Weight of the air in the tank.

[Mike1967]

Just a math check, really.

If I'm sporting a 100 cu. tank, I assume that means 3000 PSI with 100 cu. ft. sea level air.

Per the great font of all knowledge temporal and otherwise (Wikipedia) air has a sea level density of

0.0765 Lbs/ft^3.

So, the air in there has a weight of 7.65 Lbs. (Is this correct)?

So, if I climb up on the boat at 500 PSI, then I've blown off 6.375 Lbs (is this correct)?

Thanks.

Check this out
‎Diving Buddy Lite - Scuba Pre-Dive Check Aid

 

[scubadada]

0.0807 lbs/cf in any cylinder. A AL80 has 77.4 cf of air at 6.25 lbs. At 500 psi you have 12.9 cf of air at 1.04 lbs, a difference of 5.21 lbs.

 

[Alan Browne]

Have you done a buoyancy check at the end of the dive with 500 psi in your tank?

No. But I have at 1100 PSI for lack of weight because of the hotdog idiot DM in Cuba who refused my weight request. Yes, I was that stupid.

 

[Alan Browne]

Watch those zeros!

I think it is more like 12,927 grams per 1,000 liters.

366.05 grams per cubic feet

Or something like that

My brain hurts now.

While I'm a very metric guy, I kept this all in PSI, cu-ft and LBS to keep things simple since the basic reference was a 100 ft^3 tank (and then I screwed that up - 3300 PSI, not 3000).

Why I didn't even reply to the metric version.

 

[runsongas]

Because air actually weighs about 1293g/1000 liters.

only at 0C/32F.

but if you want to challenge the density of 1.225 g/L at 1.01325 bar and 15C/59F then feel free to show a source.

 

[tursiops]

only at 0C/32F.

but if you want to challenge the density of 1.225 g/L at 1.01325 bar and 15C/59F then feel free to show a source.

Maybe I was just trying to agree with you? He said 800g, you said 1225, I said 1293. Whatever.

 

[Steve_C]

The less than 3300 the OP saw could also be due in part to hot fills which would not be surprising if they are cranking through a lot of dives. Also there is nothing in it for the shop to give folks more air since crew and boat make the same on a slightly shorter dive as a longer one.

 

[BRT]

The real reason is they are filling them in groups with the AL80's and they all get filled to the same pressure. The people with big tanks tend to leave them emptier and the heat buildup iis greater than for partial fills.

 

[Alan Browne]

The less than 3300 the OP saw could also be due in part to hot fills which would not be surprising if they are cranking through a lot of dives. Also there is nothing in it for the shop to give folks more air since crew and boat make the same on a slightly shorter dive as a longer one.

The last place I dove (Roatan Fantasy Island) seemed willing to do anything reasonable to make people happy. We will be returning there this March (95%). I've asked them to convert those 100's to Nitrox. Ria replied that it should be okay, but no confirmation so far (I assume they have the expertise/qualifications to do so). I'll also ask that they be filled to 3300 if possible since I tend to go through air fast (that's the next thing I'm working on - better buoyancy and trim to reduce air consumption).

While we were there, I believe the tanks were charged the day before, so plenty of time to cool ( I didn't look into their pressure station to see if there were water baths but must assume there were...). On the boat the tanks were at least 3000 Lbs (don't recall if I saw any at 3300 ... but definitely above 3000).

They were definitely patient about people getting in full dives - no pressure from DM's to get back to the boat. I'd often be one of the first on the boat and it could be 20 minutes or more for the last divers to get back on board.

The boat captain, John, used to dive as well (fishing - a nasty and dangerous business from his telling) so extremely patient.

 

[Alan Browne]

The people with big tanks tend to leave them emptier and the heat buildup iis greater than for partial fills.

Guilty as charged. I'd sometimes hit the safety stop at 400 PSI. In 15' of water that lasts quite a while...

I'd assume though, that when filling, they have water circulating to pick off heat...