Tire Pressure and Altitude

[JohnD10]

Originally Posted by TexasGolfBum

What do we that have nitrogen filled tires do?

Put air in them instead and save your wasted money on the nitro for beer.

[TitanMike]

Originally Posted by elchilero53

Claiming that tires act as a molecular sieve for O2 leaving N2 behind is a lie, but people today are so utterly unsophisticated that they fall for this garbage.

Sorry but this phenomena has been demonstrated several times.

A company named Branick Industries sold for years a "Nitrogen Generator" that was no more than a reverse osmosis membrane system that separated larger N2 molecules from the smaller atoms/molecules of the remaining gases in "Air".

Tests of tires over a two year period proved that the same happened inside tires with percentage of Nitrogen actually increasing.. The tire industry has for almost a century attempted to achieve a perfect barrier between the air in the tire and the outside atmosphere to keep them from losing air. Even today they have come up short as elastic materials have enough space between molecules to allow passage of smaller molecules like Oxygen and atoms like Argon, Helium, etc.

What's strange is that a company that manufactured a device to sell to the tire industry so everyone could jump on the Nitrogen bandwagon actually through their own research proved it was totally unnecessary.

Btw, I didn't come by this information second hand, I got it straight from the leadership at Branick Industries as I was testing one of their Nitrogen generators in our own training facility.

[johntaylor]

Originally Posted by Joe_GA

Altitude does not change the pressure of a gas (air) in a closed container. According to Boyle's Law, pressure in a closed container changes if:

1.) temperature changes
2.) number of molecules increases or decreases
3.) volume changes

In other words, temperature and pressure of a gas in a closed container are directly proportional. If the temperature decreases, the pressure decreases and vice versa. Yes, it is true that atmospherical air pressure decreases as altitude increases, but that is air that is not contained in a closed container.

Atmospheric pressure IS contained in the tire, together with the pressure measured by the pressure gauge.

Everything must be calculated using ABSOLUTE pressures and temperatures. The air in the tire is not at gauge pressure, it is at atmospheric + gauge = absolute pressure. Your pressure gauge does not measure the air pressure in the tire, it measures the DIFFERENCE between the air pressure in the tire and the atmospheric pressure. That's why it's called gauge pressure.

Similarly, deg F is an arbitrary scale. Temperatures must be factored in absolute units, with 0 at the absolute zero of temperature.

In post #12, I added the gauge and atmospheric pressures to determine the ABSOLUTE pressure of the air in the tire, factored that for the absolute temperature ratio between the two conditions, and then deducted the atmospheric pressure at the new altitude to get back to GAUGE pressure.

The pressure/temperature relationship is Gay-Lussac's law. When you add air, it's an adiabatic process. Boyle's law (pressure-volume variation at constant temperature) is irrelevant to tires, as is Charles' law (volume-temperature variation at constant pressure).

[JohnD10]

Originally Posted by johntaylor

Atmospheric pressure IS contained in the tire, together with the pressure measured by the pressure gauge.

Everything must be calculated using ABSOLUTE pressures and temperatures. The air in the tire is not at gauge pressure, it is at atmospheric + gauge = absolute pressure. Your pressure gauge does not measure the air pressure in the tire, it measures the DIFFERENCE between the air pressure in the tire and the atmospheric pressure. That's why it's called gauge pressure.

Similarly, deg F is an arbitrary scale. Temperatures must be factored in absolute units, with 0 at the absolute zero of temperature.

In post #12, I added the gauge and atmospheric pressures to determine the ABSOLUTE pressure of the air in the tire, factored that for the absolute temperature ratio between the two conditions, and then deducted the atmospheric pressure at the new altitude to get back to GAUGE pressure.

The pressure/temperature relationship is Gay-Lussac's law. When you add air, it's an adiabatic process. Boyle's law (pressure-volume variation at constant temperature) is irrelevant to tires, as is Charles' law (volume-temperature variation at constant pressure).

A whole lot easier to just read the side of the tire(s) in question and fill it to what it says.

[johntaylor]

Originally Posted by TitanMike

And if you keep topping your tires off as they loose air naturally, that percentage of Nitrogen will slowly increase to about 92-95 % which is about the best concentration you get when a tire is initially inflated with pure Nitrogen.

Exactly! Best argument against buying nitrogen (aside from the fact that for road vehicles, it's a scam). Oxygen permeates out faster than nitrogen, which means the nitrogen concentration in the tires slowly increases over time.

[johntaylor]

Originally Posted by JohnD10

A whole lot easier to just read the side of the tire(s) in question and fill it to what it says.

Oh, exactly. I'm the author of a text book on the subject of aerostatics, with hundreds of equations on the pressure-temperature-volume-humidity relationships of gases, but I just fill my tires to what it says on the sidewall too. But the guy did ask the question ...

[mwdilday]

Air pressure at sea level is 14.7 PSI. Air pressure at 10,000 feet is 10.1 PSI. So you will have an INCREASE in tire pressure reading of 3.6 PSI if you travel from sea level to 10,000 feet all else being equal. You will see a temperature decrease of about 3.3 degrees per 1,000 feet. This will DECREASE you tire pressure about 1-2 PSI for every 10 degrees decrease in temperature.

https://www.engineeringtoolbox.com/a...ure-d_462.html

[Joe_GA]

Originally Posted by TitanMike

I agree with all you said but in a tire, the "closed container" is not rigid. It is somewhat elastic so as it increases in temp, pressure can increase physical dimensions somewhat, causing a variation actual measured pressure. They used to call automobile tires "balloon tires" for good reason.

Personally, I have not checked tire pressures at sea level and again at 7000 feet when the ambient air temperature remained the same. That might be difficult to find. However, if you took an inflated balloon from sea level to 7000 feet, the balloon would be larger and may even pop. The reason for that is the pressure that johntaylor mentioned. The atmospheric pressure decreases and allows the balloon to expand. So, that would be a viable explanation for a decrease in tire pressure at 7000 feet altitude as the "container" volume has increased. And, volume was item 3 of Boyle's Law.

What do you think, OldCoot?

Anyway, this could make your head hurt. Like someone said, let's just put the proper amount of air in our tires!

[Tom48]

Originally Posted by HangDiver

None! The decrease in tire pressure that you're seeing is due to lower ambient temperatures than where you started in TX. If anything, gaining altitude which reduces ambient air pressure would yield a slight increase in tire pressure.

https://www.tirerack.com/tires/tiret...jsp?techid=167

That is consistent with my experience. In fact I stopped checking COLD TIRE PRESSURE in the cold of morning because the increase in pressure when it hits 90+ out and I am rolling down the road was scary.

I check COLD TIRE PRESSURE, before I move the rig, but in a warmer part of the morning or afternoon before I roll out.

[mwdilday]

Originally Posted by Joe_GA

Personally, I have not checked tire pressures at sea level and again at 7000 feet when the ambient air temperature remained the same. That might be difficult to find. However, if you took an inflated balloon from sea level to 7000 feet, the balloon would be larger and may even pop. The reason for that is the pressure that johntaylor mentioned. The atmospheric pressure decreases and allows the balloon to expand. So, that would be a viable explanation for a decrease in tire pressure at 7000 feet altitude as the "container" volume has increased. And, volume was item 3 of Boyle's Law.

What do you think, OldCoot?

Anyway, this could make your head hurt. Like someone said, let's just put the proper amount of air in our tires!

That makes me think of a funny story - at least funny to me. Climbing out in a Cessna one day probably to about 10,000 I heard a loud pop. Never figured out what it was until I landed and found a bag of Lays potato chips had popped and they were all over the place. Yes increase in altitude causes balloons and any closed bag to expand. That is why the put vents on humidors.

[babock]

The difference in atmospheric pressure between sea level and 7000 feet is only around 3 PSI. The difference in air temps will cause more pressure change than that!

Basically, the pressure you measure with a gauge is the pressure above atmospheric pressure. That is why it is called PSIG.

[Boomerweps]

Since air is over 80 percent nitrogen, IMHO, not much difference with temperature. AFAIK, nitrogen reacts less with the tire rubber compared to oxygen so that can affect pressure loss over time.
The old thumb rule on temperature change affect on tire pressure is you lose 1 psi per 10 degree drop. Sounds like that’s just what you encountered.

[mwdilday]

Originally Posted by Boomerweps

Since air is over 80 percent nitrogen, IMHO, not much difference with temperature. AFAIK, nitrogen reacts less with the tire rubber compared to oxygen so that can affect pressure loss over time.
The old thumb rule on temperature change affect on tire pressure is you lose 1 psi per 10 degree drop. Sounds like that’s just what you encountered.

So why do tires loose pressure over time? Always wondered that.

[TitanMike]

Originally Posted by mwdilday

So why do tires loose pressure over time? Always wondered that.

The permeation of gases through the tire itself.

[MR.M]

pressure increases as you go up in altitude . it can be hot at 7500 feet . 90 this summer was a scorcher , so when i head up the mountains i don't worry about pressure as much as when i fill up at 7500' and head down . then i make it a point to fill to max cold on the first am out of the mountains

[milkman55]

This thread kind of detoured into a discussion on nitrogen filled tires. Just to report I am back in good ole Texas and the pressure in my tires returned to 61 psi front and 63 psi rear and I never touched them. When I saw they were all down a relative amount at 7,000 ft and 41f, I had to assume it was just climate. I drove about 2,500 Miles this trip and the tires and TPMS did great.

I know some recommended adding and letting out air to adjust, but I never noticed any difference in handling. I also never saw anyone in all the RV parks I stayed at in Colorado adjisting their tires. It stayed within a 10% range the entire trip.

[TitanMike]

Originally Posted by milkman55

This thread kind of detoured into a discussion on nitrogen filled tires. Just to report I am back in good ole Texas and the pressure in my tires returned to 61 psi front and 63 psi rear and I never touched them. When I saw they were all down a relative amount at 7,000 ft and 41f, I had to assume it was just climate. I drove about 2,500 Miles this trip and the tires and TPMS did great.

I know some recommended adding and letting out air to adjust, but I never noticed any difference in handling. I also never saw anyone in all the RV parks I stayed at in Colorado adjisting their tires. It stayed within a 10% range the entire trip.

It might be a good idea to add air if it drops below a pressure needed to support your load. Of course only an issue if you are loading your tires to near max load to start with.

[tlspencer]

Originally Posted by mwdilday

That makes me think of a funny story - at least funny to me. Climbing out in a Cessna one day probably to about 10,000 I heard a loud pop. Never figured out what it was until I landed and found a bag of Lays potato chips had popped and they were all over the place. Yes increase in altitude causes balloons and any closed bag to expand. That is why the put vents on humidors.

Just ask any grocery store stocker how many bags of anything they have to toss because it's bagged at near sea level and their store is "up there", LOL. I live just over a mile high. I'm always afraid of touching the chip bags that haven't exploded yet but are soooo close!

[Larry-NC]

Originally Posted by TitanMike

Sorry but this phenomena has been demonstrated several times.

A company named Branick Industries sold for years a "Nitrogen Generator" that was no more than a reverse osmosis membrane system that separated larger N2 molecules from the smaller atoms/molecules of the remaining gases in "Air".

Tests of tires over a two year period proved that the same happened inside tires with percentage of Nitrogen actually increasing.. The tire industry has for almost a century attempted to achieve a perfect barrier between the air in the tire and the outside atmosphere to keep them from losing air. Even today they have come up short as elastic materials have enough space between molecules to allow passage of smaller molecules like Oxygen and atoms like Argon, Helium, etc.

What's strange is that a company that manufactured a device to sell to the tire industry so everyone could jump on the Nitrogen bandwagon actually through their own research proved it was totally unnecessary.

Btw, I didn't come by this information second hand, I got it straight from the leadership at Branick Industries as I was testing one of their Nitrogen generators in our own training facility.

Let's see. The atomic weight of an Oxygen molecule (two atoms, its natural state) is 32 grams/mole. The atomic weight of a Nitrogen molecule (two atoms, also its natural state) is 28 grams/mole which is lighter, but the difference really isn't significant. If smaller molecules were to diffuse more rapidly, Nitrogen would leave the tire faster than Oxygen, not slower.

As Mike suggests, a more interesting test would be to see how fast the trace of helium (second smallest molecule in nature) diffuses from a tire. We all know how fast (overnight) it diffuses through a party balloon.

Larry

[babock]

Originally Posted by Larry-NC

If smaller molecules were to diffuse more rapidly, Nitrogen would leave the tire faster than Oxygen, not slower.

Nitrogen molecules are larger than Oxygen molecules because of the fact Oxygen's molecular weight is more...not the other way around. Has to do with the molecular cloud size. Oxygen has 8 protons and the electron cloud gets drawn to it more than the 7 protons in nitrogen. Interestingly enough, that isn't the case with Hydrogen or Helium which have very small molecular sizes.

What is surprising is how fast CO2 leaks out of tires. I use CO2 cartridges for flat repairs on the road with my bicycles. Tires filled with CO2 are completely flat in the morning. If you look at a molecular size chart, CO2 is smaller than both oxygen and nitrogen.