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Homework Help: Deriving an equation for the density of air

  1. Jan 20, 2004 #1
    First of all, I'm new to this forum, so please excuse any newbie etiquette errors I have made =)

    My problem is this: I'm doing a lab report on an experiment where we attempted at measuring the density of air. I won't go to the details, but basically we just measured air on different pressures.

    The task is to, given the following equations, derive an equation for the density of dry air as a function of the density of moist air, relative humidity, pressure of saturated 'steam' (can't figure a better translation..Oh yeah, I'm a Finn :) ) and the total pressure.

    The first equation:
    [tex]
    \frac{M_i^'}{M_i}=\frac{\rho_i^'}{\rho_i}=1-\frac{M_i-M_v}{M_i}*\frac{e}{p}\approx 1-0,378\frac{e}{p}
    [/tex]

    ...and the second:
    [tex]
    f=\frac{F}{F_m}*100\%=\frac{e}{e_m}*100\%
    [/tex]

    Notations:
    [tex] M_i [/tex] = molemass for completely dry air
    [tex] M_i^'[/tex] = molemass for moist air (air with some water vapour in it)
    rhos are densities of dry & moist air, respectively
    [tex] M_v [/tex] = molemass for water
    e = partitial pressure of water vapour
    p = total pressure
    f = relative humidity
    [tex] e_m [/tex] = the pressure required for the condensation of water ( again, not so sure of the translation)
    F = absolute humidity (measured in, for example, g/m^3)


    I hope I didn't forget anything. I have been trying to solve this with my mates for days and days now, and I'd really appreciate any help...

    Oh yeah, and I'm sorry, but I can't provide you with any of my sketches, as all I have are red-rimmed eyes and a heap of trash paper =)
     
  2. jcsd
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