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Online Calculatorsback to Dust Monitoring Equipment main menusaturated vapour pressure relative humidity of air density of water density of (humid) air diffusion coefficient of water vapour in air viscosity of air Note: 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: <0.1x10^{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: <0.1x10^{6} Pa.s; validity range: 054^{o}C. [Source: parabolic fit on values from Weast, 1986; page F42]  
Holsoft (c) 2001 