How much air in your BC?

[SparticleBrane]

Your facts are correct (mostly). Your conclusion is not.

Assume you have an AL tank that holds 6 lbs of air. This tank is 4 lbs negative when full and 2 lbs postive when empty.

Assume you also have a ST tank that hold the exact same amount of air. This tank is 8 lbs negative when full and 2 lbs negative when empty.

There is a 6 lb shift in buoyancy with either tank.

The reason I said your facts are mostly correct is when you said, "First of all, steel is heavier than aluminium." That is true, but perhaps a tad misleading. An AL tank is usually heavier than a ST tank of the same volume (assuming we're talking cubic feet and not liquid volume).

Walter hits the nail on the head. Deefstes is incorrect. While the absolute buoyancy will be different, the amount of buoyancy swing from full to empty in cylinders that hold the same volume of air will be roughly identical.

For example:
Luxfer Al80 (3000psi, 77.4cf): -1.4lbs full, +4.4lbs empty = 5.8lbs swing
Faber HP80 (3500psi, 80cf): -13.22lbs full, -7.22lbs empty = 6lbs swing
PST E7-80 (3442psi, 80cf): -8.5lbs full, -2.5lbs empty = 6lbs swing
PST LP80 (2400psi+10%, 80.6cf): -7lbs full, -1lbs empty = 6lbs swing


....are you getting the picture?

 

[Deefstes]

OK, this was fascinating. So let me get this straight, the advantages then of ST cylinders over AL is NOT that you get less buoyancy swing from full to empty but just that the cylinders do not become positively buoyant when empty?

 

[SparticleBrane]

OK, this was fascinating. So let me get this straight, the advantages then of ST cylinders over AL is NOT that you get less buoyancy swing from full to empty but just that the cylinders do not become positively buoyant when empty?

Generally speaking, that is the advantage -- it allows you to wear less weight since the cylinder isn't quite as positive at the end of the dive.

 

[elan]

OK, this was fascinating. So let me get this straight, the advantages then of ST cylinders over AL is NOT that you get less buoyancy swing from full to empty but just that the cylinders do not become positively buoyant when empty?

Do not be that fast in changing your opinion. I think I'm joining your camp here have made some calculations and formula stuff and it turns out you are right - AL gives more swing. Give me some time I will post it here.

Sorry, guys for questioning your being right

 

[BobinNC]

This has been a very interesting discussion and perfect timing I might add. Thanks to Deefstes for bringing it up. Last night I was diving in a quarry and weighted myself at the start of the dive as I was taught: Weight yourself to when you fully dump your BC air and exhale you start to sink and then fully inhale and should come back up to where your eyes should be at the surface. This worked great in the pool and for my OW check out dives.

In the quarry last night I had no problem getting down to 65ft and tooling around a bit but on our way back up the SS I was making sure I had all the air out of my BC and had to struggle around 20ft with a downward swimming angle of attack to maintain my depth, if I sat still I kept floating up even with fully exhaled lungs and holding. Now I know the problem and to add another 4lbs and plan for the end of the dive vs. the beginning. Thanks!

 

[SparticleBrane]

Do not be that fast in changing your opinion. I think I'm joining your camp here have made some calculations and formula stuff and it turns out you are right - AL gives more swing. Give me some time I will post it here.

Sorry, guys for questioning your being right

You can run as many calculations as you want, but the average weight of air isn't going to change.

 

[jmorriss]

I want to be negative (slightly) even at 2 feet depth. I do not ever want to rise from my safety stop to the surface at any rate faster than ultra-slow.

I'm also a fan of getting to 100 feet as fast as possible, so while I don't grossly overweight myself anymore, I do have more weight on me than it would take to be neutrally buoyant anywhere in the water column.

Negative descents rock-its like skydiving...albeit a bit slower :cool2:

I prefer to be a touch heavy anyway, makes it easier to hunt lobsters and what not. First few times we jumped off the boat I really learned to hate AL-80s

 

[Steve50]

Do not be that fast in changing your opinion. I think I'm joining your camp here have made some calculations and formula stuff and it turns out you are right - AL gives more swing. Give me some time I will post it here.

Sorry, guys for questioning your being right

take your time..

 

[elan]

You can run as many calculations as you want, but the average weight of air isn't going to change.

The thing is it's not the only component in the equations so it does not really matter that it's not going to change It is in fact not even involved in the final formula.

Here we go: As a base I took the initial formula from wikipedia which is right (I checked it myself)
Buoyancy - Wikipedia, the free encyclopedia

http://upload.wikimedia.org/math/a/9/e/a9e680082e41ce9d9675bf7dd680c5f2.png

Check it out and if you find anything wrong please let me know. I want to find the truth here

The steel tank taken is HP100 say filled to 77.4 cuf to make the mass of air the same.

 

[SparticleBrane]

The thing is it's not the only component in the equations so it does not really matter that it's not going to change It is in fact not even involved in the final formula.

Here we go: As a base I took the initial formula from wikipedia which is right (I checked it myself)
Buoyancy - Wikipedia, the free encyclopedia

http://upload.wikimedia.org/math/a/9/e/a9e680082e41ce9d9675bf7dd680c5f2.png

Check it out and if you find anything wrong please let me know. I want to find the truth here

The steel tank taken is HP100 say filled to 77.4 cuf to make the mass of air the same.

Even with all that I am failing to see how the buoyancy swing will be different in an aluminum cylinder vs. steel. The metal the cylinder is made from isn't going to affect the buoyancy from full to empty, since the metal isn't changing throughout the dive.

The ONLY thing changing throughout the dive is how much gas is inside the cylinder, which effects the weight and average density (and thus the buoyancy) of the cylinder. The inherent buoyancy characteristics of the cylinder's metal isn't going to change.


I believe your calculations are canceled out by what I posted earlier with regards to cylinder sizes vs. buoyancy swing. Read that list carefully again -- I'll post it below, plus the physical sizes of the cylinders in question:

Luxfer Al80 (3000psi, 77.4cf): -1.4lbs full, +4.4lbs empty = 5.8lbs swing
Faber HP80 (3500psi, 80cf): -13.22lbs full, -7.22lbs empty = 6lbs swing
PST E7-80 (3442psi, 80cf): -8.5lbs full, -2.5lbs empty = 6lbs swing
PST LP80 (2400psi+10%, 80.6cf): -7lbs full, -1lbs empty = 6lbs swing

Note that all cylinders in question are 7.25" in diameter, and weight quoted is when empty with no valve:

Luxfer Al80s are 26" tall and weigh 31.4lbs.
Faber HP80s are 20" tall and weigh 32.5lbs.
PST E7-80s are 20" tall and weigh 28lbs.
PST LP80s are 24" tall and weigh 34lbs.


....so here we have 4 different cylinders, each with roughly the same buoyancy swing from full to empty (the Al80 has 5.8lbs swing while the others are 6lbs swing...incidentally the weight of the 2.6cf of gas the Al80 is 'missing' vs. the others would make up for the 0.2lbs of buoyancy swing missing....imagine that!), yet they are all different sizes, weights, and one is made from a different metal.


Hopefully this makes my point that the buoyancy swing of a cylinder while diving, from full to empty, is dependent on the volume of air expelled rather than the metal or size of the cylinder in question.