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.

All it takes for the He balloon to float is that the density be less
than that of air--nothing even close to the 95% fraction would be
required so it's not a useful comparison.

I was thinking the ratio wouldn't be quite so high altho the 80% N in
air is the saving grace; just hadn't computed the actual fraction
previously.

It would be interesting to see the CR data/experiment w/ a control
population that were just backfilled w/ N alongside the purged set they
did just to see if it showed up.

After the above computation I'll concede it is sufficient to likely be
able to produce the net savings of roughly 1.3 lb less diffusion
pressure drop over a year than air; I'll still contend that isn't
sufficient difference to be of any real value to the end user.

In large part that that's so can be shown because the temperature swing
from yesterday afternoon's high of 80 F to this morning's low of 19F
(it's all the way up to 22 F now at 10 AM) results in a pressure swing
of (20+460)/(80+460)*30 psi = -3.3 psi or 2.5X that annual loss
differential. It's just not significant difference in what happens w/ N
in lieu of air for passenger car applications.

So the car this morning is underinflated but by this time two weeks from
now if were to correct for it it'll be back to seasonal normals and
they'd be over-pressured by about that much.

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