Air in BCD necessary while UW?

[Scuba Barbie]

Is it necessary to have air in your BCD while underwater?

The only time I use the BCD is while floating on the surface. Does this mean I am overweighted, underweighted, or doing something wrong?

 

[Ber Rabbit]

You don't say how deep you dive, what kind of water you're in (fresh, salt, cold, warm), what exposure protetion you're wearing, how much lead you carry with that protection and whether you dive steel or aluminum tanks. I'll give you a "best guess" based on the information you did give.

If you were very underweighted and had no air in your bc you would still float and diving would be a continuous fight to stay underwater. Depending on your exposure protection and how underweighted you were you might not be able to descend at all.

If you were very overweighted you would need air in your bc to keep you from crashing to the bottom. Your exposure protection would compress as you went deeper and you would need to keep adding air to the bc to slow your descent. The deeper you went the more air you would need.

If you are neutrally buoyant you should only need to add a little air (if any) to the bc to compensate for compression of your exposure protection.

Here's my guess: you dive warm water with little exposure protection so you don't need to carry much lead and you probably dive aluminum tanks. Your dives have probably been relatively shallow. You can easily descend when you dump the air from your bc and exhale and you can change depth by changing the way you breathe. If this is accurate (I'm no psychic) you're not doing anything wrong, you've just achieved neutral buoyancy and that is a very good thing. Diving under different conditions and in different exposure suits will probably cause you to need to put at least a little air in your bc or dump air as needed to maintain your depth.

Let me know if I'm close!
Ber

 

[MNScuba]

Well based on what you've said, here's my best guess.

If you have to struggle to get down, you're underweighted. Do a buoyancy check with 500 psi left in your tank (after a dive). You should float at eye level holding a normal breath, when you exhale you should descend.

Also, the deeper you go will determine how much air is needed to keep you neutrally buoyant. Where I dive most often there is a platform at 25 feet, I go to the platform and establish neutral buoyancy right away. This makes the rest of the dive much better.

Not sure if any of this helps but it's a start with the info you provided.

 

[MikeS]

Please take this with a grain of salt as I am still working on the buoyancy/trim issue myself. Someone please let me know if I have this screwed up.

I think that you should have air in your BC underwater for two reasons:

One, when you first start the dive you should have air in the BC equal in lift to the swing buoyancy of your air tank. As the dive progresses and air is used, your tank becomes more buoyant and air is dumped from your BC to maintain neutral buoyancy.

Some items, primarily wetsuits, compress as pressure increases with depth. To compensate for this change in buoyancy air is added to the BC as you desend and released as you ascend.

I cannot imagine any scenario where you could remain neutrally buoyant throughout a dive without releasing air to compensate for the increased buoyancy of your tank as air is consumed.

Are you walking on the bottom ?

Mike

 

[Spectre]

Scuba Barbie once bubbled...
The only time I use the BCD is while floating on the surface. Does this mean I am overweighted, underweighted, or doing something wrong?

I'm not going to _assume_ you are doing something wrong, but I would check if you can hover in one place without ascending, decending, or moving your arms or legs. If you can do that without air in your BC, and throughout your whole dive, then cool!

However if I was to guess, I would guess as others have, that you dive with little or no exposure protection, so that you don't need to deal with suit compression, and you swim to contend with the added bouyancy of your tank emptying.

If your swimming to contend with the bouyancy changes, then you could be underweighted [you swim constantly down to keep yourself from ascending]. If you swim constantly to keep yourself from landing on the bottom, then your weighting is probably fine, but instead of swimming, you can use your bc to acheive neutral bouyancy.

Definately check hovering. Also check if you can decend and arrest your descent without hitting the bottom without swimming.

Now people dove for many many years without a BC, so obviously it can be done, it's just with the BC you can dive much more relaxed.

 

[O-ring]

I would wager you are swimming to compensate one way or the other.

 

[Scuba Barbie]

"Here's my guess: you dive warm water with little exposure protection so you don't need to carry much lead and you probably dive aluminum tanks. Your dives have probably been relatively shallow. You can easily descend when you dump the air from your bc and exhale and you can change depth by changing the way you breathe. Ber"

Yes, I dive at the local "Texas swamp" and rarely get deeper than the thermocline at 25'. I do not always wear a wetsuit and I do use a rental tank. I had a very similar experience with a 3mil in the Caribbean last month.


" I would wager you are swimming to compensate one way or the other. O-Ring"

There has really been no need to stay still (hover) so I have been continually moving.

So I guess I need to practice hovering and check where I am at with 500 PSI.

Thanks for your time and expertise.
-DD