tank buoyancy

[darkmoon3d]

Did a pool dive tonight to test correct weighting and neutral buoyancy. Dove with 1200 psi and didn't think tank buoyancy would be an issue. Normally I dive with 20pnds but ended up having to use 22. At what point during tank falling to a certain does tank become noticeably buoyant? Was really off tonight as far as weighting. Dove in my 7mm fullsuit with vest/hood underneath since its what I normally dive in quarries here in midwest. Thought that 16 or 18 pounds would have been adaquet since it's 10% of my body weight but didn't seem to work too well. What should be a good weight setup for 7mm fullsuit with hood/vest, in fresh water for me?

 

[DeeperOut]

There are tables out there on the interwebz that list buoyancy for most popular tanks. In general, an aluminum tank will be negatively buoyant when full and might become positively buoyant when less than half full.
If you want to know the specific buoyancy for your specific tank, see how many weights are required to sink the tank alone, without any gear, when filled, and how many when empty.
The practical check for weighting your complete setup is seing how much weight you need to hover with a 50 bar tank at 5 or 3 meters depth, I recommend 3 meters. That means, in practice, that even with a near-empty tank you can complete your safety stop (sharing air if needed)

 

[windapp]

If you want to know the specific buoyancy for your specific tank, see how many weights are required to sink the tank alone, without any gear, when filled, and how many when empty.

The "when filled" part won't work as the tank will be negative at that point. Subtracting the difference in air weight from the "when empty" part will, however will give you this value.

 

[Ste Wart]

Try this:
UK Scuba - Cylinder Buoyancy Calculation

 

[scubadiver888]

Air has weight. If you find the buoyancy of a tank full versus empty then calculate the weight of the air in the tank, you will see that the shift in buoyancy is directly related to the full versus empty weight. For example, my HP100 is -10.0 lbs. buoyant when full, it is -2.5 lbs buoyant when empty, it holds 99.5 cu.ft. of air. The only thing which changes from full to empty is the amount of air. The size of the cylinder remains the same. Therefore 99.5 cu.ft. of air is approximately 7.5 lbs. If we do the math: 7.5 / 99.5 = 0.07537688442211. Depending on the temperature, that is about correct. I believe this number is the weight of air at 20C or 68F, i.e. 'room' temperature.

When I'm working on buoyancy I figure out neutral buoyancy for the equipment I'll be using then I figure how how much air is in the tank at that time. Whatever the weight of the air, I add that to my weight belt. This way, if in an emergency I empty the tank I still have enough weight that I'm not going to shoot to the surface. Because the temperature of the tank makes a small difference, I always round up. That is, colder tank equals denser air equals weights more. It is seldom the case that the water I dive in is warmer than 'room' temperature. If I was diving in say the Caribbean I'd have to see if I need to adjust.

 

[D_B]

cool calculator Wart
common cylinder weights ... Scuba Cylinder Specifications from Tech Diving Limited ... the X series Worthington's are the same as XS Scuba
You can see that the typical Al80 tank spends most of the dive positive

 

[Bob DBF]

For 2# it may not be the tank, if you always use AL's. A little trapped air anywhere can make the difference. My recorded weight checks are after a dive (usually over 30') at 15' and 500#. If I was going to dive less than 15' (pool), have less than 500#, or not dive deep enough or long enough for any trapped air to work it's way out of the gear, or compress the suit enough that its at it's normal rebound buoyancy I would add a few pounds since there arn't any rocks to put in my pocket on the bottom of the pool.

You should be able to manage 2# with your breathing, but maintaining neutral buoyancy in shallow water is a PITA at best.




Bob
-------------------------------------
I may be old, but I'm not dead yet

 

[Phil_C]

Bob raises a very important point - especially with heavy or multilayered neoprene in a pool. It usually isn't deep enough to compress the suit enough and force all the air out, so you may well find that it appears you need more weight in a pool than you will actually need in on an open water dive where you decend to a sufficient depth to compress things properly. I find the same with shallow shore dives- P

 

[Nemrod]

Neoprene itself will compress as you go deeper, usually beginning around 15 feet. The old Rubatex G231 was very resistant to compression but even it would compress some with depth. You will become increasingly negative as you move deeper as a result of suit compression, with a thick suit, this amount can be significant.

N

 

[spectrum]

darkmoon3d,

A few years back Uncle Riccky had a thread titled "Neural Buoyancy is a Fleeting zConcept" or something like that. You need to adjust your buoyancy, it won't just happen.

No one here can tell what you need for weight, you need to do the proper tests. See below.

You probably were using an AL80 but it does not matter. Any common cylinder will shed about 5 pounds as you breathe the contents on a complete dve. You will need to account for this change with your BC adjustments or lung volume, the latter being impractical in most cold water situations. Don't worry about the cylinder it's just one component in the buoyancy equation. Cylinder empty buoyancy is valuable for adjusting weights when changing your cylinder to a different model.

Your neoprene will compress and geta heavier as you descend and rebound to some extent as you ascend. Meanwhile all of your gear will saturate as mateials soak and air escapes. This happens as the cylinder gets lighter. It's all in flux. Even your lung volume probably decreases as you get into the dive and relax.

At any point in the dive you should expect to have some air in your BC, especially with a 7mm suit. The exception may be as you end the dive in the shallows with a cylinder near 500 PSI. At any other time you need your BC unless you are modifying your lung volume. If you were flying the BC and could not get stable then you may be severly overwighted. In some cases if lots of people are in the pool the upwelling can become disruptive.

Here's a full explanantion of what you need to deal with and how to optimize your weighting.

Pete