Questions about Buoyancy control

[JasonH20]

The method you mentioned is mainly used to estimate the amount of weight you need. Be sure to do it the other way to get it spot on. At your 15 foot safety stop, with about 500 psi in your tank, dump all air from your bc and breathe normally. If you float, you need more weight. If you sink you need less.

This is basically what I did. After a dive, I bled off air from my tank until I only had 500 psi left. Then in just 5 feet of water I played with how much weight I could remove and still be able to sink. When you get it right (with all the air out of of your BC obviously) you can stay on the bottom with an exhale, and start to rise with an inhale.

Good luck!
Jason

 

[talon39]

talon, are you using aluminum or steel tanks?

Aluminum rentals

 

[Scubaguy62]

What worked for me was to first learn what is the ultimate test for how much lead you need; if you can hover at 15 ft (safety stop) with an empty bc, and a nearly empty tank (about 500 psi), you're properly weighted. You also need to consider that on average, neoprene has 2 - 3 pounds of positive buoyancy for every pound it weighs dry; a nearly empty AL80 has about 3 pounds of positive buoyancy. So first you'll have to off set that.

I used this buoyancy calculator http://dive.scubadiving.com/html/200105buoyancycalc_chart.html,
to make a fairly accurate determination of how much lead I needed to keep neutral at all times and voila, the nearly empty tank and empty bc at 15 ft check worked like a charm. I dive with AL80s, a 3mm full suit and I weigh 214; I use 14 pounds of lead, and lately I've been looking to shed 2 more pounds.

Just my .02 psi

 

[Nomad]

I am also negatively bouyant. I sink like a stone with no wetsuit and no additional weights.

My weighting depends on my expected activities. When I will be helping with a class, (and I am wearing a 3 mil. shorty) I usually add about 4 lbs.

On most fresh water dives, I wear my shorty and 2 lbs. additional weight. Although this makes me a bit heavy, I compensate by adding a little air to my BC.

Aside from the PADI surface test you mentioned, when you are "at depth" you should be able to move 4-6 feet in altitude without any movement, simply by inhaling and exhaling...

Something important to remember, you need to be weighted so that if the dive goes "really bad" you can dump your weights and emergency acsend to the surface.

...Of course that's just my opinion...

 

[Charlie99]

An aluminum 80 may go from -4 lbs to +6 lbs. That can be a devastating swing of +10 at the end of the dive. The easiest way - and the way most of us do it is to switch to a steel tank. A steel 95 for example starts out at about -7 and ends up about -3. A +4 swing, ....

Your profile says "SSI Advanced Open Water Instructor". Truly amazing.

Can you tell me where to find these magic 80 cubic foot tanks that have a buoyancy swing of 10 pounds? Or even better, I'd like one of those magic 95 cubic foot tanks that only have a 4 pound swing.

The myths that surround aluminum and steel tanks never ceases to amaze me.

 

[JeffG]

Your profile says "SSI Advanced Open Water Instructor". Truly amazing.

Unfortunately its not.

Can you tell me where to find these magic 80 cubic foot tanks that have a buoyancy swing of 10 pounds? Or even better, I'd like one of those magic 95 cubic foot tanks that only have a 4 pound swing.

LOL

 

[Land Locked]

My tank chart shows an empty al80 at a +3.8lbs buoyant
in fresh water, never really tried to empty one.

 

[Bobby]

My tank chart shows an empty al80 at a +3.8lbs buoyant
in fresh water, never really tried to empty one.

I have emtied many a stage cylinder and trust me AL's are very buoyant empty. Steels are for the most part close to nuetral when empty or a little negative. We still use AL's for stage's though cause the steels are too negative when full. The bouyancy shift of actual weight is the same from steel to AL the difference is when you get in the H20 and you are talking positive or negative bouyant. Most wings will give you more lift than you need so to be more negative at the start of the dive with a steel still lets you carry less weight cause at the end of the dive when a cylinder is close to empty a steel does not add lift like an AL which would require you to carry more lead for the off set.

Bobby

 

[Land Locked]

Maybe I should empty them, just carried 2 to the garage for fills in the morning.
If I had drained them it would be a few less pounds I would have lugged to the
garage!! All kidding aside, if we start with a positive buoyancy of +3.8 lbs empty
and add 5.8 lbs of air couldn't we figure the tank full as 2 lbs negative at the start
of the dive? Or is it just too late in the evening for my faulty thinking?

 

[gcbryan]

The most common problem for new divers is not realizing how positively bouyant an empty aluminum tank is. An aluminum 80 may go from -4 lbs to +6 lbs. That can be a devastating swing of +10 at the end of the dive. The easiest way - and the way most of us do it is to switch to a steel tank. A steel 95 for example starts out at about -7 and ends up about -3. A +4 swing, however you still remain negative. This allows you to wear less weights (that other 6 lbs) somewhere else on your body and drops your weighting that 6 lbs completely.

Direct example... When I dive with an aluminum tank I have to wear 6lbs of lead with my 3 mil suit... to be sure I can stop at the end of the dive. With my steel tank I don't wear any weight at all. Hope this enlightens you...

Ken

This is hillarious! Not only does this instructor not know what he is talking about with the devastating 10 lb swing but he proves himself wrong with his last paragraph where he shows that he needs only an additional 6 lbs when he goes from steel to aluminum. It would require 10 lbs according to his initial explanation

Sorry dude, not trying to be harsh here but it is funny