Water PPMV value in air and effect of pressure

[DominicASP]

I am trying to find the water concentration in ppmv of an air mixture and determine the effect of lowering the total pressure of the mixture on this water concentration.

Firstly, I would like to know if the formulas used to find the ppmv value are correct. Here's my procedure:

Working conditions:
T = 23 Celsius
Relative Humidity (RH) = 44%
Ptot = 1011 mbar


Step 1 - Find the Vapor Saturated Pressure (Pws) at Temperature (T) using a Saturation Vapor Pressure over Water Table:
For T = 23 Celsius, I find 28.086 mbar

Step 2 - Find the Vapor Partial Pressure (Pw):
Pw = RH * Pws

Step 3 - Find the Air Partial Pressure (Pa):
Pa = Ptot - Pw

Step 4 - Find the Absolute Humidity (AH):
AH = (RH * 0.622 * Pws) / Pa

Step 5 - Knowing the Absolute Humidity, find the ppmv:
ppmv = (Ma / Mw) * AH * 10^6

Reorganizing last equation, we find:
ppmv = (29/18) * {[RH*0.622*Pws]/[Ptot - (RH*Pws)]} * 10^6

For my conditions, I find:
ppmv = (29/18) * {[0.44*0.622*28.086]/[1011-(0.44*28.0866)]}*10^6 = 12401


Is that procedure correct and does it have certain limits (ex: RH value, pressure value, etc.) that would make its use incorrect?
If not, how could I determine the ppmv value for a water vapor/air mixture knowing the total pressure of the mixture and the relative humidity?


Secondly, regardless if the procedure described above is right or wrong, I would like to know the effect of lowering the total pressure of the air/vapor mixture on the water ppmv value.
If I based my reasoning on the ppmv formula above, lowering the total pressure would result in an increase in the water ppmv value.

Assuming an air/water vapor mixture in a closed system with the ambient conditions described above, at constant temperature, where, using a vacuum pump, I gradually lower the pressure of the mixture. How would the ppmv water concentration value change as the pressure lowers? Would it behaves linearly? How could this be explained using the perfect gas law?

 

[chicopee]

Steps 1 thru 4 are based on thermodynamics relationships involving a vapor in a dry air-vapor mixture; so far OK assuming that the lower case "a" stands for dry air. Steps 5 and unward, the equations seem to fit those found in industrial hygiene calculations involving contaminant concentrations in air. Your procedure appear in line with established procedures.

 

[chicopee]

There is one equation that may used to double-check your answer and it is found in the fundamental of industrial hygiene. In essence that equation states that ppm(V)= vapor pressure of one constituent divided by the total barometric pressure.

I have never tried to modify the application of this equation, however, I think that as a rough estimate you could it; again use your step 2 then divide it by the total reduced pressure. With this modified equation, You still retain your temperature and relative humidity initial conditions.

 

[chicopee]

Correction: It s/b ppm(V)= (vapor pressure of one constituent divided by the total barometric pressure)*10^06