On 11/11/2014 9:50 AM, dpb wrote:
On 11/11/2014 9:30 AM, Jeff Strickland wrote:
"dpb" wrote in message ...
...

Actually, it's not that tough to get pretty good estimate...

14.7/(14.7+30) = 0.33 -- 33% air by fraction of total pressures. So
w/o purge you're only going to be 2/3rds or so N...

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That cannot be right, but I have no idea the formula that is needed.
...

It _IS_ right for what it is... _Air_:N ratio. I just didn't go ahead to
then account for the 80% N content in the entrained air (as clare
pointed out the oversight). See the followup there.

*ADDENDUM*

...It _IS_ right for what it is... _Air_:N ratio....

More precisely, it's Air:Total ratio.

And, to explain if don't see it, it's simply the absolute pressure ratio
instead of gauge pressures. 14.7 is normal atmospheric pressure; hence
when the tire gauge reads '0' that's what's there. The total pressure
when the gauge reads 30 psi is then roughly 45 psia (or 30 psig).

Dalton's Law says partial pressures are additive.

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