Bezalel

I'm surprised noone mentioned propane yet. https://www.genuineinnovations.com/co...-hang-tab.html

FBinNY 

Propane is very nice for things like portable cartridges. It's vapor pressure at 70° is about 120psi so it's liquid in the canister and boils off as needed to fill tires. This means that much lighter higher capacity canisters can be used compared to CO2. That's the bright side. OTOH, there's a flammability hazard, and I have no idea about the permeability of Butyl tubes to propane.

It makes sense for those that want CO2 style inflaters for larger section tires, or want the ability to use more than once, and are willing to accept the negatives. I still pump 80/20 nitrox by hand.

rekmeyata

I think she sounded weird without the intro of the gas. But she was cute to look at.

Mark Kelly 

One source I have says it's about 10 x that of air. This makes sense under the solution / diffusion model (propane will be highly soluble in butyl).

FBinNY 

So that's certainly long enough to get home.

rekmeyata

Actually I like flammable gas in tires, I can find leaks faster by simply waving a lighter over the tube until a small jet flame erupts; but my earlier gas experiment I mentioned works by smell instead of by sight. So if you're blind use the smell method, if you have sinus and hay fever problems use the sight method.

kamtsa

But you didn't, right? HillRider assertion still holds.

kamtsa

What is the motivation for this question? Are you pondering about improving something?

rsg

Just want to see if nitrogen would be any better for a bike tire since a some people use nitrogen for their car since it doesn't deflate as rapidly as o2 will.

My original question was, If nitrogen molecules are larger than o2 molecules, shouldn't the psi weight be different? for each gas? Looks like the answer is no from what I'm reading??

DiabloScott

You're asking for a scientific answer to a question that is not phrased very well (psi weight is a nonsense unit); but no, a tire filled with oxygen will weigh a couple grams more than the same tire at the same pressure filled with nitrogen. And that has nothing to do with the permeation differences of nitrogen vs oxygen through the rubber which is partly due to the physical size (not mass) of the diatomic gas molecule.

https://www.getnitrogen.org/pdf/graham.pdf

Airburst

Look at your periodic table, nitrogen has an average atomic weight of 14, oxygen has one of 16 (ignoring different isotopes, which make up a fraction of a percentage of each). Double them both, as they both form molecules with two atoms, and it's 28 vs 32.

The pressure of a gas is basically unrelated to the mass of the molecules, but it is affected by how many molecules there are. The same number of molecules is required for a given pressure, regardless of what molecules those are.

If you had two identical tyres inflated to the same pressure, one filled with oxygen and one with nitrogen, the nitrogen one would be slightly lighter.

The science I've used here involves the ideal gas law, but I don't think real gases differ much from that until you start dealing with massive molecules.


The second point people made was that it was pointless doing it, because the air you put in your tyres is mostly nitrogen anyway.

FBinNY 

Do not confuse the weight of a given amount of gas in a container (tire) with the pressure at which it's pumped into that that container.

If you're interested in the technical answer, for any container, number of molecules contained within will be proportional to the (absolute) pressure. So the weight of the various gasses in containers is proportional to the molecular weight and pressure. Twice as much pressure, doubles the weight, or a gas with double the molecular weight will weigh double at the same pressure.

The molecular weights of Nitrogen and oxygen (N2 & O2) are 28 and 32 respectively so for two identical tires filled to the same pressure, the gas in nitrogen filled tire would weigh 7/8ths what of an oxygen filled tire.

The gas in a typical road tire weighs roughly 7grams or so,at 100psi, so you could save about a gram with the lighter gas.

Mark Kelly 

That would be true of the comparison was with oxygen but it's with air. Pure nitrogen is about 3% less dense than air (1.25 kg.m^-3 @ STP vs 1.29 kg.m^-3 ) so the difference would be about 0.2 g on the first fill, less thereafter.

FBinNY 

Absolutely right, but I was writing for the OP, and it was complex enough as it was. I also figured that a potential weight savings if 1 gram (at most) was low enough to make the point.

In any case the entire nitrogen debate is totally pointless since selective osmosis already ensures that most of us are riding on nearly pure nitrogen as it is.

cycle_maven

Nitrogen- Monster cables for your bike.

JPMacG

Ah yes... nitrogen flled cable dielectrics. For those who just hate having money in their wallet.

rekmeyata

Well, don't you know, that nitrogen filled solid perfect surface silver time aligned counter spiralling geometry directional 4 gauge conductive dielectric wire incorporating spread spectrum conductors with 1 inch thick of silicone insulation incorporating copper winding shielding will make any sound system that previously sounded like a $12 transistor radio sound like your in the front row of a concert? Sure I use those, and you can get some too for your stereo, they only cost about $21,000 for 3 meter length. You need to pony up the cash and get some for your system...but be prepared for a great sonic awakening.

Don't forget you need to break these wire in first by using a special CD that uses special frequencies that you play at moderate levels for 30 days 24 hours a day. Also you need to use cable elevators to keep the wire off the floor. As a side benefit of playing the special frequencies for 30 days all rodents, insects, birds, and small mammals will be purified from your home.

Bill Kapaun

Will Helium reduce rolling resistance?

FBinNY 

I hope you're just breaking balls. If not, the super technical answer is yes, but the effect is so microscopic that for all practical (measurable) purposes its really NO.

AEO

but they are at a higher pressure than STP or SATP.
You actually need to calculate the total mass of the nitrogen while it is under pressure, which is a lot higher than 1.25kg*m^-3


PV=nRT

Mark Kelly 

No, you don't.

FB's estimate of 7g is reasonable for a 700C tyre of 23-25mm width at about 700 kPa, assuming it approximates a torus.

The difference in density is about 3% per my figures (1.29 / 1.25 ~= 1.03)

3% of 7 g is about 0.2 g.

FBinNY 

I agree with this math, but since you started with my estimate, I decided to recheck it. It's a bit off.

I quoted the 7gr. figure based on my memory of an ad I ran some 35 years ago when I made bicycle tire pressure gauges. Rechecking, I'm off by a factor of two because the 7gram figure was for both ties, not just one.

a 700x25c tire has a volume of about 21 cubic inches, and at 7atm (8ata), would hold 8 times that, or 166 cubic inches of air (at surface pressure) and at .02grams per cubic inch, 3.3 grams per tire. (doubled, and rounded to 7 for the ad 35 years ago).

So the savings of replacing the 20% oxygen component of air with nitrogen = 1/8 x .2 or .025 of that 3.3grams or roughly .08grams, but double that for the pair.

BTW- can we agree that this horse is not only dead, but that the flies are starting to get dense on the carcass?

Mark Kelly 

I didn't take your result at face value I checked it first.

Volume of torus = 2.Π^2.R.r^2, so for a 700C wheel where R is around 325mm and r is around 10mm (remember this is inside the tube) the internal volume is about 6.35 x 10^-4 m^3. Air at 700kPa has a density around 10.3 kg/m^3 so the net result is a mass of 6.6g.

7 is close enough.

That horse whiffs a bit, Doug.

GrandaddyBiker

You guys killed a horse? I am contacting PETA.

Meantime I am going out in the garage and letting air out my bike tires and refilling with fresh nitrogen and oxygen. Air gets stale you know. You change oil in your car when it gets old, don’t you? You do not want old stale air in your tires do you?

Don’t say I am being ridiculous; this thread can’t get any more ridiculous.

howeeee

[QUOTE=FBinNY;14649461]

The other thing is that nitrogen bleeds through the tube much slower than oxygen, so the rate of pressure loss will be slowed, but not as much as most people think.

So that is the pitch these Tire Dealers were making when they first started using nitrogen. Later studies and tests are saying it makes so little difference there is no reason to use nitrogen.

Many tire stores that were using it stopped.