Simple bouyancy question

[Flguy76]

There are a few threads along these lines but not wanting to hijack someone else's I thought I would just ask. Theoretically speaking, in terms of maintaining neutral buoyancy most of us are taught to dump all of our air during initial decent and then add small amounts as we near the bottom to establish neutral buoyancy again. Wouldn't it make more sense or would it work at all to just establish neutral buoyancy at the surface and do a tuck roll and just swim down to the bottom. If you are neutral at the surface (provided you aren't wearing exposure protection which I normally don't) wouldn't you be neutral at depth as well since there is nothing to compress and make you negative. Why not just get your weighting dialed in, do a float check at eye level like we are taught in our initial cert course and then just swim down. It seems like you shouldn't even need to fumble with your bc until you being your ascent? Again this is diving in shorts and t shirt not a 7 mil wetsuit. I'm a warm water guy.

 

[CT Sean]

If there's any air in the BC at all - and generally there's a couple lbs to compensate for the gas in a full cylinder - it will be compressed at depth and you'll lose the buoyancy it's providing.

 

[spt29970]

For many people it is harder to clear your ears when you are head down. That is why it is best to descend in a mostly vertical orientation. Instead of dumping all of the air from your BC on the surface, it is best to dump enough to become slightly negative and make small adjustments as you descend, controlling your descent rate, becoming neutral at depth.

 

[TSandM]

In the days before BCs, this is more or less how things went. You weighted yourself so that you wouldn't be too positive at the surface to swim down, and wouldn't be so negative at depth that you couldn't swim up, and you controlled everything else with your fins and your breathing.

Nowadays, we prefer to weight ourselves so that we are neutral at the END of the dive, when the tank is low; that means that, at the beginning of the dive, we are negative by the amount of gas we intend to use (which, in an Al80, is around five pounds). To be neutral at the beginning of the dive, you either have to keep your lungs quite full (which gets uncomfortable and is not a way to exchange gas efficiently in the lungs) or you have to put a little air in your BC. Since that air will compress with depth, in order to remain neutral, you will have to adjust the volume of the BC with depth changes.

When you use exposure protection, you also have to compensate for compression. But even in swim trunks, you have to do something about gas weight.

 

[pisoiu]

If you are neutral at surface, at the beginning of the dive with your BC empty and no protection suit, then you will be neutral at depth, since there is nothing to compress and alter your buoyancy. But the weight of the gas in cylinders will dissapear at the end of the dive and then you will be positive. It will be hard to make the safety stop or maintain level since your BC is already empty.

 

[rongoodman]

In the 'good old days', before BCs, safety stops weren't done either.

 

[rick00001967]

short answer = yes......but your method should be used with a cylinder that is low on gas. most will say to do this with about 500 lbs in your tank. but since i rarely if ever breath my tank below 1000lbs, i do the method you describe with about 1000lbs in my tank to set my weight. remember that different tanks also have different buoyancy characteristics as well. so if you set your weight with an allum 80 and then one day you are using a LP steel tank, your weighting will change

 

[Flguy76]

Thanks for the input. I do dive an al80 and i weight myself by doing a standard weight check on the surface and then adding about 4 pounds to compensate for the gas loss as I breathe the tank down. i know it's probably better to just weight myself using a tank that is low on air or near empty but i dont normally have one handy at the beginning of a dive. i agree the bc will compress at depth, this seems to have slipped my mind when i posted the question. I plan on going to the beach tomorrow just to play around with this.

 

[Steve_C]

then add small amounts as we near the bottom to establish neutral buoyancy again.

If you are doing deeper dives and wait too long it can become difficult to stop your descent. On bottomless dives it is possible to get a descent that is too fast. Under normal circumstance maybe better to start before you are near the bottom unless these are shallow dives.

 

[kwinter]

most of us are taught to dump all of our air during initial decent and then add small amounts as we near the bottom to establish neutral buoyancy again

Sorry, but I have a problem with the initial post. Who is teaching to dump all of the air from a wing or BCD to start a dive? You only need to dump as much as necessary to begin descending. Even if you are weighted perfectly, you will still have some gas in your wing to compensate for your weight and buoyancy differences at the end of the dive. The more tanks you are carrying, the greater the weight difference will be.

And as for adding gas as you near the bottom, the rate you do that all depends on how deep you go and how fast you want your descent. I usually want a quick drop and don't add much until I'm nearing bottom, then blast it full force and come to a gentle stop. Yes, I have crashed into a few wrecks when the vis is less than 1 foot, but that's part of the fun.