Relative humidity as the ratio of vapor partial pressure in the air - to the saturation vapor partial pressure if the air at the actual dry bulb temperature.
φ = pw / pws 100% (1)
where
φ = relative humidity [%]
pw = vapor partial pressure [mbar]
pws = saturation vapor partial pressure at the actual dry bulb temperature [mbar]. This is the vapour pressure at maximum content of water gas in air, before it starts to condense out as liquid water.
Vapor saturation pressures at different temperatures:
Temperature | Saturation Vapor Pressure [10-3 bar] | |
---|---|---|
[oC] | [oF] | |
-18 | 0 | 1.5 |
-15 | 5 | 1.9 |
-12 | 10 | 2.4 |
-9 | 15 | 3.0 |
-7 | 20 | 3.7 |
-4 | 25 | 4.6 |
-1 | 30 | 5.6 |
2 | 35 | 6.9 |
4 | 40 | 8.4 |
7 | 45 | 10.3 |
10 | 50 | 12.3 |
13 | 55 | 14.8 |
16 | 60 | 17.7 |
18 | 65 | 21.0 |
21 | 70 | 25.0 |
24 | 75 | 29.6 |
27 | 80 | 35.0 |
29 | 85 | 41.0 |
32 | 90 | 48.1 |
35 | 95 | 56.2 |
38 | 100 | 65.6 |
41 | 105 | 76.2 |
43 | 110 | 87.8 |
46 | 115 | 101.4 |
49 | 120 | 116.8 |
52 | 125 | 134.2 |
If the water vapor pressure in the air is 10.3 mbar, the vapor saturates on a surface with 45oF (7oC).
Note! The atmospheric pressure of air is 1013 mbar (101.325 kPa, 760 mmHg). As we can see the maximum water vapor pressure - the saturation pressure - is relatively small.
Example: Relative Humidity and Vapor Pressure
From the table above the saturation pressure at 70oF (21oC) is 25.0 mbar. If the vapor pressure in the actual air is 10.3 mbar the relative humidity can be calculated as:
φ = 10.2 [mbar] / 25.0 [mbar]* 100[%]
= 41[%]
Relative humidity can also be expressed as the ratio of the vapor density of the air - to the saturation vapor density at the the actual dry bulb temperature.
Relative humidity by density:
φ = ρw / ρws 100% (2b)
where
φ = relative humidity [%]
ρw = vapor density [kg/m3]
ρws = vapor density at saturation at actual dry bulb temperature [kg/m3]
A common unit for vapor density is g/m3.
Example: Relative humidity at a given temperature and known vapor density and saturation density
If the actual vapor density at 20oC (68oF) is 10 g/m3 and the saturation vapor density at this temperature is 17.3 g/m3, the relative humidity can be calculated as
φ = 10 [g/m3] / 17.3 [g/m3] *100 [%]
= 57.8 [%]
Relative humidity can also be expressed as the ratio at actual mass of water vapor in a given air volume - to the mass of water vapor required to saturate at this volume.
Relative humidity can be expressed as:
φ = mw / mws 100% (2c)
where
φ = relative humidity [%]
mw = mass of water vapor in the given air volume [kg]
mws = mass of water vapor required to saturate at this volume [kg]
Barometric Altitude [mbar] | Abs. Altitude | Correction Factor for φ | |
---|---|---|---|
[m] | [ft] | ||
1013 | 0 | 0 | 1.000 |
1000 | 108 | 354 | 0.987 |
989 | 200 | 656 | 0.976 |
966 | 400 | 1312 | 0.953 |
943 | 600 | 1996 | 0.931 |
921 | 800 | 2624 | 0.909 |
899 | 1000 | 3281 | 0.887 |
842 | 1500 | 4922 | 0.831 |
795 | 2000 | 6562 | 0.785 |
Moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content.
The drying force of air depends on the air moisture holding capacity and the water surface to air evaporation capacity.
Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.
The mass of water vapor present in moist air - to the mass of dry air.
Maximum water content in humid air vs. temperature.
The moisture holding capacity of air increases with temperature.
Actual air compressor capacity (ACFM) vs. standard air capacity (SCFM) and inlet air capacity (ICFM).
Dry Bulb, Wet Bulb and Dew Point temperatures can be used to determine the state of humid air.
The optimum relative humidity ranges for bacteria, viruses, fungi, mites, asthma.
Recommended indoor relative humidity vs. outdoor relative humidity and temperature.
Cooling and dehumidifying processes of moist and humid air - sensible and latent cooling.
The pressure in a mixture of dry air and water vapor - humid or moist air - can be estimated by using Daltons Law of partial pressures.
Humidity ratio of moist air to humidity ratio of saturated moist air.
Density of moist air vs. pressure ranging 75 - 1000 mmHg.
Dry and wet bulb temperatures, saturation pressure, water vapor weight, specific volume, heat and more.
Dry and wet bulb temperature, specific volume, relative humidity, enthalpy and more.
Specific humidity of moist air vs. relative humidity, water vapor and air density.
Vapor pressures vs. dry and wet bulb temperatures in moist air.
Recommended relative humidity in production and process environments - like libraries, breweries, storages and more.
Calculating heat removed with air by measuring the wet bulb temperature.
A salt solutions can be used maintain a particular value of relative humidity.
Online calculator, figures and tables with water saturation (vapor) pressure at temperatures ranging 0 to 370 °C (32 to 700°F) - in Imperial and SI Units.
The Wet Bulb Globe Temperature can be used to measure the general Heat-Stress index.
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