Why do you need more lift when cold water diving?

bkopec · Oct 25, 2012

Sorry, n00b question, but can't seem to make sense out of it. I understand that you need more weight to compensate for the extra positive buoyancy that comes from wearing a 7mm or drysuit. What I don't understand is why you need more lift in the BC to then further compensate for that extra weight. Whether you use 4 lbs w/ no wetsuit or 22 lbs w/ a 7mm suit, isn't that added weight ultimately giving you a small "net" amount of negative buoyancy (just enough to get you to sink)? Isn't the lift of your BC solely counteracting that "net" negative buoyancy and not the total amount of weight you added to get there? Am I missing something where a 7mm suit actually somehow becomes more positively buoyant at greater depths, so the extra weight is needed to account for that? Thus at the surface, you now need more BC lift to help compensate for that extra-extra weight? Please let me know what I am missing.
Thanks!
bkopec

00wabbit · Oct 25, 2012

At the surface you are very positively bouyant. You need extra weight to offset that extra bouyancy from the thick wetsuit. As you descend the wetsuit compresses and you become less positively bouyant. So you need to add air to the BC to offset the loss of bouyancy from the compressed neoprene.

The surface area of neoprene on a large 6'2" 290 guy like myself is a lot. At the surface I have to breath out very hard to descend, but after aobut 10-15 ft I will sink like a rock as the neoprene starts compressing.

If you weren't wearing all that neoprene you may not need to add air to your bc at all.

dumpsterDiver · Oct 25, 2012

A thick suit may undergo 15 lbs or more of suit compression at depth.

An old, worn out 3 mm suit may experience 2-3 lbs of loss of buoyancy at depth. For warm water diving with little wetsuit compression, you change in buoyancy is going to be a few lbs plus the weight of the air consumed in the tank...

padesc · Oct 25, 2012

Any BC needs to meet two criteria:

1) Be able to float your rig at the surface with full bottles.

2) Be able to compensate for the maximum change in buoyancy of your exposure suit.

More suit, more change in buoyancy.

3D diver · Oct 25, 2012

It's a trick question...if you're diving really cold water you're likely going to get a drysuit, wear lots of lead to compensate for the bouyancy of your undergarment, then need more BC bouyancy to compensate for all the lead if your drysuit floods.

How's that for a run-on sentence?

Glenn08 · Oct 25, 2012

Do you remember your gas laws ? At a first approximation, a suit that has 30 lbs buoyancy at the surface, 1 atm, 14.7 psia, will have only 15 lbs buoyancy at 33 ft sea water, 2 atm, 29.4 psia. At the surface you need more weight, and since there is more suit to compress at depth, there is more buoyancy loss, so more BC inflation is necessary.

TSandM · Oct 26, 2012

I think you've been answered, but I'd like to try to make it even simpler and clearer.

At the surface, you are wearing thick neoprene, which is very buoyant, so you need a lot of lead to get under the water.

At depth, that neoprene is compressed, and it's no longer buoyant. But you're still wearing the lead you had to have to sink at the surface. Somebody I know took a 7 mil suit down to 100 feet, and found it lost 23 pounds of buoyancy. So, if you were neutral at the surface, you are now 23 pounds negative at 100 feet! Your BC has to COMPENSATE for that buoyancy loss, so it has to have at least 23 pounds of lift (actually more, because most of us weight ourselves neutral with EMPTY tanks).

If you are wearing no exposure protection, or very thin exposure protection, you have to wear little or no weight to sink in the first place, and you have very little buoyancy you can lose at depth.

If you are diving dry, you have to have enough lift to make up for a flooded suit, which can be a bad situation.

fnfalman · Oct 30, 2012

I think that the equipment configuration also has a lot to do with more lift as well.

A lot of people nowadays use weight integrated BC. When you put a bunch of lead on your BC...well...you need more lift.

If you were to use a weight belt for either your whole dive weight or split it between your weight belt & your BC's integrated weight pouches, then your BC doesn't need as much lift because your exposure suit (compressed as it is at depth) still has some positive buoyancy characteristics.

3D diver · Oct 30, 2012

If the total weight is the same, then where you wear it has no bearing on the amount of BC lift needed.

fnfalman · Oct 30, 2012

3D diver:
If the total weight is the same, then where you wear it has no bearing on the amount of BC lift needed.

Sorry but that's not correct. If you put all your dive weights on your BC instead of splitting it up between your body and your BC then your BC will need bigger lift to account for all these weights.

Why do you need more lift when cold water diving?

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