Posted by Indiana Ken on February 20, 2013 at 17:50:39 from (66.249.235.112):
In Reply to: Winter Gas /Summer Gas posted by John in La on February 19, 2013 at 19:43:49:
Yes, I understand what you are saying. My issue is how you compare atmospheric pressure to RVP. Perhaps I am nit picking however, allow me to explain.
RVP is a gauge pressure and has the units of PSI (pounds per square inch). Pressure gauges are calibrated at a reference pressure of 1 atmosphere (there are exceptions) and indicate the pressure above that reference point. That is to say the RVP gauge will read zero PSI when exposed to a pressure level of 1 atmosphere.
One atmosphere is 14.7 PSI however it is referenced to a pressure level of zero, that is the pressure, if the earth had no atmosphere. Since the presssures are referenced to difference values, to compare, the atmospheric pressure must always be added to the gauge pressure. This is the called the absolute pressure; one can think of it as the total pressure. Absolute pressure has the units of PSIA (pounds per square inch absolute), absolute, denoting the reference point is from zero pressure.
Therefore, a RVP of 14.7 PSI would be equal to 14.7 (atmospheric) + 14.7 (gauge) = 29.4 PSIA; 29.4 PSIA is not the "same" as one atmosphere of pressure (14.7 PSI). And, a RVP of 0 PSI would be equal to 14.7 (atmospheric) + 0 (gauge) = 14.7 PSIA which is the "same" and equal to one atmosphere of pressure.
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