Miracle gas

[Steve Brown]

I've read some of the threads about filling tires with nitrogen and just wanted to give some feedback on effectiveness, though not on my airplane yet.

I bought a setup from Harbor Freight, which has two sizes. My initial application was filling bicycle tires. I recently started commuting by bicycle, and the morning flow works best if I keep my bicycle in the house. Keeping and using a compressor in the house is obnoxious so I decided to go with nitrogen.

I knew it was purported to reduce leaking, but I thought it may be one of those things which is theoretically true, but results in only incremental improvement.

My commuter bike has 120 PSI rated tires which I fill to 130. I probably loose about 5 of that in the filling process. Previously, I needed to fill the tires twice a work week, and still noticed softening. That means a fill would last about 2 days.

After switching to nitrogen, I only need to fill once a week, with no noticeable softening of the tires.

We also filled our mountain bike tires with nitrogen to 30PSI / 40PSI for front/rear. After 2 weeks, loss ranged from 1PSI to 1/2 PSI on the 4 tires.

So, I'm sold on this. I bought the big tanks just in case I would want to use this for the airplane. As soon as I get my hanger at RHV I'm moving the big tank to the airport and I'll get a small one for the bikes.

I expect this will translate into negligible pressure loss between oil changes.

[airguy]

There is some very slight differential in the leak rate between plain air and nitrogen, but the biggest difference comes from the lack of oxygen inside the tire that is available to slowly oxidize the rubber inner tube, and the lack of moisture which can promote bacterial attack.

From a chemistry/physics perspective, the only difference between air and nitrogen (since air is 78% nitrogen) with regard to the pressure loss is that oxygen molecules are considerably smaller (even though they are close on the periodic chart, the molecular interaction is way different) so it will perfuse through the rubber (leak down) faster than the nitrogen molecules will. By filling with pure nitrogen you avoid losing the oxygen in the first place, so your leak-down rate is lessened by 22%. The remaining trace gases in atmospheric air do not contribute appreciably to the issue.

[fl-mike]

Originally Posted by airguy

From a chemistry/physics perspective, the only difference between air and nitrogen (since air is 78% nitrogen) with regard to the pressure loss is that oxygen molecules are considerably smaller (even though they are close on the periodic chart, the molecular interaction is way different) so it will perfuse through the rubber (leak down) faster than the nitrogen molecules will. By filling with pure nitrogen you avoid losing the oxygen in the first place, so your leak-down rate is lessened by 22%. The remaining trace gases in atmospheric air do not contribute appreciably to the issue.

If the oxygen molecules leak out faster, wouldn't you eventually end up with mostly nitrogen anyway? (assuming you continually top off rather than refill).

[Michael Brown]

Also, Nitrogen pressure does not change with temperature.

[n5lp]

Originally Posted by Michael Brown

Also, Nitrogen pressure does not change with temperature.

When nitrogen is a gas (almost all the time) it follows Boyle's Law. http://en.wikipedia.org/wiki/Boyle%27s_law

[breister]

Originally Posted by fl-mike

If the oxygen molecules leak out faster, wouldn't you eventually end up with mostly nitrogen anyway? (assuming you continually top off rather than refill).

The permeability of a substance (rubber) to a gas is generally in relation to the size of the molecule. Some will leak out faster, some slower than Nitrogen - but not necessarily. Oxygen is usually O2 (two atoms bound together), while Nitrogen (I believe) tends to be inert and come as simply a single atom N. So, I'll just go with the empirical evidence here and say, sure. That said, I suppose that if you overfill your tires with air to 120% rated pressure that, once the other gasses leak out, your pressure should be perfect.

[Dave Cole]

Nitrogen exists in the atmosphere as N2, which is a molecular inert gas.

[BrentHumphreys]

Originally Posted by Michael Brown

Also, Nitrogen pressure does not change with temperature.

Mr Boyle would disagree.

[Steve Brown]

Originally Posted by airguy

........ By filling with pure nitrogen you avoid losing the oxygen in the first place, so your leak-down rate is lessened by 22%. ........

Greg,

The practical benefit in terms of how often the tires need to be filled is at least 2:1 in my unscientific experiment.

Nevertheless, another possible contributing factor to my great success with this could be temperature.

With the compressor, I tend to turn it on to fill the reservoir, then fill the tires. So, the compressed air is warm. Cooling due to pressure reduction is minimal, since my compressor only fills the tank about just over 130PSI.

With the nitrogen tank, the gas starts at ambient temperature, and probably has some significant cooling going from over 1500 PSI down to 130 PSI. Then, as the gas warms up to ambient in the tire, the pressure would actually go up from where I filled it.

I'm not familiar enough Boyle's law to fully understand where in my filling system the energy (heat) goes, but my understanding is that the gas must cool with the pressure reduction.

Anyway, unscientific or not, all I care about is how often I need to fill the tires. Once I get some results for the airplane tires I'll report back.

[BrentHumphreys]

Originally Posted by Steve Brown

Greg,


I'm not familiar enough Boyle's law to fully understand where in my filling system the energy (heat) goes, but my understanding is that the gas must cool with the pressure reduction.

Boyles law as we are discussing doesn't really apply to filling. The statement was made that Nitrogen pressure doesn't vary with temperature.

The combined gas law more applies

That statement isn't consistent which says that given a fixed mass of gas the product of the temperature and volume of the gas will remain constant. (note that this is an ideal gas, which Nitrogen behaves as an ideal gas at temperatures in this discussion)

From this law we can determine the volume or temperature of a gas as it is heated and cooled using the relationship P1V1/T1=P2V2/T2

So, assuming the volume of the tire will remain constant we are left with.

P1/T1 = P2/T2

Temperatures here must be Absolute.

So if we take 1 atm of pressure at 0deg C (273 Deg K). And raise it's temperature to 20deg C. (293 deg K)

We get..

1/273 = P2/293

Solving for P2 = 1.073

For those more interested in imperial units.

Raising a fixed volume of a gas at atmospheric pressure (14.7 psi) from 32 degrees F to 68 degrees. The pressure will rise to 15.772574 psi