I came across this the other day: http://powertank.com/index.html . It would be much preferable to my small air bottle. I put off putting together a small nitrogen tank setup all summer.

Do any of you who who are in an ME or other such program have a handy resource for the practically expressed coefficients of thermal expansion for CO2, Nitrogen, and Dry Air? I've gone thru all my texts and failed.

The Powertank lit says for CO2 it's "similar" to dry air (approx 1 psi per 17 degrees F), which seems pretty reasonable. I presume its even lower for Nitrogen, but by how much?

Scott, who is always finding something else he could use.....

 

Funny you mention this.

I think I'm going to run nitrogen bottles next year. I crew for a friend of mine who has 2 big nitrogen bottles in his van, and just one bottle lasts an entire sprint race season, complete with air tools and numerous tire fill ups.

$15 to refill at a welding shop. Just gotta fork out for the original bottle (usually exchange them).

I may run 2 smaller ones though. His bottles are huge. 220 cubic feet. That's like a 5 foot tall bottle!

Warren

 

So Scott, do you think Kleinubing runs Nitrogen?

 

LOL! Andrew!

I know someone that used to race formula ford and used it for that reason...very little expansion (if any)

Its inert...so why not run it, and not worry about Hot tire pressures.

 

Hmmm, I thought PK was just a regular air breathing hominid....but maybe he runs on nitrogen....what, you think he has a battle chassis underneath human flesh?......that would explain alot.

Scott, who's pretty sure nitrogen pressure goes up as temp goes up....it's a gas law....it's moisture that really kicks the pressure of typical compressed air...that's latent heat / change of state stuff......

 

Oh. Ok Thanks.

 

OK, I did a little more searching and found this: http://www.goodyearaviation.com/tech/tireinf.html

So, Nitrogens 1% pressure gain per 5 degrees F means that at 30 psi initial pressure at 100 degrees F run to 200 degrees F (which is the max recommended operating temp for Hoosiers) means that we'd gain 6 psi and so end up at 36 psi.

CO2 at 1 psi per 17 degrees F is awfully close. And air is 78% nitrogen and 21% oxygen.

The other sites and documents I found suggest that oxygen attacks tires and that tire life is improved with nitrogen. I don't think this could be much of a factor on a tire that only lives for about 5 hours.

I like the safety advantages of the CO2 setup, and it looks like total system cost is about the same for either.

Anybody know anything else about this stuff?

Scott, who refuses to use tire-liefs....

 

Other than a possible storage benefit - i.e. lots of 'air' in a small package - this is useless for trying to keep pressures constant. Just because its intert doesnt mean anything, air is too mostly. Remember the ideal gas law from chem class?

PV = nRT

If you heat a gas up, at a constant volume (like a tire out on a track) the pressure will go up... simple as that. Air, nitrogen, oxygen, etc... all the same.

-Ryan

 

I don't think he was addressing that point with the "inert" comment. He seemed to be addressing the fact that since its inert, it can't hurt anything to use Nitrogen. This is in contrast to something like Hydrogen (Not a noble gas), which would not be very wise to use in a situation you are increasing temperatures.


[Modified by mojoGSR92, 2:53 AM 12/19/2001]

 

Put a drier filter containing descant on the outlet line.

 

The only thing I'm looking for is the ability to carry more inflatant to the track. We all know that lots of people use nitrogen because it's dryer, but nobody ever knows if there are different coefficients of thermal expansion and what they are, or if CO2 is a suitable substitute. It appears that where tires are concerned CO2 would be just fine. The reason for selecting it over nitrogen would be on the basis of the lower storage pressure of mostly liquid CO2 and hence the lower weight of the bottle (aluminum vs steel), and the greater practical capacity of a CO2 bottle of equal size. The fact that this CO2 setup comes with a carrying cage / regulator protector, whereas I'd have to make one up if I put together a nitrogen bottle setup is attractive too.

The next question is what about shock charging. Is CO2 ok for that? I don't want to have two setups at twice the money.

Scott, who always has another question......

 

Hmm. I saw somebody carrying one of these around @ the 12 Hours @ Thunderhill. We were just about to do a tire/driver change so I forgot about it. Might be a great way to run airtools. I wonder how long one would last. I am going to post on the NASA yahoo list.

 

Scott:
I remember watching NASCAR Garage (yes, I'm a closet nascar watcher ) and they mentioned that all the teams use nitrogen b/c it doesn't expand as much when compared to regular air.

yoshi - who goes off to do a search at tnn's nascar site

 

Let's look at that equation, though. We're comparing the behavior of two gasses at the same temperatures (well, change in temperatures) from the same starting pressure. Volume will remain mostly constant (the tire doesn't get much if any bigger, and the size change should be the same regardless of gas) so we'll leave that out. R is a constant so let's leave that out, too. That leaves us with:

P = n * T

n is the number of moles of gas. So, what this means, is that IF n is differnet between, say, air, CO2, and N2 (nitrogen is N2, right?), then the speed at which pressure varies with temperature will be different.

So, is n different? Let's find out:

PV = nRT goes to n = PV / RT. Pressure doesn't depend on the gas (cuz you're inflating to the same initial temp), V is the same for each gas, R is the same, and T is the same for each (ambient air temp). Therefore, n *should* be the same regardless of gas, since it's a function of a bunch of "constants" (here I mean the values aren't variable with gas).

Now, the question becomes less clear when you're talking about air, because there's other stuff in air, like pollution and water vapor and stuff. Unfortunately, I don't even know where to begin analyzing that. However, I would conclude that no matter what kind of gas you put in your tires, as long as that gas is pure, it will have exactly the same thermal expansion behavior.

-Mike

 

Ahhh, but you're referring to the number of moles of a gas.

http://quiz2.chem.arizona.edu/prepro...ole_of_gas.htm - read the "DISCUSSION" portion

Wanna play? http://jersey.uoregon.edu/vlab/Piston/

Summary: all moles are not created equally - some are bigger than others.

All that, and my GF (who is a chemist) says so. ;-)

Jarrod

 

I only read the first URL, but, I don't see how it actually contradicts anything I wrote above.... It seems to agree that the ideal gas law does, in fact, say that 1 mole of any gas does in fact occupy the same volume at the same temperature and pressure. At first, I thought the basketball sized glove sounded a little small until I remember that it's probably at significantly above 1atm pressure being that the glove is increasingly resisting expansion.

Also, I *know* some moles are *heavier* than others. But the ideal gas law says they all take up the same amount of space when they're a gas at the same temperature and pressure, unless I'm mistaken.

So anyway, I guess I'm still missing something. Wanna try again?

-Mike

 

I have a powertank and they work Great I used it for about a year now. They hold alot of pressure. You can run air tools if needed.

Glenn

 

The ideal gas equation is just that...idealized. It was developed by assuming that the compressibility factor Z is equal to 1, or that pv = RT (be careful which R you use... the one in the common pv=nRT is actualy R-bar, or R * molecular weight). This relationship is not exact, particularly for higher reduced pressures (p/pc > .05) or lower reduced temperatures (T/Tc < 15). Typical values of ambient pressure and temp fall within the acceptable range for using the ideal gas law, but the small differences in expansion under increased temperature are due to the slightly different compressibility factors.

 

I only have a little to add to this, don't want to rock the boat

Yes, it's the "ideal" gas law. But it works perfectly fine for 99% approximations. There would be no measurable difference in the pressure of a tire filled with pure oxygen, pure nitrogen, pure CO2, or any combination of these on the track after a session.

The real culprit here is the absorbed water in compressed shop air. Compressed air holds a lot of water, and this water probably exhibits a consoderable vapor pressure inside the tire when it starts to heat up.

Remember that the real value of the ideal gas law is in calculating changes in a closed system, which requires discussion of neither moles nor the gas constant.

Take a tire. V is constant. PV=nRT. P1V=nRT1. P2V=nRT2. P1/T1= P2/T2. P2= P1*T2/T1.

Ah, this leads us to a wonderful experiment! Take any mounted tire on a rim. Put in known pressure (P1) of maybe 35psi out in the garage on a cold day. Note the temperature (T1). Bring it inside and leave it for 24 hours, then note the temperature (T2) and measure the pressure (P2).

I'll bet P2(actual) > P2 (theoretical), and it's from the water vapor pressure.

Repeat experiement with bottled (dry) N2 or any other gas. I'll bet you $5 that P2(actual) = P2(theoretical).

PS - be sure to use T in degrees Kelvin

(edit - damn smileys!)

 

Now, to reply to the original question

The product Scott mentioned is *liquid* CO2. A liquid will yield far more gas at atmospheric pressure than any compressed gas will. CO2 liquefies at a much lower pressure than O2 (or air), so liquid CO2 tanks are cheaper than liquid O2/air tanks.

It actually looks like a pretty good product. Like a propane grill tank, as long as there is liquid in the tank, the regulator can supply constant high pressure. I bet the promary bottle pressure is really, really high before the regulator.

 

OK, new day, and my brain may be working better....

The biggest reason you see the jump in pressure when tires get hot (and they have water vapor in them) is because of the boiling point of water - 212 degrees F, which happens to be in the neighborhood of optimum operating temperatures.

As soon as the water changes state from vapor (just water in the air) to gas (past the boiling point), it requires magnitudes more space.

I'll read back and try to figure out where my brain was yesterday, Mike. ;-)

Jarrod

 

Damn! Finally something I can contribute. When water (solid) converts to steam (gas) it expands 1700X (give or take). The expansion & the Law of Latent Heat of Vaporization are why water is still one of the best ways to put out fires.
Mike, Local 1965, MVFD