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Water concentration in air in terms of PPMV

Water concentration in air in terms of PPMV

(OP)
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?

RE: Water concentration in air in terms of PPMV

Volume concentration is expressed in ppmv units. For moisture in air, this unit refers to volume of water vapor in certain volume of air. Changes in pressure or temperature do not affect concentration, as long as there is no phase change (= no partial condensation of water from vapor phase). However please note that relative humidity and dew point will change, because the ability of air to hold moisture changes with pressure and temperature.

If you start pulling vacuum on a system that contains X ppm of water in air, you would be evacuating air together with water molecules. Assuming that the air-water mixture is homogenous, the resulting ppm of water in air will remain unchanged. Think of this as if you have 1 liter bottle of 5% solution of NaCl in water. Now start decanting the solution and filling an empty glass with this solution. Will the NaCl concentration in the bottle change? No.

For psychrometry calculations, check http://www.humidity-calculator.com/index.php and http://www.daytonashrae.org/psychrometrics_si.html

Dejan IVANOVIC
Process Engineer, MSChE

RE: Water concentration in air in terms of PPMV

(OP)
Thank you very much for your answer. I am currently doing an experiment where I try to determine the water concentration in a dry air / water vapor mixture. I am measuring a "parameter" and I want to link the values of this parameter to the water concentration inside my system.

I work in a closed system, at room temperature. I basically start at atmospheric pressure and progressively reduce the pressure inside my system until I reach 0.4 psi while taking measurement throughout the process.

For my initial working condition at:
T = 23 Celsius
Relative Humidity (RH) = 44%
Ptot = 1011 mbar

I have noticed that as I decrease the pressure inside my system, I seem to notice that the effect of water on what I am measuring decreases (linear trend with R2 = 0.99) but I am still trying to understand why. From an Internet source:

If a system is isothermally compressed (compressed with no change in system temperature) then the relative humidity of the system increases because the partial pressure of water in the system increases with increasing system pressure.

So if I lower my pressure, the relative humidity should decrease. Therefore, I should not be worry about having a saturated vapor mixture, ie. no phase change.

Based on what you said previously, the PPMV (concentration of water in my mixture) should not change as the pressure lowers but the RH will. Since the total pressure goes down, and Ptotal = Pvapor + Pair, Pvapor also goes down. Therefore, RH will go down since RH = Pvapor / Psaturated at that Temperature.

RE: Water concentration in air in terms of PPMV

Yes, that's about it.

Dejan IVANOVIC
Process Engineer, MSChE

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