Relative Humidity (RH)

by Dasa . 27 No 2021
Enter Values from Humidity Ratio Page:
Standard atmospheric Pressure in psia (p):
Dry bulb temperature in °R:
Wet bulb temperature in °R:
Saturated Pressure @ DBT in psia (Pdbt):
Partial Pressure Of Water Vapour in psia (VP):
METHOD-1
Relative Humidity in % (RH):
METHOD-2
Relative Humidity in % (RH):

Follow the instructions and get the Humidity Ratio from the below link.

Calculation for Humidity Ratio OR Specific Humidity.

Assume the calculated value W = 0.0131 lb/lb @ 75°F DBT, 68° WBT, & 10 ft sea level.

Method-1:

Saturation pressure @ given temperature

ln(pt)=C8t+C9\ln(p_{t}) = \dfrac{C_{8}}{t} + C_{9}
+\quad\quad+ C10t+C11t2+C12t3C_{10}*t + C_{11}*t^2 + C_{12}*t^3
+\quad\quad+ C13ln(t)C_{13}*\ln(t)
Saturation pressure @ DBT = Saturation pressure
ps=pdbtp_{s} = p_{dbt}
ln(pdbt)=10440.397534.67+11.29465\ln(p_{dbt}) = \dfrac{-10440.397}{534.67} + -11.29465
+\quad+ 0.027022355534.67+1.28904E05534.672-0.027022355*534.67 + 1.28904E-05*534.67^2
+\quad+ 2.47807E09534.673-2.47807E-09*534.67^3
+\quad+ 6.5459673ln(534.67)6.5459673*\ln(534.67)

ps=pdbt=0.43psiap_{s} = p_{dbt} = 0.43 psia

Humidity ratio at Saturation

Ws=0.621945psppsW_{s} = \dfrac{0.621945 * p_{s}} {p-p_{s}}
Here psp_{s} represents the saturation pressure of water vapor in the absence of air at the given temperature and it differs slightly from the vapor pressure of water in saturated moist air.

Humidity ratio @ DBT = Humidity ratio at Saturation
Wdbt=WsW_{dbt} = W_{s}
Wdbt=0.6219450.4314.6910.43W_{dbt} = \dfrac{0.621945 * 0.43} {14.691-0.43}
Ws=Wdbt=0.0188W_{s} = W_{dbt} = 0.0188

Relative Humidity (RH)

Φ=WWs\Phi = \dfrac{W}{W_{s}}

=0.01310.0188=0.697=69.7=\dfrac{0.0131}{0.0188} = 0.697 = 69.7 %

Method-2:

Partial pressure of Water vapour

pw=pW0.621945+Wp_{w} = \dfrac{p * W} {0.621945 + W}

Where,
pwp_{w} - Partial pressure of water vapour (VP) in psia
WW - Humidity ratio in lb/lb
pp - Standard atmospheric pressure at given altitude(sea-level) in psia (Refer Humidity Ratio calculation for details)

pw=14.6910.01310.621945+0.0131=0.303psiap_{w} = \dfrac{14.691 * 0.0131} {0.621945 + 0.0131} = 0.303 \, psia

Relative Humidity (RH)

Φ=pwps\Phi = \dfrac{p_{w}}{p_{s}}

=0.3030.43=0.705=70.5=\dfrac{0.303}{0.43} = 0.705 = 70.5 %


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