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Online Calculators
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saturated vapour pressure
relative humidity of air
density of water
density of (humid) air
diffusion coefficient of water vapour in air
viscosity of air
Note:
Temperatures on this page are on the Celcius scale. If you are more familiar with Fahrenheits, you may want to see what a value in Celcius would be in Fahrenheit: use our F/C Converter.
saturated vapour pressure (SVP) [top]
Saturated water vapour pressure in air; empirical relation:
Temperature T in Celcius [ = {Kelvin} - 273.15]); validity range roughly 0-80oC.
[Source: Monteith and Unsworth, 1990; some parameters slightly adjusted ]
relative humidity of air [top]
Relative humidity is the ratio of partial vapour pressure and saturated vapour pressure at ambient temperature:
If humidity is to be determined by dry- and wet-bulb temperatures (Td and Tw, respectively; Celcius scale), partial vapour pressure is calculated by (at ambient pressure 1 atm):
where psat,w is saturated vapour pressure at the wet-bulb temperature. Finally, saturated vapour pressure at dry-bulb temperature (i.e. normal ambient temperature) must be calculated, and the pressure ratio gives relative humidity.
[Source: calculator for saturated vapour pressure (see above) and relation between bulb temperatures and vapour pressures (List, 1958)]
density of water [top]
Empirical relation for T=0-30oC, error <0.1 kg/m3:
For a wider temperature range (T=0-50oC, error <0.1 kg/m3) use instead:
Density
w in [kg/m3].(Note: at 3.98oC water has its highest density).
[Source: polynomial fit on values from Weast, 1986; page F-10]
density of (humid) air [top]
Empirical relation:
Temperature T in Kelvin [K]; barometric pressure B in [Pa]; partial vapour pressure pv in [Pa];
a: density [kg/m3]; validity range: 'environmental' values. (Note: 1 atm = 1.01325 x 105 Pa)[Source: Weast, 1986; page F-8; modified for SI units]
diffusion coefficient of water vapour in air [top]
Empirical relation:
T: temperature, Celcius [C]; Dv: diffusion coefficient [m2/s].
Error: <0.1x10-6 m2/s; validity range: 0-45oC.
[Source: linear fit on values given by Gates, 1980 (Appendix 3)]
viscosity of air [top]
Empirical relation:
T: temperature, Celcius [C]; viscosity in Pascal x seconds [Pa.s]>.
Error: <0.1x10-6 Pa.s; validity range: 0-54oC.
[Source: parabolic fit on values from Weast, 1986; page F-42]
Holsoft (c) 2001
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