Talk:Vapour Pressure Deficit
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I have tried using the equation given in the present article to calculate atmospheric VPD from air temperature and RH and found the answer to be clearly wrong (off by many orders of magnitude). I then followed the second reference link to "Ohio State University Extension Fact Sheet" and followed thier equation and found the asnwer to be reasonable. I then clicked on the first reference link to autogrow.com and found a VPD calculator. The VPD calculator gave me yet a different answer. Here is my math (note I took out the scientific notation in case excel was choking on it and also tried punching this out on my TI-82 which confirmed that excel was not the problem).
Temperature: 24.04 C
RH: 44.75%
(these readings were recorded on a datalogger in a greenhouse)
Temperature conversions:
To K: 24.04 + 273.15 = 297.19 K
To F: (212-32)/100*24.04+32 = 75.272 F
To R: (what is R scale for temperature?) 75.272 + 459.67 = 534.942
Using the Wikipedia equation for VP sat:
VPsat = EXP((-18800/297.19) - 13.1 + (-0.015*297.19) + (0.0000008*(297.19^2)) + (-0.0000000000169*(297.19^3)) + (6.456*(LN(297.19)))) = 7.907E-20 kPa
VPair = 7.907E-20 x (44.75/100) = 3.538E-20 kPa
VPD = VPsat - VPair = 7.907E-20 - 3.538E-20 = 4.37 kPa
Using Ohio State University Extension Fact Sheet equation:
VPsat = EXP((-10440/534.942) - 11.129 + (-0.02702*534.942) + (0.00001289*(534.942^2)) + (-0.00000000247*(534.942^3)) + (6.456*LN(534.942))) = 0.29196 psi
VPair = 0.29196 x (44.75/100) = 0.13065 psi
VPD = 0.29196 - 0.13065 = 0.1613 Psi
0.1613 * 6.89475729 = 1.112 kPa
Using the autogrow.com VPD calculator:
VPD = 1.65 Kpa
___________________________
After quite a bit of research, I suggest the equations used by LiCor Inc. (www.licor.com) in thier LI-6400 device. These equations give results that seem to match up well with the calculations done by autogrow.com's calculator.
Reference: http://www.licor.com/env/Products/li6400/6400_manuals.jsp Click on "instruction manual" and go to book 3 pg 14-10.
The above link will give you the equation for VPsat. You must then multiply by the RH to get VPair and then subrtact VPair from VPsat to get VPD (as done in above examples).
