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Engineering ToolBox > Water Vapor and Saturation Pressure in Humid Air
Water vapor is almost always present in our surrounding air.
The maximum saturation pressure of the water vapor in moist air vary with the temperature of the air vapor mixture and can be expressed as:
pws = e(77.3450 + 0.0057 T - 7,235 / T) / T8.2 (1)
where
pws = water vapor saturation pressure (Pa)
e = the constant 2.718.......
T = temperature of the moist air (K)
The density of water vapor can be expressed as:
ρw = 0.0022 pw / T (2)
where
pw = partial pressure water vapor (Pa, N/m2)
ρw = density water vapor (kg/m3)
T = absolute dry bulb temperature (K)
The saturation pressure and density of water vapor for a common temperature range:
| Temperature | Saturation Pressure | Density | |||||
| (oC) | (oF) | (Pa) | (mmHg) | (psia) | (inHg) | (kg/m3) | (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 |
The Saturation pressure of water vapor in moist air at 25oC can be calculated as:
pws = e( 77.3450 + 0.0057 (273 + 25) - 7,235 / (273 + 25) ) / (273 + 25)8.2
= 3,130 Pa