Try this

http://members.nuvox.net/~on.jwclymer/wet.html

or you might find something useful at:

http://www.martindalecenter.com/Calculators.html

Regards,

You need an equation for vapor pressure of water vs. temperature, such as the Antoine equation (do a Google search, or fit it yourself from steam tables.

From dry bulb temperature, you calculate the equilibrium (saturated, or 100% humidity) vapor pressure of water. That times the actual humidity (%/100) gives the actual partial pressure of water. Then invert the vapor pressure equation to find the (wet bulb) temperature corresponding to that vapor pressure.

Hi cogen,

The procedure described by DickRussell will give you the Dew Point and not the Wet Bulb Temperature. But you need the Dew Point anyway to get to the WB Temp.

The correct procedure is described well at
http://meted.ucar.edu/awips/validate/wetblb.htm

Unfortunately it is an iterative procedure. I'm not too clever at doing iterative things in a spreadsheet, but on this web site it is claimed that 10 iterations is enough to get the correct answer.  I would just copy the formula into 10 adjacent columns where each one feeds its answer into the next column, and then I would take the answer from the last column. With a bit of experimentation you will quickly see if the answer is converging within the 10 iterations.

I happen to have needed to calculate WB Temp for an evaporation-pond project so I followed katmar's link and ran through 10 iterations on 36 temperature/humidity pairs.  The average WB Temp at the end was 8.5K different from the start and a couple of values changed by over 25K (8%).  

My error function was rapidly approaching an asymptote after 5 iterations, the last 5 didn't really add much.

With the "precision" of my starting data (i.e., a standard NOAA average of averages, blended into a mean), I'm not sure that it was worth the added complexity of the spreadsheet, but with better data the iterative process looks to be worthwhile.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

David,

Thanks for the feedback.  I was too lazy to test the rate of convergence, so I appreciate your efforts.

Harvey

Dave,

Have you tried to use Solve or Goal Seek?  Either should be able to directly iterate the calculations.

TTFN

I use goal seek all the time, but I don't know how it could be applied here.  It requires the variation of a single value and I don't know which value would fit.

In most equations, you would guess at Wet Bulb temp and goal seek on an error function.  This process uses a pretty complex guess to start the process.  I can see how to do it in MathCad, but I think that Excel lacks the horsepower to do this one.

David

Dave,

Can you post your sheet on your website?  If not, let me know and I'll email you at your address from the website.

I'd be interested in looking at it; just out of curiosity.

Thanks...

TTFN

It is a work in progress and pretty messy, I don't think I'll post it on my website.  No problem sending it if you e-mail me.

David

On a range Tdry bulb=15..40[deg C] and rh=10...90[%]you can calculate the Twet bulb[degC] at P=1006[mbar](=755 Torr)with max relative error~6% with an approximative polynomial model that you can put in a cells. For lower Tdry and rh the relative error will be higher.
Twet bulb= A*Tdry + B*rh + C*Tdry^2 + D*rh^2+ E*Tdry*rh + F
 where
A=5,391260E-01
B=1,047837E-01
C=-7,493556E-04
D=-1,077432E-03
E=6,414631E-03
F=-5,151526E+00

regards
m777182