Moisture Measurement

1-6vfs

1-6v

1-5f

kg J x10834.2LLL n,sublimatio ofheat Latent

kg J 2.5x10L ion, vaporizatofheat Latent

kg J 3.34x10L fusion, ofheat Latent

Sensible heat and Latent heat

Water vapor in the Air

Equation of state, gas law of dry air

K kgJ

dd

ddVM

287R air,dry for constant gas :R

T;R p T;R p

1. Water vapor pressurevapor.for water constant gas ,461R humidity; absolute : K kg

Jvv

p

,p M const;:V

,p V const;:M

const ,p T const;:VM,

TR e TR p

T,......,R p T;R p T;R p

vvOHVM

OH

COVM

COOVM

ONVM

N

2

O2H

2

22CO

222O

222N

2

OHCOON 2222P...PPPP

TR e vv

2. Mixing ratio, r

622.0 ;

r

dv

dd

dd

v

dv

dv

RR

pe

epe

pe

RR

TRp

TRe

VV

)(Mair dry of mass)(Mor water vapof mass

v

3. Specific humidity, q

622.0 ;

q

dv

vdd

v

vdv

vdv

RR

pe

e)1(pe

TRe

TRp

TRe

)V(V

)M(Mair moist of mass)(Mor water vapof mass

Relationship between Vapor Pressure and Saturation

Saturated water vapor pressure, E

r ratio, mixing Saturated s

s

s

r r E,e :air saturatedr r E,e :air dunsaturate

E(T)E

4. Clausius-Clapeyron equation

K 16.273T ,kg J X105.2L hPa, 11.6E

),TT

T-TRLexp(EE ;

TRE L

TE

01-6

0

0

0

v02

v

6% E -------------C1t 0

5. Relative humidity, h %qq

rr

Eeh

ss

6. Dew-point, the temperature to which air is cooled to become saturated at constant pressure.

T K 16.273 ,Jkg 2.5X10L hPa, 11.6E

),-RLexp(Ee ;

Re L

de d

01-6

0

0

0

v02

v

T

TT

7. Moist virtual effect

e, p p d

)]1(pe1[

TRP

TRe

TRe-p

VMM

dvdvd

vd

0.608q)T(1q)-1T(1)1(

pe-1

TT ,TRp vvd

Tv: the temperature that dry air would need to have at the given pressure in order to have the same density as moist air, assuming idea gas behavior.

)r0.608r(1 lv TT

rl is the liquid water mixing ratio

If it’s saturated,

Moisture measurement

wTTWet bulb temperature

Dry bulb temperature

The change of enthalpy is more useful in thermodynamics

VppVUH

The first law of thermodynamics WQU

Work may be represented byW VpW

Thus, pVQH

For isobaric process QH

QTCp

Enthalpy pVUTCH p

U: internal energy (in joules), p: pressure of the system, (in pascals)V : volume, (in cubic meters)

Wet bulb temperature

QTTc ifp )(

)( wv

pTofs TT

crr

w

wTT

)(622.0 w

v

pTofs TT

pcee

w

Since )( Ds tee

)( ws Tee

TTT wD

)()( rrTTc wvwp

Other Humidity Measurements

Physical principles: condensation

Sling psychrometer

heating

How to measure moisture below freezing ?

1. A fan draws outside air past a thermometer to measure the ambient dry-bulb temperature, 2. A heating device, 3. A second thermometer to measure the dry-bulb temperature of the heated air,4. A wet-bulb thermometer

)(,: 1 humidityrelativehTeairheated heat

)(: humidityrelativehTairambient

Hair tension hygrometer

Electronic hygrometers

Capacitive sensor

Resistive sensor

Polymer membrane

Capacitive sensors sense water vapor by measuring the change in capacitance caused by the amount of water vapor present

Resistive sensors use a thin film that changes conductivity according to absorbed water.

Extinction of light

Lyman alpha hydrometer

At certain wavelengths the absorption and scattering of light can be a strong function of water vapor density.