First of all sorry for my bad English ...
I have to calculate the specific weight of air knowing that in the air there is actually a small percentage of water vapor.
Therefore the specific weight of the humid air is:
dry air + water vapor
I want to know specific weight of air with temperature 293.15 K, pressure 101325 Pascal and RH 10% (relative humidity)
I found formulas but I don't know if they are correct...
1) I have to calculate the saturation pressure of the steam:
"Psat"= 6.0178*(10^(((7.5*T)-2048.25)/(T-35.85))) . .
2) I have to calculate the partial pressure of the steam:
"Pv"= ("RH"/100)*"Psat" . .
3) Specific weight of dry air:
"Pd"=("P"-"Pv")/(287.058*T) . . .It would be the ideal gas formula as a function of Temperature (T), gas constant (R), molecular weight (PM)
4) Specific weight of steam
"Pv2"="Pv"/(461.495*T) . .
Where data of humid air are:
a= 0,1358 costant van der waals b= 0,000036 costant van der waals PM= 28,962788 molecular weight R= 8,314472 costant P= 101325 Pa pressure T= 293,15 K temperature RH= 10% realtive humidity
Where data of water are:
a= 0,548 costant van der waals b= 0,000031 costant van der waals PM= 18,0153 molecular weight R= 8,314472 costant P= 230,36 Pa pressure T= 293,15 K temperature
My doubts are also about the constants and the molecular weight of humid air and water.
Anyway with these formulas and these data we have:
Saturation pressure of steam: 23,113848 Pa
Partial pressure of the steam: 2,3113848 Pa
Specific weight of dry air: 1,204057292 kg/m3
Specific weight of steam: 0,000017085 kg/m3
Final specific weight of humid air is: 1,20407437 kg/m3
So formulas are correct? a and b constant (van der Walls) e molecular weight are correct ?
Thanks a lot and sorry for my english :)