WIXLIB

WelcomeVaisala Humidity 101 – Humidity Theory, Terms & DefinitionsYumi AlanolyVaisala ApplicationEngineer

Agenda1.2.3.4.5.Why does it help to understand humidity?Dalton’s LawVapor pressuresRelative humidityTd, x, ppm, Tw, hPage 2 / 11/10/08 / BEMD / Vaisala

Water Vapor Theory - H2O- Exists in the three phases- Which phase depends on the amount of thermal energy that is presentPage 3 / 11/10/08 / BEMD / Vaisala

American Meteorological SocietyGlossaryHu-mid-i-tyHumidity1. Generally, some measure ofthe water vapor content of air.Page 4 / 11/10/08 / BEMD / Vaisala

Dalton’s Law“The total pressure of a gas is equal to the sumof the different gases’ partial pressures”Pt P1 P2 PnEnglish chemist,meteorologist, physicist(1766 – 1844)air around usPt PN2 PO2 Pw Pmisc.Page 5 / 11/10/08 / BEMD / Vaisala

Practical Example of Dalton’s LawNitrogen.Oxygen .Water vapor .Other gases .77%21%1%1%1000 mbar 770mbar 210mbar 10mbar 10mbarHow does this change in Denver?840 x 77%Pt 840 mbar840 x 21%840 x 1%8 Pw 8 Other647 N2 177 O2 Pt PPage 6 / 11/10/08 / BEMD / Vaisala840 x 1%w Pdry

DefinitionsPsychrometry

Partial Pressure of Water Vapor(psi,mbar,hPa,inhg )Pw“The key parameter that affects all other humidityparameters”Note:The only two properties that can affect a change in Pw adding or removing water vapor changes in system pressurePage 8 / 11/10/08 / BEMD / Vaisala

Saturation Vapor Pressure(psi,mbar,hPa,in hg )Pws1200 evaporation and condensationare in equilibrium and occur atthe same ratePw (mbar)1000800On the saturation curveSaturation Curve Pw Pws600 dewpoint temperature400 wet bulb dry bulb200 RH 100%0010203040506070Temperature ( C)Page 9 / 11/10/08 / BEMD / Vaisala8090 100Note: The only property thataffects Pws is temperature

Pws Saturation Vapor PressurePws - maximum vapor pressure or amount of water vapor that canexist at a given temperature. Expressed in units of pressure.1200Pws 480 mbarPw re ( C)Page 10 / 11/10/08 / BEMD / Vaisala8090 100

Relative Humidity (%)RHRelative humidity is the ratio of water vapor partial pressure presentin a gas (Pw) to the saturation vapor pressure of water at thattemperature [Pws(t)]orThe amount of water vapor present in air (gas) expressed as apercentage of the amount needed for saturation at the sametemperature.Page 11 / 11/10/08 / BEMD / Vaisala

Bucket AnalogyPws bucket size or max amount of waterPw amount of water in the bucketRelative fill 1/1100%Relative fill 1/1010%Page 12 / 11/10/08 / BEMD / VaisalaRelative fill 1/520%

Relative humidity1200PwPws (t)200%RH 100 x 480 42%(t 80)1000Pw (mbar)%RH 100 x800600400Pw 200Pws 48020000102030405060708090 100Temperature ( C)Note: Relative humidity is strongly proportional to temperature and its measurement isvery sensitive to temperature differences.Page 13 / 11/10/08 / BEMD / Vaisala

Temperature and Relative Humidity1200%RH 100 xPwPws (t)Pw (mbar)100080060040020000102030405060708090 100Temperature ( C)Ta 60 CPws 200 mbarPw 70 mbarRH 100 x 70/200 35 %RHPage 14 / 11/10/08 / BEMD / VaisalaTa 90 CPws 700 mbarPw 70 mbarRH 100 x 70 / 700 10 %RH

Temperature and Relative Humidity1200%RH 100 xPwPws (t)Pw (mbar)100080060040020000102030405060708090 100Temperature ( C)Ta 60 CPws 200 mbarPw 70 mbarRH 100 x 70/200 35 %RHPage 15 / 11/10/08 / BEMD / VaisalaTa 40 CPws 70 mbarPw 70 mbarRH 100 x 70 / 70 100 %RH

Temperature and Relative Humidity– Rule of Thumb #1Rule of Thumb #1*-As temperature increases, air becomes drier (RH decreases)-As temperature decreases, air becomes wetter (RH increases) drier and wetter are relative terms; applies to a closed system where pressure and water vaporcontent do not changePage 16 / 11/10/08 / BEMD / Vaisala

What about pressure and RelativeHumidity?Recall Dalton’s Law of Partial PressuresIfthensoPt Pw Pdrydouble total pressure;2(Pt) 2(Pw Pdry) 2Pw 2PdryPw changes proportionately to overall pressure changesremember that Pws remains unchanged because T is unchangedPage 17 / 11/10/08 / BEMD / Vaisala

Pressure and Relative Humidity1200Pt 1000 mbarsoPt 2000 mbarWhat happens to Pw? Pws?Pw (mbar)We double the total pressure100080060040020000102030405060708090 100Temperature ( C)Ta 80 CPws 500 mbarPw 200 mbarRH 100 x 200/500 40 %RHPage 18 / 11/10/08 / BEMD / VaisalaTa 80 CPws 500 mbarPw 400 mbarRH 100 x 400 / 500 80 %RH

Pressure and Relative Humidity –Rule of Thumb #2Rule of Thumb #2*-As pressure decreases, air becomes drier (RH goes down)-As pressure increases, air becomes wetter (RH goes up)* drier and wetter are relative terms; applies to a closed system where temperature and water vapor content do notchangePage 19 / 11/10/08 / BEMD / Vaisala

Relative Humidity Application ExampleRelative humidity is the common parameter in HVACapplications where comfort balanced with efficiency is themain concern.Page 20 / 11/10/08 / BEMD / Vaisala

TdDewpoint (ºC,ºF,ºK)TdThe temperature to which a given portion of air must be cooled at constantpressure and constant water vapor content in order for saturation to occurThe temperature at which a moist gas is saturated with respect to a planesurface of pure liquid waterPage 21 / 11/10/08 / BEMD / Vaisala

DewpointTnTd m 1 Pw log A changes with water vapor changes with pressureNote: Dewpoint is not a temperature dependent parameterPage 22 / 11/10/08 / BEMD / Vaisala

Beer temperature 38FTd 20FGlass temperature above the dewpoint – no condensationPage 23 / 11/10/08 / BEMD / Vaisala

Beer temperature 38FTd 45FGlass temperature below the dewpoint – condensation appearsPage 24 / 11/10/08 / BEMD / Vaisala

Dewpoint & PressureRule of ThumbRule of Thumb-As pressure increases, dewpoint temperature rises, air becomesmore moist (RH increases)-As pressure decreases, dewpoint temperature goes lower, airbecomes drier (RH decreases) drier and wetter are relative terms; applies to a closed system where water vapor content doesnot changePage 25 / 11/10/08 / BEMD / Vaisala

Td/fFrostpoint(ºC,ºF,ºK)Td/fThe temperature to which a given portion of air must be cooled at constant pressure andconstant water vapor content in order for saturation to occurThe temperature at which a moist gas is saturated with respect to a plane surface ofpure iceNote: Td/f is a Vaisala term which means dewpoint above 32ºF and frostpoint 32ºF and belowPage 26 / 11/10/08 / BEMD / Vaisala

Dewpoint versus FrostpointFrostpoint-0.10 C-5.00 C-10.00 C-20.00 C-30.00 C-40.00 C-50.00 C-60.00 C-70.00 C-80.00 C-90.00 CPage 27 / 11/10/08 / BEMD / VaisalaDewpoint-0.11 C-5.64 C-11.23 C-22.25 C-33.09 C-43.74 C-54.24 C-64.59 C-74.88 C-85.29 C-96.37 CAIRcondensationICEevaporation

Td/fFrostpointTdorTd/fTd/f – gives you dewpoint at 32 degrees (F) and above and frostpoint below 32 degrees (F)Td – gives you dewpoint across the entire range of temperatures and assumessupercooled water below 32 degrees (F)Page 28 / 11/10/08 / BEMD / Vaisala

Application Example - compressed aircompressor picture courtesy of Atlas CopcoDewpoint is the common parameter for measurement incompressed air systems and plastics production feed dryingPage 29 / 11/10/08 / BEMD / Vaisala

xMixing Ratio or Humidity Ratio(g/kg, gr/lb)x–the ratio of the mass of water vapor per unit mass of dry airto which it is associatedNote: mixing ratio is an absolute measure, not affected by temperature or pressurePage 30 / 11/10/08 / BEMD / Vaisala

Application Example – drying processMixing ratio can be used as a measure to help determine drying timewhere moisture content of a product is important like paper drying ordog biscuit drying.Page 31 / 11/10/08 / BEMD / Vaisala

ppmv ppmwparts per million (volume/weight)ppmv- volume of water vapor per total volume of dry gasppmw- mass of water vapor per total mass of dry gasNote: ppm is an absolute measure, not affected by temperature or pressureMw is molecular mass of water ; Md is molecular mass of dry airPage 32 / 11/10/08 / BEMD / Vaisala

ppmv and -5.00PPMw14244269113181284439Page 33 / 11/10/08 / BEMD / VaisalaPPMv233967111181290456706Mw/Md .621980

Application Example – glove boxppm is sometimes used in dry environments where very precise absolutemeasurement is required such as in a glove box or clean roomPage 34 / 11/10/08 / BEMD / Vaisala

aAbsolute Humidity (g/m , gr/ft , lbs/MMcf)3a- the mass of water vapor per unit volume of moist air- the density of the water vaporPage 35 / 11/10/08 / BEMD / Vaisala3

Application Example – natural gasAbsolute humidity is the common parameter for measurement of moisturecontent in natural gas (in the U.S.)Page 36 / 11/10/08 / BEMD / Vaisala

TwWet bulb temperature (ºC,ºF)Twthe temperature indicated by a thermometer sheathed in a wet cloth as air is passedover itPage 37 / 11/10/08 / BEMD / Vaisala

Application Example– evaporative cooler or swamp coolerBy comparing the wet bulb temperature to the dry bulb temperature we candetermine cooling capacity of an evaporative cooler.Page 38 / 11/10/08 / BEMD / Vaisala

hEnthalpy (kj/kg; btu/lb)h Measure of the total energy in a moist gas heat content sum of the latent heat sensible heatPage 39 / 11/10/08 / BEMD / Vaisala

Application Example - HVACEnthalpy is a useful measurement for determining HVAC equipment size andefficiencyPage 40 / 11/10/08 / BEMD / Vaisala

Psychrometric Terms––––––––––relative humiditypartial pressure of water vaporsaturation pressuredewpoint/frostpointabsolute humiditymixing ratio/humidity ratiowet bulb temperatureppmvppmwenthalpyPage 41 / 11/10/08 / BEMD / VaisalaRHPwPwsTd/faxTw[%RH][mbar; in.Hg, etc.][mbar; in.Hg, etc.][ C; F][g/m3; gr/ft3][g/kg; gr/lb][ C; F]h[kJ/kg; Btu/lb]

Summary1.2.3.4.5.6.7.Water vapor theoryDalton’s law of partial pressuresPw & PwsRH Pw/PwsTemperature and RH – Rule of thumbPressure and RH – Rule of thumbAbsolute parameters – x, ppmPage 42 / 11/10/08 / BEMD / Vaisala

Vaisala Humidity Resources On-line Humidity Calculator www.vaisala.com/humiditycalculator Slide Rule Calculator to order – http://forms.vaisala.com/forms/RequestSlideRule Psychrometric Chart - http://forms.vaisala.com/forms/RequestPsychChart Humidity Conversion Formulas - http://forms.vaisala.com/forms/humidity conversionFor expert assistance with your humidity measurementRequest info: Click here to fill out ‘Request Contact’ formDirect telephone: 800-408-9454Website: www.vaisala.comPage 43 / 11/10/08 / BEMD / Vaisala

Next Webinar – Humidity SensorTechnology - TutorialWednesday, June 26th, 9:30AM MDT Humidity Sensor TechnologyFor full Webinar Schedule info please click here.Everyone who registered for Humidity Theory will get the invitationfor Sensor Technology.You will receive a follow up email with all of the resource links & linkto recording.Page 44 / 11/10/08 / BEMD / Vaisala

Thank you!This concludes the webinar.Follow-up email will arrive shortly with the resource links &further contact information.Page 45 / 11/10/08 / BEMD / Vaisala

Relative humidity is the common parameter in HVAC applications where comfort balanced with efficiency is the main concern. Page 21 / 11/10/08 / BEMD / Vaisala T d Dewpoint (ºC,ºF,ºK) The temperature to which a given portion of air must be cooled at constant