×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

formula for relative humidity from dewpoint and tempetature
3

formula for relative humidity from dewpoint and tempetature

formula for relative humidity from dewpoint and tempetature

(OP)
Is there and approximate formula that can compute relative humidity from dewpoint and tempetature?

Thanks
Pete

RE: formula for relative humidity from dewpoint and tempetature

Look at this link:

http://www.theweatherprediction.com/habyhints/186/

Take your wet bulb and dry bulb temperatures, calculate the saturation water vapor pressure, the RH is the ratio of the two.

RE: formula for relative humidity from dewpoint and tempetature

2
Pete -

A nice program (downloadable, free) that does it for you:

http://www.pmilligan.net/pmtherm.htm

After downloading and installing, select "PMTherm.exe." Go to "psychro" on the menu and select "psychrocalc." Screen turns up that allows you to enter any two parameters, press "calc" and any other needed psychrometric paramters appear.

Only trouble is that if you have a data set, PMTherm will only do one value at a time. I believe I have a spreadsheet at work with the equation built in - email me: cbean@eheinc.com

RE: formula for relative humidity from dewpoint and tempetature

(OP)
TD2K - I think I knew that once but it has been awhile. So now the problem turns to one of calculating vapor pressure from temperature.  Is there any easy formula for that? I'm looking to plug it into a spreadsheet.

cb - It looks like PMTherm won't help because I want to do it in a spreadsheet. I'll e-mail you to see if you have the spreadheets handy.

Sorry if I'm being  a litte lazy. I'm sure this info is easily available somewhere on the internet. I haven't searched very hard yet, but I figured you guys would help me get there a little faster.


any other thoughts?
Thanks

RE: formula for relative humidity from dewpoint and tempetature

ElectricPete, drop me an email at testdog2000@yahoo.com, I've got it already in a spreadsheet if you want to cheat .

RE: formula for relative humidity from dewpoint and tempetature

(OP)
Thanks TD2K and CB for sending me spreadsheets. Both gave the same answer for a test case. TD2K's was a little easier for me to understand. Below are the equations that I extracted from his spreadsheet:

# Symbols are as follows:
> # TwbF - wet bulb temperature in degrees F
> # TdbF - dry bulb temperature in degrees F
> # TwbK - wet bulb temperature in degrees K
> # TdbK - dry bulb temperature in degrees K
> # Vwb - Vapor pressure associated with saturation conditions at wet bulb temperature.
> # Vdb - Vapor pressure associated with saturation conditions at dry bulb temperature.
> # RH - Relative Humidity
 
> TwbK:=(TwbF-32)*5/9+273;
                      TwbK := 5/9 TwbF + 2297/9
> TdbK:=(TdbF-32)*5/9+273;
                      TdbK := 5/9 TdbF + 2297/9
> Vwb:=6.11*exp((2453000/461)*(1/273-1/TwbK));
Vwb := 6.11*exp(2453000/125853-2453000/461/(5/9*TwbF+2297/9))
> Vdb:=6.11*exp((2453000/461)*(1/273-1/TdbK));
Vdb := 6.11*exp(2453000/125853-2453000/461/(5/9*TdbF+2297/9))
> RH:=Vwb/Vdb;
RH := exp(2453000/125853-2453000/461/(5/9*TwbF+2297/9))/exp(2453000/125853-2453000/461/(5/9*TdbF+2297/9))
> RH:=simplify(Vwb/Vdb);
RH := exp(239446.854663774*(TwbF-1.*TdbF)/(5.*TwbF+2297.)/(5.*TdbF+2297.))

The above expression for RH is suitable for plugging directly into an excel formula if you change the TwbF and TdbF to cell references.
An example calculation using TwbF=80,TdbF=90, yields RH= .724

Thanks again.

RE: formula for relative humidity from dewpoint and tempetature

(OP)
Looking a little closer, I see this is simply an implementation of the equations given at TD2K's link, above.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources