Water Vapor - Other Formulas
My programs implement a number of formulas which claim to compute saturated water pressure
at a given temperature.
However, from time to time I run across new formulas - and occasionally, flat out wrong formulas.
Since I don't want to be continuously updating my program,
I will simply add these "new" formulas here.
The Engineering ToolBox
The Engineering ToolBox
is a source of fairly reliable data.
As a result, I was shocked (and disappointed) to find that their
Saturation Pressure of Water Vapor
formula is totally different than all the other formulas I've collected.
pws = e(77.3450 + 0.0057 T - 7235 / T) / T8.2
T in kelvins
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In general, the results agree fairly well with GolfGratch up to about 0°C,
then it begins to strongly diverge. For instance,
(based on this table)
T (C) GolfGratch (mb) EngToolBox (mb)
-15 1.91 1.9
- 9 3.09 3.0
2 7.05 6.9
25 31.652 31.30
46 100.84 101.4
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Only GISS-AR4 (NASA) is worse.
Unfortunately, they do not reference their source for this equation -
it is merely presented as "a statement of fact".
A Google search for that equation
returned numerous hits.
Unfortunately, all the examples I checked are obviously just quoting The Engineering ToolBox
- either an explicit reference or the identical text and formatting.
In addition, on their
Dry Bulb, Wet Bulb and Dew Point Temperatures
page they say
The SI unit is Kelvin (K). Zero Kelvin equals to -273°C.
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which is, of course, wrong!
It should be -273.15°C.
(Yes - it does make a difference! Particularly in a reference book.)
Author:
Robert Clemenzi