Loss of human body volume vs depth (buoyancy)

[MaxTorque]

The reason for this question is as follows:

i mostly dive dry, #becauseUK ;-) and my dry suit obvs provides the redundant buoyancy should i experience a BCD failure

But i dive with a couple of different setups in terms of the scuba unit. Might be a single 10l and rec bcd, might be a 12l and a BPW, but it also might be my double 12's and i may or may not carry a pony cyl.

But sometimes, for messing arouind i the pool training, or when the weather is REALLY nice (ok, this one never happens in the uk...) or when i'm on my holibobs, i will be diving wet.

So my cunning plan was to try to make each system reasonably neutrally buoyant, for a wet dive, and then simply add a weight belt for dry dives to account for the extra lift.

After playing around in the pool, by adding 2kg of lead directly to my single cyl BPW, i need no extra weight for wet dives, and about an additional 4 to 5kg (depending on undersuit thickness) to dive dry.

That means i just take what ever scuba unit best suits the dive, and if i take my drysuit, i also take my weight belt.


The downside, is the wet diving has not ditchable weight, hence my question. In reality, my wet diving is likely to be recreational, shallow, and relaitive benign, so what i wanted to establish is that if say i can demonstrate that with an empty bcd i can swim up easily from say 5m, how deep a max depth threshold should i set for that config in order to maintian that capability? Is it likely that within std rec limits (lets say 30m on the single cyl) i could be too negatively buoyant ?

 

[lowwall]

The downside, is the wet diving has not ditchable weight, hence my question. In reality, my wet diving is likely to be recreational, shallow, and relaitive benign, so what i wanted to establish is that if say i can demonstrate that with an empty bcd i can swim up easily from say 5m, how deep a max depth threshold should i set for that config in order to maintian that capability? Is it likely that within std rec limits (lets say 30m on the single cyl) i could be too negatively buoyant ?

How positive is your wetsuit at the surface? Worst case is your suit goes to neutral. If it is neoprene, it won't, but neutral is a conservative approximation. Can you swim up the difference?

 

[rick00001967]

Define "some".

i used the word "some" because many divers who need a lot of weight simply cannot configure there rig with it all being ditchable. and i am not saying 100% of it has to be.
however, this is the "basic" scuba forum so i assume we are discussing entry level divers where most use a typical rec bcd with most (if not all) of the lead in removeable pockets.
the amount chosen depends on a lot of factors. are they diving a single 80 or double 100's ? are they actually weighted properly, or are they over weighted ? etc
every diver has to decide "if" they want any ditchable weight but many don't (our students are trained to carry ditchable weight) and then decide, based on their needs, how much can be ditched.
i always hear discussions about being able to swim up if you loose your wing for example. but i rarely hear anyone mention anything about being able to remain at the surface with your head comfortably out of the water.
so ditching may not be part of the divers plan in order to reach the surface, but maybe it should be part of the plan for what to do once they are at the surface. just my opinion.

 

[MaxTorque]

How positive is your wetsuit at the surface? Worst case is your suit goes to neutral. If it is neoprene, it won't, but neutral is a conservative approximation. Can you swim up the difference?

It takes 3kg to sink me in my wetsuit at the surface, as long as i breath out. At 2kg, i can't leave the surface no matter how much i breath out.

I checked the buoyancy of my built up single cyl scuba unit, and with a full (200 bar) 12l steel cylinder, and 2kg of weight on the waist band, with my single reg set attached, and with the BCD fully deflated, that has a negative buoyancy of 5.5kg. With the around 2,400 litres of air, weighing about 3kg, that means it could have a minimum negative buoyancy of 2kg when the cylinder is close to empty. I think this will mean that worst case is i can just hold a 5m safety stop with an empty cylinder, allowing for a bit of compression on the wetsuit.

 

[boulderjohn]

I am not a freediver.

About 5 years or so ago I said something in a thread that was consistent with what the majority in this thread are saying about the body not compressing at depth. A couple freedivers responded as if I had written the most ignorant, stupid thing in the history of ScubaBoard. How could I know so little about free diving? One person in particular essentially told me to hide my face in shame forever, but, then, that was the topic of many of his posts anyway.

Having learned my lesson, I recently wrote a post in which I mentioned the body compressing at depth, and someone responded as if I had written the most ignorant, stupid thing in the history of ScubaBoard.

I am still not a freediver, and I have no intention of becoming one, but I would sure like to find out so I can stop being perceived as the person writing the most ignorant, stupid thing in the history of ScubaBoard.

 

[sea_ledford]

Just wear all the gear you would use in open water (including weight) in the pool minus your wetsuit. If you can suspend yourself mid water with no air in your BC for a reasonable amount of time (ie a couple minutes to ascend from depth plus a few more for a safety stop should you choose), you should be fine with no ditchable weight. If you can't keep yourself off the bottom at all or without exhaustion, you should be carrying ditchable weight.

 

[johndiver999]

One relatively simple way to look at the situation (for diving with a wetsuit) is to put it into a mesh bag and add lead until it sinks in the pool. Swish around to make sure trapped air is eliminated. Then you ASSUME that the entire buoyancy of the suit is lost at depth. For a thin warm water suit, you might be looking at 4-7 lbs. then assume an aluminum 80 has about 5 lbs of air in it. So your max negative condition should be the sum of the wetsuit compression and the mass of air in a full tank.

So worst case, you sink straight down deep with a full tank at the start of the dive and you BC completely fails and your suit is completely crushed and you have no ditchable lead. Can you swim up? Are you a strong enough swimmer to kick up the wetsuit shift and the weight of the air in the tank? If NOT, then you might want to consider having SOME ditchable lead. The word "some" means no less than the amount that you need to drop so that you can swim up.

You could test in a deep pool by wearing maybe 12 lbs of extra lead and no air in the BC and just see how that feels kicking up.

A better test is to actually do a (deep) dive, dump all the air from your BC and try to kick up at the start of the dive. Easy to test and more accurate than any formulas, assumptions or pool tests.

 

[Angelo Farina]

When I started diving in the seventies, no BCD was used, and the wet suit was a luxury. It was quite common to dive "naked", with just a swimming suit.
10 years later I started working as an instructor in resorts at Maldives, where the water is really warm, so I immediately ditched the wet suit. At that time (1986) the usage of a BCD was already widespread, but not mandatory yet.
So, when diving without customers, as in "exploration dives" which we were doing routinely for discovering new diving sites, I and my wife did use a very minimal equipment: no BCD, no suit, a pair of very long free diving fins (highly recommended at Madives due to incredibly strong currents in reef channels), a small tank (10 liters) and no other equipment hanging around. Being extremely streamlined and neutrally-buoyant it was possible to explore much larger areas, swimming very fast and efficiently. In these exploration dives we usually did never exceed 40m depth, and we were doing no planned deco.
So a 10 liters tank was enough.
Going from 0 to 40m did cause substantially no variation in buoyancy (and this answers to the OP: the human body has no signficant buoyancy variation with depth, except of course the lungs in case of a free diver - but lung's volume is kept unaffected by ambient pressure when scuba diving). Emptying the tank from 200 bars to 50 bars (2000 liters to 500 liters) did cause an excess buoyancy of less than 2 kg, which was easily compensated with lungs. So the BCD was really not necessary...
And due to the super-streamlined equipment, naked body, and long free-diving fins, in case of complete failure of the scuba system it was relatively easy to perform a CESA even from 40m. I did never need to do that, indeed, but we were feeling confident that, in case, it was feasible without significant problems.
At the time I was able to free dive down to 30m, so an ascent from 40m in substantially neutral buoyancy and with a lot of air expanding in my lungs appeared trivial.

 

[Wibble]

i understand the reasoning some people use. i certainly do not want to open up that can of worms, especially in this forum. i just think the ease of having some ditchable lead far outweighs any down side. just my opinion.

If you're doing pure recreational dives, that's fine.

If you've a decompression obligation it's most definitely not good. Take redundant buoyancy with you -- drysuit, couple of SMBs, plenty of gas...

 

[lowwall]

Prof. Farina should have settled the issue of human compression at depth. If not, let's call in some folks who've been down to dozens of atmospheres.

How about it, @Akimbo and @Pressurehead? Do you start tripping over your bell bottoms as they bring you to working pressure?