Talk:Vapour-pressure deficit

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

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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 link above does not work! —Preceding unsigned comment added by 203.25.1.208 (talk) 04:00, 22 October 2010 (UTC)

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).

X
The X in the equation is so stylized that I thought it was a variable. A note should be added that the X is the times symbol but a better solution would be to rewrite the equation shown. (Even while writing this I find it hard to believe such a problem exists.) — Preceding unsigned comment added by 198.103.184.76 (talk) 19:39, 7 March 2012 (UTC)