Water in Air calculation

[Tablet]

I am trying to locate the formula that will let me calculate the amount of water in air. Either in pounds or grains
I have a spread sheet with Wet bulb & dry bulb temperatures in the same room for 6 months of working days

thanks

 

[MintJulep]

ASHRAE fundamentals, psychrometrics chapter.

 

[CESSNA1]

THE AMOUNT OF MOISTURE IN THE AIR CAN BE DETERMINED FROM THE PSYCHROMETRIC CHART. BEAR IN MIND THAT THE USUAL PSYCHROMETRTIC CHART IS FOR STANDARD AIR PRESSURE. IF THE AIR IS SIGNIFICANTLY HIGHER OR LOWER THAN STANDARD, YOU WILL NEED A DIFFERENT PSYCHROMETRIC CHART.

REGARDS
DAVE HALL

http://WWW.WSEC1.COM

 

[GBAILEY]

Here is a link to an online calculator:

http://www.connel.com/freeware/psychart.shtml

Or like Mint* pointed out, Chapter 6 2001 ASHRAE Fundamentals. If you go this route don't expect an easy answer! Most engineering libraries have a copy, and my local public library also has one...

 

[25362]

Perry VI, chapter 22, gives example 8 on how to correct for atmospheric pressures other than 1 atm.

 

[quark]

There is a nice excel spreadsheet here at,

http://www.hvacware.net/engrproj.html

Regards,

 

[25362]

The procedure to calculate the humidity in lb water/lb dry air, is based on an enthalpy balance in the process of cooling air from the DBT to the WBT. For that purpose one needs to know the enthalpies of water at both temperatures, plus the vapor pressure at the WBT, and the prevailing atmospheric pressure. The specific heat of air is generally assumed to be constant at 0.24 Btu/(lb.^oF)

As one can see a program can be worked out with the help of steam/water thermodynamic tables, but a single formula, I'm afraid, wouldn't be practical.

 

[JamesWinchester]

Take a look at

http://www.rhsystems.net

where a number of excellent papers on the subject are posted for download. There is one in particular updating the Wexler Formula for water vapor pressure to the ITS-90 temperature standard that should be of benefit.