Re: How do I calculate GPP if I know the %RH and Dry-Bulb Temp?

Hi Pat,

This is probably the most specific question I've had to answer, and I've 
gotta say that without google I'd be 
totally stumped about your question.  

The best answer I've been able to find is courtesy of the 
 American Society of Heating, Refrigeration and Air-Conditioning 
Engineers.  Not only are there lots of free programs available to do 
the calculations, but David Butler says:

     First, you need to create a look-up table for Saturated Vapor
     Pressure over the temperature range of interest. A good reference
     is the 2001 Fundamentals, Chapter 6, Table 3, page 6.9, under 
     column marked Absolute Pressure (psi).
     Next, you need to calculate the Partial Vapor Pressure, based on 
     conditions:
     PVP = SVP * RH * .01
     where,
     PVP = Partial Vapor Pressure (Psia)
     SVP = Saturated Vapor Pressure (Psia)
     RH = Relative Humidity (%)
     Next, you need to calcuate the Humidity Ratio (pounds of water 
     vapor per pound of dry air):
     W = 0.62198 * PVP / (BP - PVP)
     where
     W = Humidity Ratio (lbs/lb)
     BP = Barometric Pressure (Psia)
     0.62198 = commonly used ratio of the molecular mass of water 
     and dry air
     Note: BP(psia) is approximately BP(inHg) / 2.036. If you don't 
     know the BP, then just use 14.696.
     Once you have W, you can convert to grains of water vapor per 
     pound of dry air by multiplying by 7,000.

Another option for you is that since you're already using a 
thermohygrometer, if you've got $295 to spend, you can buy a new 
one that measures GPP for you.

I hope this helps.
Jeff Yap
Mad Scientist