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Calculating partial pressures Pa and Pw

  1. May 27, 2012 #1
    The humidity level is 80% and T=303Kelvin. So I can use the equation RH = Pw/Pws to calculate the Pw = Partial pressure of water vapor, by finding Pws with the following equation:

    Pws=e^(77.3450+0057T-7235/T)/T^8.2 = 4200Pa, where T is in Kelvin.
    So Pw=0.8*4200Pa=3360Pa

    I can find Pa by using two equations for the density of dry air:

    pa=0.0035*Pa/T, where pa = density of dry air
    I rearrange this into Pa=paT/0.0035

    Now I calculate the density of dry air by using the ideal gas law:

    pa= P / Rspecific*T which gives me: 101325 Pascal / 287.058*303 Kelvin = 1.1644Kg/m^3

    So Pa=1.1644*303/0.0035=100803Pa or 100.8 KPa. But this seems like too much.

    Because the equation for evaporation rate is W = (95+0.425*V)*(Pw-Pa)/Y

    And Pw-Pa in this case gives me a negative number.
    So what am I doing wrong?
     
  2. jcsd
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