Tanks and buoyancy...

[mudguppy]

In regards to buoyancy, are all tanks created equal
....meaning....
in comparing a steel verses an aluminum, will they both loose the same 4-5 lbs (80 tank) as they get empty?
I am looking to buy my own tank and wanted a little info on the pros and cons of each and which would be better for the best neutral buoyancy...or does it make any difference?
MG

 

[jonnythan]

If you put 80 cf of air into a steel tank, it will gain the same weight as if you put 80 cf into an aluminum tank, a submarine, or whatever.

However, steel and aluminum tanks vary a good bit in their buoyancy.

http://www.sportdiverhq.com/tkcht.htm will give you a good idea of the relative buoyancy characteristics.

 

[nickjb]

This is quite good for calculating the buoyancy of various configurations:

http://www.subaqua.co.uk/cgi-bin/cylinder-buoyancy.cgi

 

[dmdalton]

As previously stated the shift in bouyancy caharteristics is relative to the weight of the gas in the tank.

A more important point is what type of exposure suit do you dive, and are you diving single or double tanks. AL 80's are about 4 # positive when empty while steel tanks go from pretty much neutral to negative 4 # per tank depending on size and manufaturer. If you dive double steel tanks you may find that you can dive without a weightbelt. This is fine if you are diving a drysuit whose bouyancy characteristics do not change from the surface to depth. It is not OK if you are diving a wet suit which requires 20 # of weight to make you neutral at the surface because as the suit compresses you become more and negative and could end up 20 # negative at 100 fsw plus the weight of the air in your tanks adding on another 12-16 # (for a set of doubles). That could make you 36 # negative at the begining of your dive with no ditchable weight. Bad situation if you have a wing failure, and even if you don't you have to add so much air that your drag in the water increases.

Soemthing to think about.

Dave D

 

[Rick Murchison]

Tank buoyancy has more bad, seemingly authoratative information floating around the internet - and in "authoratative" scuba texts - than anything else about scuba. The errors are remarkable in their frequency and in their variability.
I have found that the only reliable source of buoyancy information for a scuba cylinder is to take the tank to the pool and measure it yourself. Then apply the weight of gas added or subtracted from what you measured to get the full and empty numbers.
Rick

 

[Brad B]

From personal experience I will tell ya to watch your weighting if you are trying out different types of tanks.I had been diving steel LP 120's until last Sunday when I switched to AL 80's.I had no problem at all descending but at about 30' on my first ascent it became real apparent I was to light. Ended up doing the rest of the ascent and about 2 minutes of safety stop with an empty BC, my head down and swimming pretty hard to keep my ascent rate down.My buddy was real impressed by my very unique ascent technique.I am a new diver and I know I screwed up on this. Just wanted to share this so maybe some other new diver won't make the same mistake.

 

[mer]

Brad B once bubbled...
From personal experience I will tell ya to watch your weighting if you are trying out different types of tanks.I had been diving steel LP 120's until last Sunday when I switched to AL 80's.I had no problem at all descending but at about 30' on my first ascent it became real apparent I was to light.

If you are renting tanks, here's a tip for you. Most dive shops will know the buoyancy characterstics of their tanks (how many lbs neg/pos when empty) - ask them for it. If you record this tank buoyancy info in your weighting log it becomes much easier to figure out how much weight you need (within a few pounds) when diving with a different tank.

i.e. I wore 18 lbs with a steel tank that is neg 7 lbs when empty. So "true" weight on that dive was 25 lbs. I will (theoretically) need this same 25 lbs of true weight for every dive with that exposure suit setup. If I want to dive with an aluminum that is pos 3 when empty, I need 28 lbs on the belt. If I dive with a steel tank that is only 2 lbs neg when empty, I need 23 lbs on the belt. Get it...?

PS I tend to plan my buoyancy for empty tanks, since I want to be able to maintain a safety stop and good buoyancy control for the shallower portions of shore dives. If doing much deeper dives you do need to weight yourself more carefully so you don't get runaway negativity with suit crush (as mentioned by dmdalton earlier this thread).
Hope this helps.
m

 

[Ari]

Different Tanks havs significantly different buoyancy characteristics. Naturally, it will significantly affect your buoyancy.

There is a lot of info about this on past threads. Try to search for it.

Ari