Loss of human body volume vs depth (buoyancy)

[MaxTorque]

Question: Is there any established rules or thumb of formula's for the loss of human body volume vs depth (compressive pressure)??

I believe in freediving one weights for the max depth to which you are dropping (so persumably one has to swim down harder to start with when you are going deeper?) but on a single (surface) breath they clearly don't have the AGE risk as a diver on SCUBA does in the event of a rapid ascent.

Is most of the volume lost from our exposure protection (probably the case with thick wetsuits) or does our body also compress a bit due to disolved gases in un-accessable (from non perfused) tissues?

If one establishes that say it is possible to swim up from say (arbitrarily) 15m depth with a fully deflated bcd with the weighing being carried, what does that actually mean from a max depth you could swim up from perspective?


The question arises with respect to having non-ditchable weights on a scuba system without redundant buoyancy ie wetsuit + bcd, rather than drysuit + bcd (not considering a dSMB as extra redundant buoyancy, although that is one possible option clearly)

 

[arew+4]

People are largely incompressible, air spaces, lungs when not on scuba, gas in the alimentary canal compress, but otherwise it's your gear. Weight for neutral at the surface. Then factor in your tanks.

 

[Divin'Papaw]

The body itself is essentially incompressible. The airways are kept open via the scuba unit delivering air at ambient pressure. So no, you do not need to account for the loss of human body volume with depth as their is none or it is so small as to be irrelevant to the weighting calculation. At least that's the diving physics as I understand them.

 

[wetb4igetinthewater]

The body itself is essentially incompressible. The airways are kept open via the scuba unit delivering air at ambient pressure. So no, you do not need to account for the loss of human body volume with depth as their is none or it is so small as to be irrelevant to the weighting calculation. At least that's the diving physics as I understand them.

Id like to do a test where I am not wearing exposure protection, am negatively buoyant, empty wing and I measure how negative I am with a fish scale just under the surface, go down to 100 feet amd measure again. The difference in gas will be negligible.

I refuse to do this in local waters however.

 

[MaxTorque]

I was planning on simply setting up a test in the pool, in safety, fully emptying my bcd and checking i could swim up to the surface.

If it works from 3m in the pool, then try it from 5 and 10m which are the easily available depths at my local inland dive site.

But i was wondering if there was a rule of thumb to estimate the reduction in buoyancy with depth?

 

[rick00001967]

the discussion about not using ditchable lead can be a hot topic online. pls be careful. if you are diving wet (i see you are in UK) you can loose a substantial amount of bouyancy due to the compression of the neoprene suit, hood, and gloves you will be wearing.

obviously this depends mostly on the thickness of the neoprene and the depths you are in.

this is why you need to have redundancy when it comes to bouyancy. if your bcd fails for some reason, you want to be able to compensate for that.

go diving in a 7mm suit, hood, and gloves to 130 feet, dump all the gas out of your bcd, and try to swim up. good luck with that.

personally i see absolutely no reason why any diver could not easily carry at least some ditchable lead without any risk or trouble at all. so why not just do it ?

 

[Wibble]

Compressibility of your wetsuit is far more significant.

Unless your pool is tens of metres deep, you won’t notice anything, certainly not in a 2m pool

A drysuit has a suit inflate hose and valve to counteract "squeeze"

 

[Divin'Papaw]

i see absolutely no reason why any diver could not easily carry at least some ditchable lead without any risk or trouble at all. so why not just do it ?

Because with a balanced kit I don’t need it. Why clutter up my kit unnecessarily by adding a weight belt or harness? I dive a BP/W and a weight belt or harness significantly throws off the balance if my weight distribution.

I do always have two DSMBs prerigged, one in each pocket. IF I lose all buoyancy at the beginning of the dive I have plenty of gas & time to deploy one and ascend up the line. I always dive over a hard bottom. No wall diving.

So that’s why.

 

[johndiver999]

Freedivers experience very significant changes in body volume associated with compression at depth. If a wetsuit is worn, then wetsuit compression is also an additional factor and it can be much more significant if the suit is thick and the dive is deep.

The compression of the freediver's body is associated with lung compression. In general, a typical recreational freediver will wear sufficient lead to float quite high at the surface, attain neutral buoyancy at around 30 feet and will be sinking faster and faster as they descend deeper.

Freedivers who are diving to less than 30 feet might wear a little more lead, just enough to allow themselves to be neutral at their target depth. Very deep freedivers might wear less lead (i.e., attain neutral a little deeper than 30 ft).

I have never measured the actual lost of buoyancy (from chest compression) between the surface and 60 feet, but for me it is probably on the order of 5-6 lbs - at least that is what it feels like.

 

[rick00001967]

Because with a balanced kit I don’t need it. Why clutter up my kit unnecessarily by adding a weight belt or harness? I dive a BP/W and a weight belt or harness significantly throws off the balance if my weight distribution.

I do always have two DSMBs prerigged, one in each pocket. IF I lose all buoyancy at the beginning of the dive I have plenty of gas & time to deploy one and ascend up the line. I always dive over a hard bottom. No wall diving.

So that’s why.

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.