Relative Humidity of Air
The relative humidity of air can be expressed by partial vapor and air pressure - density of the vapor and air - or by the actual mass of the vapor and air
Sponsored Links
Humidity is the quantity of water vapor present in air. It can be expressed as an absolute, specific or a relative value.
Relative humidity can be expressed by partial vapor and air pressure, density of the vapor and air, or by the actual mass of the vapor and air.
Relative humidity is usually expressed in per cent and abbreviated by φ or RH.
Relative Humidity and Vapor Partial Pressure
Relative humidity can be expressed as the ratio of the vapor partial pressure of the air - to the saturation vapor partial pressure of the air at the actual dry bulb temperature.
Relative humidity by partial pressure
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)
The table below can be used to determine the vapor saturation pressure at different temperatures:
| Temperature | Saturation Vapor Pressure (mbar, millibar, mb) | |
| (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 |
- 1 bar = 1000 mbar = 105 Pa (N/m2) = 0.1 N/mm2 = 10,197 kp/m2 = 10.20 m H2O = 0.9869 atm = 14.50 psi (lbf/in2) = 106 dyn/cm2 = 750 mmHg
If the water vapor pressure in the air is 10.3 mbar the vapor saturates on a surface with 45oF.
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 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 and Vapor Density
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. If the actual vapor density at 20oC is 10 g/m3 and the 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 and Vapor Mass
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)
Relative Humidity Chart - degrees Fahrenheit
Relative Humidity Chart - degrees Celcius
Sponsored Links
Related Topics
- Air Psychrometrics - The study of moist and humid air - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more .
Sponsored Links
Related Documents
- Cooling and Dehumidifying Air - Cooling and dehumidifying processes of moist and humid air - sensible and latent cooling
- Density of Moist Air - Density of moist air - pressure ranging 75 - 1000 mm Hg
- Drying Force of Air - The Drying Force of air may be expressed as the moisture holding capacity of the air and the evaporation capacity from a water surface to the air
- Heat Removed by Air - Heat removed with air - dry and wet bulb temperature
- Humidity Ratio of Air - Humidity ratio (GPP) is the ratio between actual mass of water vapor present in moist air - to the mass of dry air
- Moist Air and Degree of Saturation - The ratio of humidity ratio of moist air to humidity ratio of saturated moist air - the Degree of Saturation
- Outdoor to Indoor Temperature Rise and Relative Humidity - The relationship between outdoor and indoor temperature rise - and the effect on the relative humidity
- Psychrometric Terms - Dry Bulb and Wet Bulb Temperature, Specific Volume, Relative Humidity, Enthalpy and more ..
- Recommended Relative Humidity in Production and Process Environments - Recommended Relative Humidity - RH - in production and process environments like libraries, breweries, storages and more
- Relative Humidity and Health - Bacteria, viruses, fungi, mites, asthma and range of relative humidity
- SCFM versus ACFM and ICFM - Actual air compressor capacity (ACFM) versus standard air capacity (SCFM) and inlet air capacity (ICFM)
- The relationship between Specific and Relative Humidity of Air - Calculating specific humidity of moist air knowing relative humidity, water vapor and air density
- Vapor Pressure in Moist Air - Vapor pressures in moist air from dry and wet bulb temperatures
- Water Saturation Pressure - Water saturation pressure at temperatures ranging 0 - 100 oC and 32 - 700 oF - Imperial and SI Units
