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Calculating specific humidity/mixing ratio from temp/RH

David Meiland | Posted in Mechanicals on

First off, let me admit either partial or total unclarity regarding absolute humidity, specific humidity, and mixing ratio, and the differences between them…. but what I THINK I want is a way to calculate mixing ratio in grains per pound given temp F and RH %.

I have batches of data in Excel spreadsheets… see attached. I’d like to add a formula to each row that displays grains per pound. I know this might require more than one calculation for each data point. Can anyone offer guidance?

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Replies

  1. Michael Blasnik | | #1

    I guess no one is else is going to answer....it may be easiest to set up 2 columns for this calculation -- one for the vapor pressure and then one to convert that to humidity ratio in grains per pound.

    Pw = (RH/100)*EXP(17.863-9621/(T+460))

    W= 7000*0.62198/(29.92/Pw-1)

    where RH= relative humidity % and T= Temperature (dry bulb)

    You could combine these two equations and simplify slightly if you want. The Pw is an approximation but should be accurate within about 1% over the likely range of use. You can look up more precise but complicated equations in ASHRAE Fundamentals and elsewhere but it probably isn't necessary depending on your use.

  2. GBA Editor
    Martin Holladay | | #2

    Thanks Michael! If I had to come up with those equations, it would have taken a lot of online study and phone verifications before I felt confident enough to post them. I'm glad you volunteered to answer...

  3. David Meiland | | #3

    Thanks for the great answer to this. I did some online study, and it appeared that a lookup table was the best way to get the vapor pressure, rather than a calculation. No idea how to implement something like that in Excel, but I can plug in these equations and see what I get.

    All I am trying to do is get a general idea of whether or not we are removing moisture from a water-leak jobsite, and how quickly. The dataloggers I have do not do GPP, and the graphs they output are hard to read quickly as the dehumidifier causes a lot of heating, so the RH moves up and down. I have a Protimeter MMS that displays in GPP, but it won't log data on its own.

    This is not at all critical to the success of the job, it is a matter of curiosity for me, and obviously well above my pay grade. In a lot of situations I want to know whether we are adding or removing moisture from the environment, and RH is inconvenient unless you can hold the temperature steady.

  4. John Brooks | | #4

    David:

    I want to know whether we are adding or removing moisture from the environment

    David,
    If you only care if Absolute Humidity is going up or down....
    why not just look at the dewpoint temperature?

  5. David Meiland | | #5

    John, thanks for pointing that out, I hadn't thought about that. I'm just wrapping my head around some of this stuff, and it has made for very interesting and useful learning. If I'd been interested in this stuff as a kid I'd probably have a physics degree now.

    My handy meter displays GPP every time I turn it on, so I've gotten accustomed to thinking in terms of that number. Of course it also displays dewpoint, which is equally useful.

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