Weight of Compressed Air

[Thordic]

Using this formula:

Air has an average molecular weight of 29 (28 for N2*.8 + 32 for O2 * .2)
1 mole of gas at STP occupies a volume of 22.4 L, therefore, 29g {~1 oz)of air at STP occupies 22.4 L

Take you tank volume in Liters (ft3X28.3 or cu in X .0164) and correct to STP (forget the temperature, just multiply the volume by 204 or 306{3000psi/14.7psi; 4500/14.7psi}. This gives you the gas volume at STP.

Divide this volume by 22.4 and you get the weight of air in oz.

Using that, you get the following information:

3000 PSI = 204 atm
4500 PSI = 306 atm

47ci = 0.77 L
68ci = 1.11 L
88ci = 1.44 L
114ci = 1.87 L

So now, just plug it into what I said above, and you get these weights:

47ci 3000PSI tank: 7.0 ounces of air
68ci 3000PSI tank: 10.1 ounces of air
88ci 3000PSI tank: 13.1 ounces of air
114ci 3000PSI tank: 17.0 ounces of air

47ci 4500PSI tank: 10.5 ounces of air
68ci 4500PSI tank: 15.1 ounces of air
88ci 4500PSI tank: 19.7 ounces of air
114ci 4500PSI tank: 25.5 ounces of air

As you can see, the weight of compressed air is not negligible, especially in higher pressure setups.

In any case, feel free to comment on it.

[the JoKeR]

Hey, I just found a monkey wrench! Let's see how it flies!


I assume these calculations are for sea level. Do they change based on altitude, or since we are dealing with a compressed gas does it matter?

[Thordic]

Temperature and air pressure have negligible effects on these calculations. I'll try to explain

Temperature in these calculations is in Kelvin, where room temp. is ~300* for STP. Any temperature you play in will be less than say ~30*K. Thats a 10% max, but more likely you are looking at closer to a 5% max. Once you run that through the numbers, you'll see that the temperature of where you are playing has very little effect on the results.

As for pressure, at pressures this high, it has very little effect. Look at it like this - Even when your tank is empty, it STILL has 14.7 PSI in it. Everything around you is under a constant 14.7 PSI. So your true tank pressure is 3014.7 when you fill it. The extra atmosphere of pressure, that 14.7, gets effectively negated by atmopheric pressure. Just like taring a scale.

Now lets assume you are playing in, say, Colorado. Lets assume the atmospheric pressure is .8 ATM, or 11.76 PSI. I don't care what the pressure is really like in CO, just work with me. That means that the pressure in your tank at top pressure is 3011.76 PSI.

3014.7 - 3011.76 = 2.94

Thats one thousandth of a percent of the tank capacity. If you want to weigh your tank in milligrams or something that will detect that, feel free

The only time atmospheric pressure would become a problem is with low-pressure storage systems. If you are storing 100 PSI, then that 3 PSI becomes 3%, which is a statistically significant number. 0.098% isn't very significant

[the JoKeR]

Sorry, but I didn't really expect you go to through that much trouble to explain it. I knew that it would be minimal, if any difference based on altitude. It is nice to get a refresher in all the stuff I forgot from high school. Guess some people really do use what they learn! Then again, you could have made all that stuff up and I'd be in no position to argue.

[Thordic]

1 year high school physics + 1 year of studying mechanical engineering + an hour on the phone with my dad, a chemical engineer = what you just saw.

My dad was the one who gave me the formula and backed up my numbers.

Besides, this is kinda the point of Deep Blue, right? I know I find these types of discussions really interesting.

Next thing I'm diving into is gas effeciency. I may need some help with that one, though, to get data on different guns.

[the JoKeR]

Originally posted by Thordic
Next thing I'm diving into is gas effeciency

I had Mexican food for lunch, so I'm all about gas!

Yes, not real Deep Blue type stuff, but I gotta be me!

[cphilip]

Good stuff Thordic. And I was one of those who had always dismissed the weight of a tank with or without air as negligable and never thought of it as being worth bothering to add into the mix. But its surpringly heavy isn't it?

[animal]

You talk about using stuff you learned... I scared myself the other day when I actually used calculus & trig while building a subwoofer enclosure. Who knew I'd use those 5 semesters of calc.

[cphilip]

Hey thordic, Let do something else with this. Calculate the difference when using pure compressed nitrogen verses using compressed "air". See what we get then? I am wondering if we can save an ounce of weight going with compressed air or nitrogen. I could guess but I would probably be way off.

[bjjb99]

If you can find a tank that will maintain a decent seal to contain it, why not go to compressed helium (cost)? A 68/3K fill would weigh a whopping 1.4 ounces. Everyone coming off the field after a heavy firefight would have squeaky voices.

For that matter, fill your opponents tanks with compressed radon gas and watch their arms fall off as they lug around 78 ounces (nearly five pounds!) per 68/3K fill.

BJJB

[clockworkmiller]

Very nice work. I like the work. The numbers look right.

Helium would be interesting, seeing as how it would be the lightest of the safe gases you can use. I would love to see someone try that.

[Donnyluv]

Helium, hmmm.
Helium is the only liquid that can not be soilidified by lowering the temperature. It remains liquid down to absolute zero at ordinary pressures, but it can readily be solidified by increasing the pressure. Solid 3He and 4He are unusual in that both can be changed in volume by more than 30% by applying pressure.

30% increase in volume,...very interesting!

[clockworkmiller]

I could be wrong, but wouldnt increasing the pressure cause it to change from a liquid to a gas?

If you increase pressure, you also icnrease heat.

[Donnyluv]

Helium is weird though. Kinda like water expanding when it freezes; nothing else does.

[Donnyluv]

Normally increasing pressure would cause a gas to change to a liquid, not the other way around. Like CO2 (even though CO2 actually goes straight from a solid to a gas).