelectrode)andanode(thesilver,metalsurro undingthelowerportion

of the inner body) become discolored or appear corroded, polish them with the

polishing strip that is provided with the sensor. Perform this operation only as

needed to restore electrode performance

(necessary circa once every year).

Remove the membrane cap. Thoroughly rinse the inner elements of the sensor

with distilled water to remove all filling solution. Cut a 2‐cm piece from the

polishing strip provided. Wet  the dull  (abrasive) side of the polishing strip with

distilled water. Using a circular motion,

 gently polish the center glass element of

the cathode (on the very end of the electrode). Polish only enough to restore a

bright, clean surface to the center element. Next, polish the silver anode located

around the base of the electrode inner element. Polish only enough to restore a

silver

appearance.



NOTE:Aggressivepolishingwilldamage thesensorinnerelements.Besure touse

only gentle pressure when performing the polishing of the anode and cathode.

When you have completed the polishing, rinse the cathode and anode elements

thoroughlyanddrywithalabwipe.



Withnormaluse,theDissolvedOxygen sensorwilllastforyears.Themembranecap

will, however, require replacement after about 6 months of co ntinuous use.

ReplacementofthemembraneisrecommendedwhenyourDissolvedOxygensensor

willnolongerrespondrapidlyduringcalibrationorwhentakingD.O.readings.Useof

your

 Dissolved Oxygen Sensor in samples that are non‐aqueous or in those that

contain oil, grease, or other coating agents will result in shortened membrane life.

ReplacementmembranescanbeobtainedfromCMA.



ML75mDissolvedOxygenSensorUser’sGuide |9

MaintainingandreplenishingtheSodiumSulfiteCalibrationSolution

Havinganoxygen‐freesolutiontoperformazero‐oxygencalibrationpointisessential

for accurate readings with your Dissolved Oxygen sensor. The Sodium Sulfite

Calibration Solution that was included with your sensorwill lasta long time, but not

indefinitely.

Herearesomesuggestionsformaintainingandreplacingthissolution:



 After your first use of the solution for calibration, the solution will no longer be

brim full (some overflow results when the sensor is inserted into the solution). If

you cap thesolutionwith an airspace abovethe sensor, oxygen gas in thespace

will dissolve in the

sodium sulfite solution—as a result, the solution may not be

oxygen free. To prevent this from occurring, before putting on the lid, gently

squeezethebottlesothelevelofthesolutionisattheverytopofthebottleneck;

with the solution at this level, screw on the lid. The

 bottle will remain in this

“collapsed”position.Usingthisprocedure,the2.0MNa

shouldremainoxygen

free for a long period of time. If the calibration voltage reading displayed during

thefirstcalibrationpointishigherthanin previouscalibrations,itmaybetime to

replacethesolution,asdescribedbelow.

 The 2.0 M sodium sulfite  (Na

) solution can be prepared from solid sodium

sulfite crystals: add 25.0 g of solid anhydrous sodium sulfite crystals (Na

) to

enough distilled water to yield a final volume of 100 ml of solution. The sodium

sulfitecrystalsdonotneedtobereagentgrade;laboratorygradewillworkfine.

Preparethesolution24hoursinadvanceofdoingthecalibrationtoensurethatall

oxygen has been depleted

. If solid sodium sulfite is not available, you may

substitute either 2.0 M sodium hydrogen sulfite solution, (20.8 g of NaHSO

 per

100 ml of solution) or 2.0 M potassium nitrite (17.0 g of KNO

 per 100 ml of

solution).

UsingtheDissolvedOxygensensorwithothersensors

TheDissolvedOxygenSensorinteractswithsomeothersensors,iftheyareplacedin

the same solution (in the same aquarium or beaker, for example), and they are

connected to the same interface. This situation arises because the Dissolved Oxygen

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

 affect the reading of

anothersensor.

TheConductivity,SalinityandpHsensorscannotbeconnectedtothesameinterface

asaDissolvedOxygensensorandplacedinthesamesolution.





Addingoneortwocrystalsofsolidcobaltchloridesolid(CoCl

)to100mlofthesodiumsulfite

solutionwillcatalyzetheinitialremovalofoxygen.

10|ML75mDissolvedOxygenSensorUser’sGuide

Effectofsaltconcentrationondissolvedoxygen(atsalinitylevels>1000mg/l)

TheDissolvedOxygenconcentrationforair‐saturatedwateratvarioussalinityvalues,

DO(s),canbecalculatedusingtheform ula:



DO(s)=DO–(k*s)



where

DO(s)istheconcentrationofdissolvedoxygen(inmg/l)insalt‐watersolutions.

DO is the dissolved oxygen concentration for air‐saturated distilled water as

determinedfromTable1.

s isthesalinityvalueinppt(partperthousand).Salinityvaluescanbedetermined

usingtheConductivityorSalinitysensor.



k isaconstant.Thevalueofkvariesaccordingtothesampletemperature,andcan

bedeterminedfromTable3.



Temp

(°C)

Constant,

k

Temp

(°C)

Constant,

k

Temp

(°C)

Constant,

k

Temp

(°C)

Constant,

k

1 0.08796 8 0.06916 15 0.05602 22 0.04754

2 0.08485 9 0.06697 16 0.05456 23 0.04662

3 0.08184 10 0.06478 17 0.05328 24 0.04580

4 0.07911 11 0.06286 18 0.05201 25 0.04498

5 0.07646 12 0.06104 19 0.05073 26 0.04425

6 0.07391 13 0.05931 20 0.04964 27 0.04361

7 0.07135 14 0.05757 21 0.04854 28 0.04296



Table3.Salinitycorrectionconstantvalues



Example:

DeterminethesaturatedDOcalibrationvalueatatemperatureof23°Candapressure

of750mmHg,whentheDissolvedOxygenSensorisusedinseawaterwithasalinity

valueof2.0ppt

(2000mg/lNaCl).

First,findthedissolvedoxygenvalueinTable1(DO=8.55mg/l).ThenfindkinTable3

at 23°C (k =  0.04662). Substitute these values, as well as the salinity value, into the

previousequation:



DO(for2ppt)=DO‐(k* s)=8.55‐(0.04662*2)=8.55‐0.09324=8.46mg/l



Usethe value8.46 mg/lwhenperformingthesaturatedDOcalibration point (water‐

saturatedair)as describedintheCalibrationsection.TheDissolvedOxygenSensorwill

bethencalibratedtogivecorrectDOreadingsinsalt‐watersampleswithasalinityof

2.0ppt(2000mg/lNaCl).







partsperthousand

ML75mDissolvedOxygenSensorUser’sGuide |11

Important: For most dissolved oxygen testing, it is not necessary to compensate for

salinity;forexample,ifthesalinityvalueis0.5ppt(500mg/lNaCl),using25°Cand760

mmHg,thecalculationforDO(s)wouldbe:



DO(for0.5ppt)=DO–(k*s)=8.36‐(0.04498*0.5)=8.36‐0.023=8.34 mg/l



Atsalinitylevelslessthan1.0ppt(1000mg/l),neglectingthiscorrectionresultsinan

erroroflessthan0.2%.



TechnicalSpecifications



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

(on‐sensordigitalconversion)

communicationviaI2C

Measuringrange 0..15mg/L

Resolution 0.0002mg/L

Accuracy

Typical2%aftercalibration at25

C

Maximalsamplingrate

1Hz

Respondtime

95%offinalreadingsin30s,98%in45s

Temperaturecompensation Automatic5‐35°C

Pressurecompensation Manual

Salinitycompensation

Manual

Storagetemperature

0~50

Minimumsampleflow 20cm/s



Note: Automatic temperature compensa tion does not mean that the reading for a

saturatedsolutionwillbethesameattwodifferenttemperatures‐thetwo solutions

have different concentrations of dissolved oxygen, and the sensor reading should

reflectthisdifference.

12|ML75mDissolvedOxygenSensorUser’sGuide



Warranty:

TheDissolvedOxygensensorML75m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applyif

thesensorhasbeendamagedbyaccidentor

misuse.



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

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

Rev. 10/06/2012

CMA ML75m User guide

CMA ML75m User guide

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