Relative Humidity Measurement

This article defines terms related to relative humidity and discusses methods involved in the measurement of relative humidity

1. Relative Humidity
Measurement
ER.FARUKBINPOYEN
FARUK.POYEN@GMAIL .COM

2. Content:
Definition
Coefficient
Units
Measuring Devices
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3. Relative Humidity
Relativehumiditydependsontemperatureandthepressureofthe
systemofinterest.Thesameamountofwatervapourresultsinhigher
relativehumidityincoolairthanwarmair.Arelatedparameteristhatof
dewpoint.Itisnormallyexpressedas%.
RelativeHumidity(RHorφ)ofanair-watermixtureisdefinedasthe
rationofthepartialpressureofwatervapourinthemixturetothe
equilibriumvapourpressureofwateroveraflatsurfaceofpurewaterat
agiventemperature.
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4. Relative Humidity
Absolutehumidityisthemassofwatervapourdividedbythemassof
dryairinacertainvolumeofairataspecifictemperature.Itisdefined
asthemassofvapourcontainedin1kgofdryair.
Specifichumidity,(massofwatervapour)isaunitmassofmoistair,
usuallyexpressedasgramsofvapourperkilogramofair,or,inair
conditioning,asgrainsperpound.Thespecifichumidityisanextremely
usefulquantityinmeteorology.
At100%RH,theairissaturatedandisatitsdewpoint.
Dewpoint:Thetemperature(dependingonpressureandhumidity)
belowwhichwaterdropletsbegintocondenseanddewcanform.
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5. Relative Humidity
Psychrometryisthescienceofstudyingthethermodynamicproperties
ofmoistairandtheuseofthesepropertiestoanalyseconditionsand
processesinvolvingmoistair.
DryBulbTemperature(�
��):Thedry-bulbtemperature(DBT)isthe
temperatureofairmeasuredbyathermometerfreelyexposedtotheair,
butshieldedfromradiationandmoisture.
WetBulbTemperature(�
��):Thewet-bulbtemperature(WBT)isthe
temperaturereadbyathermometercoveredinwater-soakedcloth(wet-
bulbthermometer)overwhichairispassed.
At100%relativehumidity,thewet-bulbtemperatureisequaltotheair
temperature(dry-bulbtemperature)anditisloweratlowerhumidity.
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6. Relative Humidity
Byusingbothwet-bulbtemperatureanddry-bulbtemperaturethe
relativehumiditycanbecalculated.
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7. Relative Humidity –Measurement
Ahygrometerisaninstrumentusedtomeasuretheamountofhumidity
andwatervapourintheatmosphere,insoil,orinconfinedspaces.
RelativeHumiditycanbemeasuredusingthefollowingtypesof
devices.
1.ResistiveHygrometer
2.CapacitiveHygrometer
3.CrystalHygrometer
4.Thermal
5.Gravimetric
6.Optical
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8. Relative Humidity –Resistive Hygrometer
Theresistance-typehygrometerutilizestheprinciplethatelectrical
resistancevariesinamaterialthatabsorbsmoisture.
Specialresistivesensorsmeasuretheelectricalresistancebetweenthe
electrodes.
Suitableinmassproductionandforuseinhomeappliances.
Accurateisaconcern.
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9. Relative Humidity –Resistive Hygrometer
Theconductingfilmoftheresistivehygrometerismadebythelithium
chlorideandthecarbon.
Theresistanceoftheconductingfilmvarieswiththechangeinthevalue
ofhumiditypresentinthesurroundingair.
Iftherelativehumidityishigh,thelithiumchloridewillabsorbmore
moistureandtheirresistancedecreases.
ThechangeinthevalueofresistanceismeasuredbyapplyingtheACto
thebridge.
TheDCisnotusedinthebridgeastheybreakdownthelayeroflithium
chloride
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10. Relative Humidity –Capacitive Hygrometer
Acapacitance-typehygrometermeasureshumiditybydetectingthe
changeincapacitanceofathinpolymerfilm.
Thistypeofsensorissufficientlyaccurateinmostcasesandisused
frequentlyinindustry.
Itismadebyplacingthehygroscopicmaterialbetweenthemetal
electrodes,quicklyabsorbingwateranddecreasingthecapacitance
value.
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11. Relative Humidity –Crystal Hygrometer
Incrystalhygrometer,thehygroscopiccrystalorthecrystalhavingthe
coatingofhygroscopicmaterialisused.
Whenthecrystalabsorbsthedropsofthewater,themassofthecrystal
changes.
Thechangeinmassisproportionaltothetotalwaterabsorbedbythe
crystals.
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12. Relative Humidity –Other Methods
Thermal-Thechangeinthermalconductivityofairduetohumidityis
measured.Thesesensorsmeasureabsolutehumidityratherthanrelative
humidity.
Gravimetric-AGravimetrichygrometermeasuresthemassofanair
samplecomparedtoanequalvolumeofdryair.Thisisconsideredthe
mostaccurateprimarymethodtodeterminethemoisturecontentofthe
air.
Optical-Anopticalhygrometermeasurestheabsorptionoflightby
waterintheair.Alightemitterandalightdetectorarearrangedwitha
volumeofairbetweenthem.Theattenuationofthelight,asseenbythe
detector,indicatesthehumidity,accordingtotheBeer–Lambertlaw.
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13. References:
https://circuitglobe.com/hygrometer.html
https://en.wikipedia.org/wiki/Hygrometer
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