## Wet bulb calculation

## Wet bulb calculation

(OP)

I have the dry bulb temperature and relative humidity in % and would like to calculate the wet bulb. it is 15 years of data or 4800 rows of Excel, therefore a formula will be very helpful instead of using psychrometric chart.

Regards

Cogen

Regards

Cogen

## RE: Wet bulb calculation

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

or you might find something useful at:

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

Regards,

## RE: Wet bulb calculation

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.

## RE: Wet bulb calculation

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.

## RE: Wet bulb calculation

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

## RE: Wet bulb calculation

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

Harvey

## RE: Wet bulb calculation

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

TTFN

## RE: Wet bulb calculation

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

## RE: Wet bulb calculation

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

## RE: Wet bulb calculation

David

## RE: Wet bulb calculation

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