On 11/10/2014 5:58 AM, trader_4 wrote:
....

And Consumer Reports did a test:

http://www.consumerreports.org/cro/news/2007/10/tires-nitrogen-air-loss-study/index.htm

"The test started on September 20, 2006 and the final measurements
were taken on September 20, 2007. The results show nitrogen does
reducepressure loss over time, but the reduction is only a 1.3 psi difference
from air-filled tires. The average loss of air-filled tires was just 3.5
psi from the initial 30pressure setting. Nitrogen-filled tires lost an
average of 2.2 psi from the initial 30 psi setting.


....

Now at least that is some real data. Over a year, ~ 1 PSI difference in
pressure. And that was with CR purging the tires 3 times to get all the
air out before the final nitrogen filling. And that is supposed to
translate to less warranty claims at Costco? Less blowouts? Less carnage
on the roads? Give me a break.

+25

I rarely find anything in CR that I think very useful, but here's a case
where they actually seem to have done something of value.

And they illustrate the trouble with "statistics" produced by
proponents--or at least one needs be aware of how can be twisted along
the way.

That 1.3 psi average differential in pressure drop sounds like a lot
when expressed as a percentage of the air-filled --

(2.2/3.5 - 1)*100% = -37%

That seems sizable but if recast as the loss relative to the air the
increase in rate is

(3.5/2.2 - 1)*100% = +59%!!!

It's the latter number the proponents are fond of bandy'ing about to
make one think it's a really, really, big deal.

But, as you point out on the realistic scale of what does it mean to
actual tire pressure, it's only 1.3 psi out of 30 psi or

(3.5-2.2)/30*100% = 4.3%

And, on a more timely basis, that translates to only something barely
over a tenth of a pound difference per month.

It all just "shows to go ya'" one has to be aware of what the divisor is
in such comparisons--decreasing it grossly inflates the percentages as
1/x--inf as x--0 very rapidly as x gets smaller.

So in the larger scheme of things that is what really matters; namely
the actual tire pressure, it just isn't that big of a difference to be a
significant factor in whether a tire is/isn't under-inflated. It's
still going to be the maintenance the owner/operator supplies that's the
controlling factor by far.

The one question I've not taken time to actually calculate is what would
be the water vapor contribution of, say, 50% RH at 70F cold inflation
pressure to see what drop there is owing to the freezing of the water to
essentially remove that contribution. Of course, for highway driving
it's unlikely except in the truly most extreme cold that air temperature
wouldn't rise to above freezing, anyway, restoring a significant
fraction of that loss...

--