View Full Version : Weight of air?

Divin'Hoosier

October 27th, 2006, 09:16 AM

All of the tank specs tell you how much a tank weighs when empty. After looking around, I haven't been able to find the formula to use to approximate how much a full tank weights. How much does compressed air weigh? Is there a common value per cf of compressed air? Does it differ by max pressure (2640 v. 3000 v. 3442)?

victor

October 27th, 2006, 09:27 AM

1 cubic foot of air weighs 0.0807 lbs at standard preasure and temperature

there is approx 80 cubic feet of air in a normal rental tank at 2400 psi

so the air in the tank weighs approximatly 6.4 lbs

If you enter at 2400 and exit at 500 then you will be about 5.1 lbs lighter than when you went in.

SparticleBrane

October 27th, 2006, 09:41 AM

^ Most rental tanks here in the US are Al80s, thus they have 77.4cf at 3,000psi, not 2400. ;)

victor

October 27th, 2006, 09:59 AM

Sorry more used to BAR

Weight of air 77.4 cubic feet = 6.25 lbs

Swing between 3000 psa and 500 psi = 5.2 lbs

xiSkiGuy

October 27th, 2006, 10:09 AM

You can roughly say that every 13cf of air weighs one pound. So a E8-130 has 10lbs of air in it when full.

donacheson

October 27th, 2006, 08:04 PM

All of the tank specs tell you how much a tank weighs when empty. After looking around, I haven't been able to find the formula to use to approximate how much a full tank weights. How much does compressed air weigh? Is there a common value per cf of compressed air? Does it differ by max pressure (2640 v. 3000 v. 3442)?

Each 13 cubic feet of air (at 1 atmosphere and normal temperature) weighs about 1 pound. Hence, an aluminum 80 holds about 80/13 = 6 pounds of air. For the metric folks, an 11 liter tank filled to 200 bar holds about 2200 liters of air (at 1 atmosphere and normal temperature), which weighs about 2.6 kilograms, or roughly 1.2 kilogram per 1000 liters.

padiscubapro

October 28th, 2006, 04:56 PM

Each 13 cubic feet of air (at 1 atmosphere and normal temperature) weighs about 1 pound. Hence, an aluminum 80 holds about 80/13 = 6 pounds of air. For the metric folks, an 11 liter tank filled to 200 bar holds about 2200 liters of air (at 1 atmosphere and normal temperature), which weighs about 2.6 kilograms, or roughly 1.2 kilogram per 1000 liters.

also remember a tank filled with nitrox or trimix will have a different weight than air.. Nitrox is not significantly different but a high He mix will be considerably lighter..

see this thread at rbworld

http://www.rebreatherworld.com/decompression-gas-choices/813-how-much-does-gas-weigh.html?highlight=weight+GAS#post7356

jeckyll

October 28th, 2006, 05:32 PM

All of the tank specs tell you how much a tank weighs when empty. After looking around, I haven't been able to find the formula to use to approximate how much a full tank weights. How much does compressed air weigh? Is there a common value per cf of compressed air? Does it differ by max pressure (2640 v. 3000 v. 3442)?

http://www.techdivinglimited.com/pub/tanks.html

That should give you a fair bit of data :)

donacheson

October 28th, 2006, 09:51 PM

also remember a tank filled with nitrox or trimix will have a different weight than air.. Nitrox is not significantly different but a high He mix will be considerably lighter...

True, but probably well beyond the scope of the info Divin' Hoosier needs. <G>

Divin'Hoosier

October 28th, 2006, 10:11 PM

Great info. Thanks. Everyone has provided me the info that I need, and then some. Thanks again.

JAMIE MCG

October 29th, 2006, 11:33 AM

Myth Busters had an episode where they informed if you put a cylinder around the Eiffel Tower the air inside the cylinder would weight more than the iron,( material ), that floored me.

Divin'Hoosier

October 29th, 2006, 12:42 PM

Myth Busters had an episode where they informed if you put a cylinder around the Eiffel Tower the air inside the cylinder would weight more than the iron,( material ), that floored me.

Not quite sure I understand. So if the space under the Eiffel Tower were somehow made into a compressed air cylinder, the weight of the air would weight more than the steel in the cylinder, or the steel in the Eiffel Tower? Please explain more. I'm just curious.

SparticleBrane

October 29th, 2006, 01:38 PM

I disagree with MythBusters.

Specifically...

The Eiffel Tower (including the spire) is 1,052ft tall and at it's widest point, 423ft wide.

Assuming a perfect cylinder (pi*[radius^2]*height), that's (pi)((423/2)^2) = 140,530.5 sq. ft. as the base, times 1,052ft for the height = 147,838,094.4cf inside the cylinder

At 0.08lbs/cf that's 11,827,047.55lbs of air. The metal in the tower weighs 7300 tons; if you include everything it's 10,100 tons. That's 14,600,000lb or 20,200,000lbs depending on if you want to include just the metal or the entire structure.

JAMIE MCG

October 30th, 2006, 06:12 PM

Don't believe everything you see on T.V.

Thanks SparticleBrane for working out the equation, and I'll stop spreading this lie, still they were only a few million pounds off still mind bottling when you think about air being that heavy:confused:

SparticleBrane

October 30th, 2006, 07:38 PM

Definitely...the stuff is significantly heavy!

A set of double 130s, between 0psi and full, is a difference of almost 21lbs.

The Kraken

October 30th, 2006, 07:54 PM

Ya know, every now and then, and so rarely, a thread comes along that is just absolutely delightful . . .

and this is one of them.

Thanx . . .

the K

WarmWaterDiver

October 30th, 2006, 09:04 PM

If you raised the air pressure in the cylinder encapsulating the tower to 11.8 PSIG (the equivalent of 28 fsw depth) the compressed air would equal the weight of the entire structure given the figures above.

If you raised the pressure to 4.3 PSIG (about 10 fsw depth equivalent) it would equal the weight of the steel given the figures above.

Mythbusters just needed to top off the tank a bit . . .

I'm using 60 degrees Farenheit air temperature