Hit 50Bar becoming uncontrollable positive bouyant problem ..

[boulderjohn]

Are you diving an Aluminum 80 cylinder? These are known to be positively buoyant as the air in them is depleted.

Maybe I read your message in a different way than intended. But the material of the tank doesn't change the buoyancy shift between the beginning and the end of the dive.

Okay, let's settle this common misconception.

Your buoyancy depends upon all the factors of your dive--your body, your gear, the air in your BCD, the air you breathe--everything. Some things pull you up toward the surface, and some things pull you down. It is not dependent upon one thing alone.

If you and all your gear are neutrally buoyant at the beginning of your dive, that is because all the factors of your dive--your body, your gear, the air in your BCD, the air you breathe--weigh the same as the water you are displacing.

If you lose 6 pounds of air during the dive and nothing else changes, you will be 6 pounds more buoyant at the end. If you want to stay neutral, you will need to lose the appropriate amount of air in the BCD to make up for the lost air.

It does not matter whether your tank is steel or aluminum--if it loses 6 pounds of air during the dive, it will be six pounds more buoyant than it was at the beginning of the dive.

 

[boulderjohn]

I believe his point was that aluminum tanks will be buoyant when near empty at the end of the dive. Basically, you have a cork on your back and the lead you carry should account for that transition.

Long, long ago, I taught at a shop that kept steel (still neg on empty) on hand for customers that wanted them and that's all I would dive. More air, less weight belt, less bobbing.

Steel tanks are less buoyant than aluminum tanks. That is true at the beginning of the dive, the middle of the dive, and the end of the dive. If you are using a steel tank, you will need less weight than if you use an aluminum tank. That is true at the beginning of the dive, the middle of the dive, and the end of the dive. It has nothing to do with the amount of air the tank loses during the dive.

 

[nomadicdiver]

Steel tanks are less buoyant than aluminum tanks. That is true at the beginning of the dive, the middle of the dive, and the end of the dive. If you are using a steel tank, you will need less weight than if you use an aluminum tank. That is true at the beginning of the dive, the middle of the dive, and the end of the dive. It has nothing to do with the amount of air the tank loses during the dive.

Yup. Edited my post a little.

 

[BlueTrin]

Take a 50 bar tank on a shore or enclosed space.

Go do a weight check with a buddy.