Buoyancy

[rockystock1]

Why is it that as you go deeper you become less buoyant and need more air??
What is the science behind this and is there a point at a certain depth where no amount of air in BCD will get u back up?? A point of no return


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[Itsacon]

When you go deeper your softer tissues, your wetsuit and the air in your BCD get compressed due to the higher pressure. Since they are smaller, they displace less water, and thus provide less buoyancy.

Adding more air to your BCD will bring it back to the same volume it was before you descended, and this process will continue all the way to the bottom. Your BCD is limited by size, not by pressure (Since pressure is the same on the outside), and you won't need significantly more size (just a bit to compensate for the other two).

Obviously, if you wouldn't vent any air while ascending, your BCD would expand and expand, (causing rapidly increasing ascent speeds), and ultimately the safety valves would open to prevent it rupturing. But doing that would be dangerous, so please stick to the limit of 10m/min.

 

[Mustard Dave]

Are you a qualified diver? This is covered in the opening pages of the PADI Open Water manual - I can't imagine any other agency not teaching it on entry level programs. If you think of how heavy a bucket of water is, all that weight is above you and increases as you descend. That weight means greater ambient pressure and therefore any gas spaces become compressed. The more they are compressed, the lower the volume and the more negatively buoyant you become.

 

[tbone1004]

basic forum Dave, have to be nice....

technically the answer is yes. If you are diving with a wetsuit so thick that it loses more inherent buoyancy than the wing has, then yes there is a point of no return without ditching lead. You would have to have ~20mm of neoprene on, descent well past 100ft, and not be able to ditch any of your lead, at that point the wing has less lift than the amount of negative ballast you have on you. This is why you should always dive a balanced rig, and diving in a thick wetsuit below 100ft isn't advocated, go to a drysuit.

Practically the answer is no, at worst a 7mm farmer john wetsuit will be ~35lbs buoyant but it will retain some of that inherent buoyancy for all of the dive and rebound to normal at the surface unless you are descending to crush depth which is somewhere in the neighborhood of 500psi ish where the neoprene actually changes *this is the pressure DUI uses to crush the neoprene for drysuits*? Equivalent to about 1100feet, you aren't going there.

 

[rockystock1]

Thank u t bone, I was worried about diving in deep water and plummeting to the bottom,


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[Crush]

When you go deeper your softer tissues, your wetsuit and the air in your BCD get compressed due to the higher pressure.

With the exception of your respiratory system human tissues contain no undissolved gases and do not compress with descent. Your respiratory system remains uncompressed because you are breathing gas at ambient pressure while submerging thanks to your regulator.

 

[Mustard Dave]

basic forum Dave, have to be nice....

I wasn't criticisng the OP - the reason I asked was threefold - If he isn't a qualified diver, it is something he will go over during his training and it is best left to his instructor to answer. If he is, I'd be concerned that this was not covered properly on the course, and also what else may have been missed. The final concern is the OP may be diving without formal training and instead seeking advice from strangers on the net.

 

[tbone1004]

he's certified according to other posts, and while it is common sense to most of us and depth compensation does get discussed, it is a very real reality with new divers that they are diving overweight and if they are diving very overweight, say 10lbs or so, they could hit a point where they would have to ditch weight to get back up.... Now if you're diving properly it is a nonissue at even the deepest of technical depths, but it hits home the point of having proper weight.

7mm farmer john can have 30lbs of positive buoyancy. If you offset that with 40lbs of ballast and you only have a 30lb wing, you are A. a moron, and B. you will hit a point somewhere in the 30m-40m range where the suit has lost enough of its buoyancy to where the wing can't compensate anymore and you have to ditch.

 

[RJP]

When you go deeper your softer tissues... get compressed due to the higher pressure.

Which tissues in the human body are compressed at depth, and by how much?

 

[TSandM]

Human bodies don't lose much buoyancy at all with depth. Tissues are mostly water and don't compress; the air spaces in the body are small, with the exception of the lungs. But the magic of the scuba regulator is that it delivers air at the same pressure as the water where you are, and if the lungs are full of that pressurized air, they don't compress!

So what DOES lose buoyancy? Anything you have with you that has gas in it and doesn't have a rigid container. That means the air bladder of your BC, any padding you have on anything, and your wet or dry suit. Now, the BC bladder and dry suit are kind of like your lungs -- you can fill them with pressurized air and stop the compression. But your padding and your neoprene don't have that option, and they do compress. Thick wetsuits can lose quite a bit of lift. In the days before BCs were used, this meant the diver had to provide the additional lift by swimming -- a diver in thick neoprene in those days was rarely actually neutral.

The BC, or "buoyancy compensator" was designed precisely to solve this. By adding air to the bladder, you COMPENSATE for the loss of buoyancy that your wetsuit has experienced with depth. That way, the diver can remain neutral, which means he can also stay still. That's nice if you want to remain in place and watch octopuses fighting over a den space, as we did for quite a while on our last dive.

Of course, if you are using your BC to compensate in this way and it fails, you have a problem. You may need to swim up, as people did before BCs. If you are wearing very thick exposure protection, this may be somewhat difficult, which is why divers are advised to keep at least some of their lead weight ditchable.

So the short answer is: If your equipment is working and you are using it properly, there is no depth where you plummet to the bottom. If you have a gear failure, you will have to provide additional lift from your fins, or reduce the degree you are negative by jettisoning weight.