Buoyancy/Displacement changes with depth?

[stewybear]

Hi Guys & Gals,

One of my club instructors commented the other day that although 16Kg of lift at 20m may be fine, it could become inadequate at deeper depths.

I have tried to work out why but can only come up with the density of water changing marginally due to temperature and pressure changes. Otherwise the BCD/wing would give you the same amount of lift when inflated to the same volume.

I posed this to him but he could not defend his comments apart from saying that he has seen it happen.

Whilst I am quite happy to accept his experience, because I have alot of confidence in him, I would still like to understand why. Does anybody else have some ideas or even the real answer.

Cheers

[Juz]

I agree with you mate!

What he may of seen tho is reduced bouyancy as a neoprene suit compresses and incorrectly believed it was due to the BCD.

Juz

[stewybear]

Does a neoprene drysuit/semi-dry compression make that much difference?

I remember my first dive and was amazed at how much weight I had to dive with just because of the suit. Damn those air bubbles

Might be worth an experiment next time I'm at Stoney. I'll get my instructor to hang from a spring balance at 10m, 20m and 30m with no air in his BC and weigh him. He has a compressed neoprene suit though......hmmmm!

[Keiron]

Not sure, however, you hear stories in chamber dives (when chamber dives were allowed, that is) of 8mm neoprene (uncrushed I guess) becoming as thin as paper. Whether this is true or not, I actually don't know. So, all that extra weight you need at the surface to combat the 8mm neoprene starts acting against you as the neoprene no longer has the same buoyancy whatsoever.

How much it actually is, hopefully, somebody cleverer than I can help?

[Nick Kay]

1. OK - as a neoprene wet/semi/drysuit compresses (with greater depth), so its buoyancy properties diminish - you're compressing the air bubbles in the suit. Therefore you get progressively more negatively buoyant (to a limit) with depth. Therefore in such circumstances you'd need less weight at depth but more nearer the surface. This change is virtually irrelevant with regards to a membrane/trilaminate suit

2. The deeper you go, the more air you need. The more air you need, the more cylinder weight you carry. Therefore, you need more lift

3. And you need to be able to lift your buddy on the same configuration... Thats one of the reasons some wings (say the OMS) provide around 100 Lbs of lift

4. The more air you take, the more the air weighs. For example: 2 x 12L x 170B of air (220B - 50B rserve) = 4080L = 4.5Kg of mass you're going to "lose" by the end of the dive, i.e. that needs compensating for at the start of the dive

5. Your body compresses with depth so you become slightly more dense

6. Salinity changes with depth (and therefore buoyancy)

With regards to saying that you need more weight with depth: putting all of the above arguments to one side, then presuming you were marginally negatively buoyant, then you'd descend to the point where you'd become neutrally buoyant and stop descending or following his argument, start to ascend?

Trying to see the logic / reasons but really struggling (aside from 1-6 above)

[tag2000]

Originally Posted by Nick Kay

5. Your body compresses with depth so you become slightly more dense

I always though I felt more stupid because of narcosis at depth. Now I know better.

LOL

Tristan

[nigelH]

Originally Posted by stewybear

One of my club instructors commented the other day that although 16Kg of lift at 20m may be fine, it could become inadequate at deeper depths.

Well the density of water does not change perceptably and human beings are mostly that stuff so they don't change either. The things that really change are those that contain sealed air and the worst is a drysuit. Ignore neoprine and consider a total suit flood. You and the suit go neutral as it is all water now. All the weight you carry, the gas and the gear must now be supported by your jacket. To dive the drysuit in the pool with fins, mask etc. I need 16Kgs. Some of that will be spread about my kit so it won't be that much lead but 16Kgs of lift will make me neutral not put my head above the waves.

The problem isn't deep the problem is the gear you use deep. I wouldn't dive a twinset in the pool without a 22Kg wing. The kit you use on a 20m dive and the kit I take to 60m are kilos apart.

He's right but it's phrased badly. My Scubapro Club BCD is quite adequate for a 20m bimble. It's not up to a 50m wreck. It was a good buy for a beginner but it won't cover all my diving if what I want to see is deep. I keep the old BCD and dive it on warm water holidays in a wet suit.

[stewybear]

Great......thanks for that everybody.

Like the thing with the squishy body

It all came about when I was doing the depth progressions for my sports diver. The instructor wanted to make sure that my kit configuration was o.k.

As it was, I had a tekwing with Woz plate setup with 15Ltr and 3ltr pony. This is the same equipment (apart from maybe a 12ltr for second dives) that I had been using for about 30ish dives so I was only really refering to this setup. Appologies for not making that clear at the start.

Totally understand what everybody has said and has now given me my next internet quest to find out about twinsets etc and the wings to use.

Thanks again.