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# Moist Air - Water Vapor and Saturation Pressure

## Saturation pressure of water vapor in moist air vs. temperature.

Water vapor is almost always present in the surrounding air.

Note! - the equations below is for pure water vapor - not moist air .

### Saturation Pressure of Water Vapor

The maximum saturation pressure of the water vapor in moist air varies with the temperature of the air vapor mixture and can be expressed as:

p ws = e (77.3450 + 0.0057 T - 7235 / T) / T 8.2 (1)

where

p ws = water vapor saturation pressure (Pa)

e = the constant 2.718.......

T = dry bulb temperature of the moist air (K)

### Density of Water Vapor

The density of water vapor can be expressed as:

ρ w = 0.0022 p w / T (2)

where

p w = partial pressure water vapor (Pa, N/m2)

ρ w = density water vapor (kg/m3 )

T = absolute dry bulb temperature (K)

Saturation pressure and density of water vapor for common temperatures

Moist Air - Saturation Pressure and Density vs. Temperature
TemperatureSaturation PressureDensity
(oC)( oF)(Pa)(mmHg)(psia)(inHg)(kg/m3 )10-3
(lb/ft3 )
0 32 603 4.6 0.09 0.18 0.005 0.30
10 50 1212 9.2 0.18 0.36 0.009 0.59
20 68 2310 17.4 0.33 0.68 0.017 1.08
30 86 4195 31.7 0.61 1.24 0.030 1.90
40 104 7297 55.1 1.06 2.15 0.051 3.20
50 122 12210 92.2 1.8 3.60 0.083 5.19
60 140 19724 149 2.9 5.82 0.13 8.13
70 158 30866 233 4.5 9.11 0.20 12.3
80 176 46925 354 6.8 13.8 0.29 18.2
90 194 69485 525 10.1 20.5 0.42 26.3
100 212 100446 758 14.6 29.6 0.59 36.9
120 248 196849 1486 28.6 58.1 1.10 68.7
140 284 358137 2704 51.9 105.7 1.91 119
160 320 611728 4619 88.7 180.5 3.11 194
180 356 990022 7475 144 292.1 4.80 300
200 392 1529627 11549 222 451.2 7.11 444
• 1 lb/ft3 = 16.018 kg/m3
• 1 kg/m3 = 0.0624 lb/ft3

### Example - Saturation Pressure Water Vapor

The Saturation pressure of water vapor in moist air at dry bulb temperature 25 oC can be calculated:

First, convertion from °C to K:

( 25 °C) + 273 = 298 (K)

Then Eq. (1) is used:

p ws = e (77.3450 + 0.0057 (298 K) - 7235 / (298 K) ) / 298 [K] 8.2

= 3130 (Pa)

## Related Topics

• ### Air Psychrometrics

Moist and humid air calculations. Psychrometric charts and Mollier diagrams. Air-condition systems temperatures, absolute and relative humidities and moisture content in air.

## Related Documents

• ### Air - Drying Force

The drying force of air depends on the air moisture holding capacity and the water surface to air evaporation capacity.
• ### Air - Humidity Measurement from Dry and Wet Bulb Temperature

Relative humidity in moist air can estimated by measuring the dry and wet bulb temperature.
• ### Air - Maximum Moisture Carrying Capacity

Maximum water content in humid air vs. temperature.
• ### Air - Moisture Holding Capacity vs. Temperature

The moisture holding capacity of air increases with temperature.
• ### Dry Air - Thermodynamic and Physical Properties

Thermodynamic properties of dry air - specific heat, ratio of specific heats, dynamic viscosity, thermal conductivity, Prandtl number, density and kinematic viscosity at temperatures ranging 175 - 1900 K.
• ### Humid Air and the Ideal Gas Law

Pressure, temperature and volume in a perfect ideal gas like moist air (air with water vapor).
• ### Liquids - Vapor Pressures

Vapor and saturation pressure for some common liquids.
• ### Moist Air - Degree of Saturation

Humidity ratio of moist air to humidity ratio of saturated moist air.
• ### Moist Air - Density vs. Pressure

Density of moist air vs. pressure ranging 75 - 1000 mmHg.
• ### Moist Air - Density vs. Water Content and Temperature

Density of the mix of dry air and water vapor - moist humid air.
• ### Moist Air - Mole Fraction of Water Vapor

Mole fraction of water vapor is the ratio of water molecules to air and water molecules.
• ### Moist Air - Partial Pressure and Daltons Law

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.
• ### Moist Air - Psychrometric Table for Pressure 29.92 inHg

Dry and wet bulb temperatures, saturation pressure, water vapor weight, specific volume, heat and more.
• ### Moist Air - Vapor Pressure

Vapor pressures vs. dry and wet bulb temperatures in moist air.
• ### Removing Heat with Air

Calculating heat removed with air by measuring the wet bulb temperature.
• ### Water - Boiling Points at Vacuum Pressure

Online calculator, figures and tables giving the boiling temperatures of water in varying vacuum, SI and Imperial units.
• ### Water - Saturation Pressure and Specific Weight vs. Temperature

Vapor pressure and specific weight of water at temperatures ranging 32 to 212 oF - Imperial Units.
• ### Water - Saturation Pressure vs. Temperature

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.
• ### Wet Bulb Globe Temperature (WBGT)

The Wet Bulb Globe Temperature can be used to measure the general Heat-Stress index.

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## Citation

• The Engineering ToolBox (2004). Moist Air - Water Vapor and Saturation Pressure. [online] Available at: https://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html [Accessed Day Month Year].

Modify the access date according your visit.

8.21.8

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