Buoyancy of full AL80 vs one with only 500 psi?

[ganu76]

Title pretty much says it all. What is the difference in buouancy between an AL80 tank at 3000 psi and that same tank later at 500 psi? I seems like I've seen numbers somewhere saying that your tank is approx 5 lbs more positively buoyant at 500 than when it was full at 3k.

If this is correct then it does line up with the "1 lbs of buoyancy gained for every 500 psi used" statement I've read (if THAT is even correct).


Thanks in advance.


(Edited to reword "What are the weights..." to "What is the difference in buoyancy...". Thanks Herman.)

 

[herman]

I assume you mean bouyancy as opposed to weight, those are 2 different things. The actual weight of a tank varies with brand, model and the valve used. The tank gets lighter and more buoyant as you use air from it. Most people say the change is 5 lb from 3k to 500, a 4 lb loss is closer and what I use for.

 

[a878bob]

Air weighs about .08lbs per cubic ft. So 80cuft is about 6.4lbs. You do the math.
Bouyancy for an average al80 at 500psi is close to neutral. Depending on the rigging and 1st stage.

 

[Scared Silly]

Air weighs about .08lbs per cubic ft. So 80cuft is about 6.4lbs. You do the math.
Bouyancy for an average al80 at 500psi is close to neutral. Depending on the rigging and 1st stage.

On average a full AL80 is -2 lbs whereas empty it is +4 lbs. Thus with about 2000 psi the cylinder is neutral. At 500psi it is about +3 lbs. As said above add in a regulator and if using it as stage bottle with rigging it is about neutral. Which is why they are great as a deco bottles but otherwise plan to add some weight.

 

[ganu76]

Thanks Herman. Yeah, I misspoke about "weight". So from your experience the change is closer to 4 lbs? 5 lbs was something I'd heard from SOMEWHERE, just couldn't recall where.


Ok Bob, so with THOSE numbers how much air do we have in our tanks at 1000 psi, 2000 psi, etc? I'm assuming that the figures aren't linear (i.e. since 3k psi is 80 cu ft then 1500 = 40 cu ft, 750 psi = 20 cu ft, etc. I'm assuming that's not the case.)

 

[spectrum]

Bob was incorrect with his neutrality statement. For the purposes of this discussion you can use the .08 /CF for each cubic foot of air and a linear relationship.

Also a fill is really on 77.4 CF at 3000 PSI and the common Al80 cylinders are 4.4 or 4 pounds positive when empty...

So.......... 3000 / 77.4 = 38.75 PSI/ CF

500 / 33.75 = 14.8 CF at 500 PSI

14.8 * .08 = 1.18 pounds of air at 500 PSI

4.2 (average) positive buoyancy of empty cylinder -1.18 pounds of air = 3.02 pounds positive at 500 PSI.

 

[ganu76]

Silly and Spectrum, these formulas are going into my dive book.

 

[TSandM]

If you figure a pound for every 13 cf of air, it's close enough for government work. Figure out how much of the tank you are willing to use in cf, then you have the swing weight.

 

[dumpsterDiver]

I might even remember that one.. my pony bottle has 1 lb of air in it!

 

[Colliam7]

So.......... 3000 / 77.4 = 38.75 PSI/ CF

500 / 33.75 = 14.8 CF at 500 PSI

OK, am I missing something? Why was '33.75' used instead of 38.75? Shouldn't it be 500 / 38.75 = 12.9 CF at 500 PSI? And, therefore

14.8 * .08 = 1.18 pounds of air at 500 PSI

should it be: 12.9 * .08 = 1.03 pounds of air?

And, that would then fit nicely with

If you figure a pound for every 13 cf of air, it's close enough for government work.

since 500 PSI weighs about 1 pound, and 500 PSI in an AL80 is 1/6 of 77.4, which just happens to be 12.9 (~13) CF?

Just curious.