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Moisture Measurement
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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
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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
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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
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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
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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
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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,
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Moisture measurement
wTTWet bulb temperature
Dry bulb temperature
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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)
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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
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Other Humidity Measurements
Physical principles: condensation
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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
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Hair tension hygrometer
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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.
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Extinction of light
Lyman alpha hydrometer
At certain wavelengths the absorption and scattering of light can be a strong function of water vapor density.