Loss of human body volume vs depth (buoyancy)

[Wibble]

Exactly, like a ball bouncing off the "bottom." Down and up, no time at any level, hopefully avoiding deco.

Or 20 mins on the bottom for a deep dive (100m+), with hours on deco coming up.

 

[inquis]

Yes, exactly my point. If the diver can maintain the weight mid water with zero buoyancy, they'll be fine.

My profuse apologies -- I glossed over the "minus your wetsuit" part of what you wrote. Thank you for the correction.

 

[tursiops]

Or 20 mins on the bottom for a deep dive (100m+), with hours on deco coming up.

Not a bounce dive, by definition. Bounce dive is zero minutes on bottom.
From Wikipedia:

Bounce Dives… what to do if you find yourself doing one
Bounce diving
https://www.scubaboard.com/community/threads/what-is-a-bounce-dive-or-better-yet-why.264521
Learn to Bounce Dive

 

[GJC]

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)

As mentioned above, body tissue compression does not affect your buoyancy. For free divers, air in their lungs is compressed and buoyancy decreases with depth. Scuba divers change their buoyancy as they breath. They max change from a full inhalation to a full exhalation in an average healthy person is about 5 liters. So if your rig is neutral (or compensated with some BCD inflation to be neutral) you can vary your buoyancy by 5 kg with breathing alone

Wet suit compression and buoyancy loss is very variable depending on a number of factors. Thickness, suit size, and bubble content determine buoyancy. Comprehensibility (stiffness that maintains bubbles) of the suit will determine how much buoyancy is lost as depth increases. I don't think you will be able to find a formula to predict this very accurately which is why a buoyancy check is recommended. If you really want to know your suits buoyancy, you could take it to various depths (without you in it) with a bunch of weights to attach to it to see how much weight it needs to be neutral. Without doing that, you could conservatively assume that your wetsuit is totally compressed at depth and provides no buoyancy. In that case, all the weight you carried at the surface to sink your wetsuit will make you that much negatively buoyant at depth.

Not yet mentioned is what the maximum weight is that you can swim swim to the surface with. Lifeguard training used to include diving to 10 ft to retrieve a 5 kg rubber brick, swim it to the surface and then swim on the surface for the length of the pool. I used to be able to recover 2 bricks and swim on the surface with them (I haven't tried that recently). So a scuba diver, with fins, should be able to swim from depth to the surface with about 5-10 kg of negative buoyancy. This will totally depend on the fitness and swimming ability of the diver. Only you can decide how much negative buoyancy you are comfortable swimming to the surface in the event of BCD or drysuit failure. Any additional weight beyond that should be ditchable.

 

[arew+4]

What about gastrointestinal gas? Could a person who tends to suck air into the stomach become more buoyant during a dive, possibly developing a reverse squeeze if unable to burp? How about some one with an out of wack micro-biome. I keep picturing Charlie and Grandpa floating up towards the fan from the fuzzy lifting drinks.

 

[GJC]

What about gastrointestinal gas? Could a person who tends to suck air into the stomach become more buoyant during a dive, possibly developing a reverse squeeze if unable to burp? How about some one with an out of wack micro-biome. I keep picturing Charlie and Grandpa floating up towards the fan from the fuzzy lifting drinks.

Good thought.

Intestinal Gas Production

The answer is, in normal people, the average is about 200 ml of gases in the entire gut at any one point in time. So about 0.2 kg of buoyancy could be lost at depth. But I guess if you had a six pack of soda without belching before your dive, it could be more.

 

[Seaweed Doc]

I refuse to do this in local waters however.

C'mon, I triple dog dare you to do this in the San Juans!

More seriously, how would you control lung volume? You'd need the same volume in your lungs at the surface/shallow depth and on the bottom/deep depth. I guess you could try to exhale completely at both. When I've messed around with that, it seems like I always have a little more air left that, with concentration, I can exhale.

 

[wetb4igetinthewater]

C'mon, I triple dog dare you to do this in the San Juans!

More seriously, how would you control lung volume? You'd need the same volume in your lungs at the surface/shallow depth and on the bottom/deep depth. I guess you could try to exhale completely at both. When I've messed around with that, it seems like I always have a little more air left that, with concentration, I can exhale.

I'd just be breathing normally. Yes some gas would be consumed from my tank but a negigible amount

 

[Seaweed Doc]

I'd just be breathing normally. Yes some gas would be consumed from my tank but a negigible amount

I suspect the gas volume changing in your lungs would have a bigger impact on what you "weighed" on the fish scale than any change in body volume in other organs/tissues.

You could get around the gas in the tank by having a pony bottle for most of the transit time. Put it down when weighing. In Rosario Strait. Wearing a swimsuit and a smile....

 

[MaxTorque]

update, i managed to get some pool time last week (first time getting wet for a while thanks to C19) and i messed around with my weighting.

I added an additional 2 kg on a belt , grabbed a full cylinder (12l steel) and sank myself to the bottom (ok, just 3m deep), completely emptied my bcd, and breathed out as fully as i could, and it was pretty easy to swim up. Pushing myself off the bottom it felt like perhaps 4 to 5kg of force to lift off, and i could actually swim up with just my arms if i needed too (really in-efficient) or really easily with a decent flutter kick.


This i think means that even with a complete loss of wetsuit buoyancy, i think it will always be possible to swim up with my rig (without the additional 2kg) from pretty much any depth, meaning i don' t need ditchable weight for that purpose.

What was noticable however was that treading water on the surface, with a delfated bcd and those 2 extra kg was not hard, but certainly would become pretty tiring if one had to do it for any signifcant length of time. At thi point, with a failed bcd, i'd be looking for additional flotation, most probably from my dSMD. I'm pretty sure that if i needed to i could actually ditch the cylinder off my bcd/harness unless it was really rough surface conditions.