Buoyancy of full AL80 vs one with only 500 psi?

[ganu76]

Colliam, I am guessing that it was a fat-fingered. Using 38.75 does give us 1.03 lbs for every 500 psi, and 12.9 cf for every 500 psi.

However still seeing it broken down like it was above does make it easier to wrap my head around.

 

[tracydr]

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.

I beg to differ. I'd say its closer to +4 pounds, which is why most cold water divers choose to dive with steel.

 

[dfx]

(Since nobody has directly replied to this yet)

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.)

Yes, that is the case. Double the pressure = double the air = double the weight. Caveat is that "empty" doesn't really mean empty, but rather air at 14.7 psi. In practice that really doesn't make any significant difference though.

 

[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.

Bob was incorrect with his neutrality statement.

I beg to differ. I'd say its closer to +4 pounds, which is why most cold water divers choose to dive with steel.

a878bob's statement is correct just worded a bit awkwardly. It threw me at first, thus my post to clarify. An aluminum cylinder by itself at 500psi is about 3lbs positively buoyant. But once one adds a regulator and other rigging as in a stage bottle it is neutral. As a deco bottle that is great, and what I think a878bob was thinking. But when used as back gas they are more components to take into account (i.e. exposure suit, body mass, BCD, etc) so it does appear misleading.


FWIW Cold water divers like steel cause there is additional negative buoyancy associated with the cylinder compared to an aluminum cylinder. That additional negative buoyancy offsets the positive buoyancy of their exposure suit without adding additional weight.

 

[spectrum]

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

And, that would then fit nicely withsince 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.

You are correct, I fat fingered it. This always happens when I do physics before bed or when i just wake up. Search my posts, it's true!

Pete

 

[coldwaterlloyd]

steel pst 80 negative 9.3 lbs full / neg 3.3 empty
steel pst 100 negative 8.8 lbs full / neg 1.3 empty
alum 50 negative 2.4 lbs full / 1.3 positive empty
alum 80 negative 1.4 lbs full / 4.4 positive empty

 

[a878bob]

Let me se if I can recap, for those of you who dive without a regulator on your cylinder. You are correct. For those of us who dive with a regulator on our cylinder, I am correct? I think we are all correct.

Weight is weight. Air, steel, lead. It just depends on how you like to put it on your rig. Again we are all right. That's a hard concept I know.

But yesterday, when I took off one of my sidemounted al80 at about 450psi. It floated along with barely a finger under it.

 

[drbill]

This is why I use neutral buoyancy aluminium 80 tanks

 

[Colliam7]

This always happens when I do physics before bed or when i just wake up.

LOL! A great quote for the future, by the way.

 

[Agility]

For those used to metrics
1 m³ of air weighs 1.20 kg at 20°C