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Old 04-22-2011, 03:30 PM   #11
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Thanks everyone for weighing in. When I checked the tires, they were cold. According to the rating chart that Glenn attached, I was well within a safe range with the weight we were pulling. The tire sidewall does indicate a max 50 psi with a max weight of 1360 lbs. That info, btw, is very difficult to read but here is what is clearly legible: Made in China. I don't know about you, but I have zero confidence in the quality of Chinese products based on the horror stories that have come out of China in recent years regarding their lack of quality control.
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Old 04-22-2011, 04:26 PM   #12
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I don't know if it is relevant or not, BUT I purchased a bag of Doritos in Jacksonville, Fl. where I live, and went to Tennessee on the Blue Ridge pk.way, and that un opened bag of chips was making all sorts of funny noises inside a Wal mart bag in the back seat; we kept wondering what the heck was making that noise, and discovered that bag was about ready to burst! Apparently, the air pressure when filled and sealed in sea level Florida, was so much greater at 3500 feet(guess) above sea level, it made the bag do that. Also, I recall a kid's TV show many moons ago, where they took normally inflated balloons down in a sub, and they lost all of the air in them. Upon returning to the surface, they magically went back to normal. This same thing effect tires???? HMMMMM
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Old 04-22-2011, 04:43 PM   #13
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I am researching this thread regarding two "head scratchers."

1) What IS the coefficient delta between nitrogen and a 78/20 mix of oxygen and nitrogen. IE if air changes 5 PSI due to a temp change what would 100% nitrogen do under the same conditions.

2) Would anybody care if I did.
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Old 04-22-2011, 07:02 PM   #14
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I hope I didn't give the impression I was advocating Nitrogen for inflating tires. I was just noting that less variance with temperature is one of the claims made by its advocates. I have not figured out how much difference there would be between air which is already 78% Nitrogen and pure Nitrogen (as you noted), but there are those which swear it is so. I personally think (that's scary isn't it!) that the result is not because of it being pure nitrogen, but the fact that it is "dry", i.e no humidity/moisture. If you removed the moisture from plain old regular air, you would probably have about the same result with more consistent pressure.

Just my thinking, feel free to rally to my cause or revolt as you deem appropriate.
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Old 04-22-2011, 07:10 PM   #15
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1.)My POINT IS does low air in the tires change at a higher altitude, or not? And, if so, then if i topped off my 50 lb. tires at 50 lb. cold in Fl, and drove to say the smokies, or someplace like that, would my air pressure now be dangerously over pressurized, risking a blow out or not?
2.)I for one would like to know the answer to Herk's puzzling strange phenomenon.
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Old 04-22-2011, 08:40 PM   #16
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From everything I have read there is a slight increase in tire pressure in higher elevations, but not nearly as much as with temperature changes. I saved this formula from someone over on RV.NET which seems accurate. I have no idea if nitrogen filled tires react differently.

"
The complete formula is (P1 * V1)/T1 = (P2 * V2)/T2. It's fairly safe to assume the volume doesn't change in a relatively minor change in temperature. Normally, the "T" number is Fahrenheit Absolute, which is regular F +460.

As an example, if you set the pressure at 120 psi when it's 40 F and the recheck it at 80F, and ignore the tire volume, P2 woud be 120*540/500, or 129.6

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Old 04-22-2011, 09:37 PM   #17
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My understanding, and this is not an area in which I claim a lot knowledge, so correct me if I am wrong, is that there is no change in pressure in a SEALED container with changes in elevation assuming all other variables remain constant.

In the example of a sealed chip bag, when the chips were placed in the bag, no air was pressurized into the bag. The air inside was at atmospheric pressure, which matched the exterior pressure and so they canceled each other and the bag was pliable. Once you went up in elevation the atmospheric pressure decreased. The bag still contained the original higher atmospheric pressure, but now the pressure on the exterior of the bag has decreased and so the pressure inside the bag is greater than that outside and the bag expands under the force of the greater pressure, but the pressure in the bag stayed the same. This is assuming all other variables remain constant.

The change in pressure in tires is primarily from changes in temperature.
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Old 04-22-2011, 11:55 PM   #18
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Quote:
Originally Posted by lbrjet View Post
The complete formula is (P1 * V1)/T1 = (P2 * V2)/T2. It's fairly safe to assume the volume doesn't change in a relatively minor change in temperature. Normally, the "T" number is Fahrenheit Absolute, which is regular F +460.

As an example, if you set the pressure at 120 psi when it's 40 F and the recheck it at 80F, and ignore the tire volume, P2 would be 120*540/500, or 129.6
"
Gas laws - Wikipedia, the free encyclopedia

We could go crazy here with formulas, but the question posed has more to do with the "pure Nitrogen" vs "Air" debate of which there are over a hundred separate threads on this forum alone.

It is true, that if you keep the volume of a gas constant (ie in a fixed non-inflateable tire) and increase temperature, the pressure will rise. Decrease temperature and the pressure will fall.

Similarly, in a fixed volume, raising or lowering the pressure OUTSIDE the container; NOTHING inside the container changes. Pressure stays the same since the volume of the tire is fixed.

If the tire was flexible enough to inflate like a balloon, it would expand until the pressure inside was the same as the pressure outside (or it popped). If you put it underwater it would deflate as the outside pressure increased. However tires are not that flexible so temperature is the only thing that changes internal pressure.
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Old 04-23-2011, 07:59 AM   #19
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FWIW, it is almost impossible to get 100% (pure) N2 in tires. Unless there is some way to completely purge the existing air out of a tire, a 95% N2 fill is about all that can be expected.
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Old 04-23-2011, 10:20 AM   #20
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Lou,

Sorry, I have not been on the forum long and did not realize there was a "nitrogen vs air" debate. I wasn't trying to start anything.

Also, thank you for explaining what I was attempting to say about the laws of gas far more clearly than I did.

I generally try to confine my comments to things about which I have knowledge, but after awhile the silence gets to me and I have to say something.......regret it.......start the process over.

Glenn
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