Posted by Indiana Ken on February 22, 2013 at 17:44:06 from (66.249.235.83):
In Reply to: Winter Gas /Summer Gas posted by MarkB_MI on February 21, 2013 at 18:15:31:
MarkB_MI said: (quoted from post at 19:15:31 02/21/13) Ken, did you check out the Wikipedia article in my previous post?
"It is defined as the absolute vapor pressure exerted by a liquid at 100 °F (37.8 °C) as determined by the test method ASTM-D-323."
So vapor pressure is always absolute, NOT gauge. RVP is measured in psiA, not psiG.
Yes, I read the Wikipedia article; I did not find a reference to the pressure units. However, the link to "Another RVP Definition" defines the pressure units as PSI. Also, in the link (bottom right of first paragraph) explaining the conversion between RVP and TVP, references the units of RVP as PSI.
The reason for my question was, did you see a definition for the pressure units or conclude the units were PSIA since RVP is defined as an absolute vapor pressure?
I also checked my copy of, Internal Combustion Engines by Edward Obert, third edition, 1968 to refresh my memory. He provides a summary of the RVP test in the Fuels chapter. In the summary he specifically states the units of pressure for RVP are in PSI (gauge). It is interesting that there is no reference to RVP as an absolute vapor pressure. Perhaps the absolute title was added later or maybe he felt it was confusing enough as is - I don't know.
My take is as follows:
RVP is measured using a bomb containing 4 parts air and 1 part chilled gasoline. The bomb is sealed and immersed in a 100 degree F water bath. Once the bomb and contents have stabilized at 100 degrees F the RVP is read on the attached pressure gauge. Therefore, the pressure gauge reads only the pressure increase from the initial condition to the final condition and is in units of PSIG.
Perhaps, since the pressure reading at the final condition of 100 degrees F includes the contribution of the 4 parts air and all components of the gasoline it is called an absolute vapor pressure. I am not a chemist and don't know. I did poke around the web however I did not find anything which actually explained why it is called absolute vapor pressure. Certainly this use of the word absolute is in a different context than for units of pressure.
Finally consider the following: Assuming a typical RVP of 7 PSI; using units of absolute pressure for RVP would mean, at 100 degrees F the pressure in a closed container of gasoline would be under a partial vacuum (14.7 PSIA - 7 PSIA = 8.7 PSIA. We know this is not the case.
Correction: The math in the last paragraph is in error and should be deleted. The paragraph should read as follows:
Finally consider the following: Assume a typical RVP of 7 PSI; using units of absloute pressure for RVP would mean , at 100 degrees F the pressure in a closed container of gasoline would be 7 PSIA which is a partial vacuum. We know this is not the case.
Sorrry for any confusion this error may have caused.
Ken
This post was edited by Indiana Ken at 05:04:38 02/23/13.
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