Holsoft’s Physics Resources Pages
Online Calculators
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online calculators listed below:
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 080^{o}C.
[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 wetbulb temperatures (T_{d} and T_{w}, respectively; Celcius scale), partial vapour pressure is calculated by (at ambient pressure 1 atm):
where p_{sat,w} is saturated vapour pressure at the wetbulb temperature. Finally, saturated vapour pressure at drybulb 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=030^{o}C, error <0.1 kg/m^{3}:
For a wider temperature range (T=050^{o}C, error <0.1 kg/m^{3}) use instead:
Density _{w} in [kg/m^{3}].
(Note: at 3.98^{o}C water has its highest density).
[Source: polynomial fit on values from Weast, 1986; page F10]
density of (humid) air [top]
Empirical relation:
Temperature T in Celcius; barometric pressure B in [Pa]; partial vapour pressure p_{v} in [Pa]; _{a}: density [kg/m^{3}]; validity range: ‘environmental’ values. (Note: 1 atm = 1.01325 x 10^{5} Pa)
[Source: Weast, 1986; page F8; modified for SI units]
diffusion coefficient of water vapour in air [top]
Empirical relation:
T: temperature, Celcius [C]; D_{v}: diffusion coefficient [m^{2}/s].
Error: <0.1×10^{6} m^{2}/s; validity range: 045^{o}C.
[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: <0.1×10^{6} Pa.s; validity range: 054^{o}C.
[Source: parabolic fit on values from Weast, 1986; page F42]
