Vapour Pressure

Tue Sep 14 01:56:14 GMT 1999

>Date: Sat, 11 Sep 1999 07:40:49 -0600
>From: bearbvd at sni.net (Greg Hermann)
>Subject: Re: DIY_EFI Digest V4 #518
>
>>Date: Fri, 10 Sep 1999 15:24:29 -0700
>>From: "John Dammeyer" <johnd at autoartisans.com>
>>Subject: Vapour Pressure
>>
>>Hi All,
>>
>>Does anyone have the formulae for Vapor Pressure?  If I assume RH is 75%
>>and can
>>measure the Air Temperature inside the intake manifold what is the 
calculation
>>for vp?
>>
>>Thanks,
>>
>>John
>
>You need some combination of psychrometric charts or GOOD steam tables. A
>good steam table will include a section giving the equations upon which 
the
>values stated in the tables are based.
>
>If RH is 75%, the VP will be 75% of the VP (partial pressure of H2O vapor)
>at saturated conditions for that temp.
>
>Greg
>

John,
	To calculate the saturation vapor pressure, you need the Goff-Gratch 
formula, which is:
log10(Ew)= -7.90298(Ts/T-1) + 5.02808*log10(Ts/T) - 
1.3816*10^-7*(10^(11.334(1-T/Ts))-1) + 8.1328*(10^(-3.49149(Ts/T-1))-1) + 
log10(Ews)

where
	Ew = saturation vapor pressure in millibar
	T = abs. temp. (in K)
	Ts = steam point temp (=373.16K)
	Ews = saturation pressure of water @ steam-point temp. (=1013.246 
millibar)

	There's a handwritten correction to this formula in my reference book - 
8.1328 has been marked as 'should be 0.0081328'. To check this, you should 
get the following results

T (C)		pressure (mb)
10		12.272
15		17.044
20		23.373

This was from the Smithsonian Meteorological Tables, 1951 edition.

Relative humidity = Vapor Pressure/Saturation Vapor Pressure

I take it 75% RH refers to ambient conditions - the RH will change 
drastically as the air passes through the throttle body.

There's a whole mess of if, buts and empirical corrections in this area of 
meteorology. You'll probably need to get a hold of a good reference and 
check it out.

Cheers,
Stuart Baly.
Bureau of Meteorology, Australia.

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