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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

1982shawn
June 7th, 2015, 04:09 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.


You forgot to pressurize the tank.
(Empty Volume/14.7psia)(max psi)= compressed volume
(147,838,094.4/14.7)(2400)"well say its an LP tank"
(10057013)(2400)=24136831738 cft of compressed air in the giant scuba tank
(24,136,831,738)(0.08)=1,930,946,539lbs of air, almost 2 billion pound of air!

Steve_C
June 7th, 2015, 04:30 PM
Except of course the tower is very much not a cylinder so its volume is much less than the volume of a cylinder of the same height.
Probably come closer with a cone but even that over estimates since the sides are very much concave. So tbones calculation shows even if you widely over estimate the volume but fix the amount of steel myth busters is still way off.

halocline
June 7th, 2015, 06:18 PM
I guess its a slow sports day today.

KWS
June 7th, 2015, 11:25 PM
8# per hundred cu ft air

nitrox is so close it is the same

helium I think is about 2# per hundred


and no some of the specs don't seem to follow the 8 per hundred.

Its possible that some groups specs are weight empty no valve and the full is with a valve.

Divin'Hoosier
June 8th, 2015, 01:14 PM
Always entertaining to see threads spring back to life ... almost 9 years after the last post in the thread!!

Leadking
June 13th, 2015, 01:08 PM
I had an instructor friend tell me that the weight shift in an aluminum tank was greater than a steel tank. I told him the weight shift is equal in both tanks. He didn't believe me.

emoreira
June 13th, 2015, 01:17 PM
I found this table somewhere in Internet. The table is in spanish. however, the information is pretty clear.

Gill
June 13th, 2015, 01:43 PM
Always entertaining to see threads spring back to life ... almost 9 years after the last post in the thread!!

They know how to use the search function. Yeah, that's it.

duckbill
June 13th, 2015, 11:44 PM
I had an instructor friend tell me that the weight shift in an aluminum tank was greater than a steel tank. I told him the weight shift is equal in both tanks. He didn't believe me.

You're both correct......or incorrect.....depending.

He is right in that an aluminum 80 has more of a weight swing from full to empty than a steel 72 does.

You are right if you are simply referring to an equal swing caused by equal amounts of air being emptied.

It was probably just a disagreement based on a misunderstanding of the eachother's rationale.

dberry
June 14th, 2015, 12:48 AM
This is a very odd, confused thread, IMO. In any case, if you're talking about equal amounts of air leaving two tanks, the change in weight will be exactly the same (ca. 8# per 100 cf as normally calculated.) That would be comparing an Al 100 to a steel 100, etc. Now the PERCENTAGE change in weight of the tank would be greater for the lighter (Alum) tank, but the "swing" due to the air in pounds would be identical.

Now, something that some folks fixate on is that an Alum 80 tank changes from negative to positive buoyancy when dropping from 3000 to 500 psi, but a steel tank stays negative (although less negative at 500psi). In other words, an empty Al 80 tank will float, but an empty steel tank will sink. But in terms of the buoyancy of the diver + rig + tank, the change in weight from using 2500 psi of air is the same, so if you're properly weighted (slightly negative with 500 psi), what's the big deal?

You need less lead with a (heavier steel) tank, but otherwise, why does the change from neg to pos buoyancy of the alum TANK make any difference at all? All I can think is the trim changes, since the empty Al tank becomes a "float" on your back, but the lead is on your belt / pockets. All the more reason to put some weight in cam band pockets, I guess...)

Anything wrong with my thinking?

duckbill
June 14th, 2015, 01:18 AM
Anything wrong with my thinking?

Nope.....with one caveat........


Now the PERCENTAGE change in weight of the tank would be greater for the lighter (Alum) tank, but the "swing" due to the air in pounds would be identical.


This is true if using underwater measurements of "weight". On dry land, the opposite would be true. (Not to add to any confusion :headscratch:) This is because in most cases equivalent capacity aluminum cylinders are actually heavier than their steel counterparts. :idea:

dberry
June 14th, 2015, 07:10 AM
Wow, and I thought my steel 119s were heavy on land!

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