I was watching Mission Impossible. Secret agent had to hold his breathe 3 minutes while doing stuff deep in underwater computer lab with special thing such that metal could not traverse entryway: so no metal tanks.

Any plan which involves a man holding his breath 3 minutes while deep underwater deserves a careful rethink.

The proposed device is a very large air bladder. Possibly made from an entire hide still in the shape of an animal. It is inflated with pure oxygen and weighted. On descent and increasing pressure, the bladder decreases in size and buoyancy (and some weight must be dumped). The swimmer breathes back and forth into the bladder, consuming some of the O2 with each passage. The bladder is pressurized by the external water to match external water pressure, and so there should be no issue breathing in and out of the bladder at any depth. Pure pressurized O2 is not easy on the lungs but the life of a secret agent is full of danger and it beats trying to hold your breath.

I think that the CO2 level in the bladder and the man will get high enough to produce hyperventilation within 10 minutes. How to purge CO2? The return valve by which exhalations return to the bladder are bubbled though a water reservoir like a hookah. The charge of water in the water reservoir is purged / replaced each breath either manually or powered by the inhalation. Scrubbing of Co2 from exhaled gas is by virtue of the high solubility of Co2 in water. Nitrogen and unused oxygen return to the bag.
-- bungston, Jun 27 2016

Some of the same principle - EU(R)BA
- use local water to scrub CO2 out, and reuse air which still has oxygen in it. [FlyingToaster, Jun 27 2016]

Wikipedia on diving technology https://en.wikipedi...f_diving_technology
The stone age tech innovation in this concept is that the air (oxygen is nice but air woul dbe fine) is contained in a collapsible / expansible reservoir. A rigid box would not work. [bungston, Jun 28 2016]

I think you may wish to read up on oxygen toxicity. Or maybe you already have...

"The bladder is pressurized by the external water to match external water pressure, and so there should be no issue breathing in and out of the bladder at any depth."

I'm none too certain about this part either...You should probably experiment with a balloon in a swimming pool first. Go ahead, I'll wait here.
-- normzone, Jun 27 2016


If this was practical, it would have been done about 4000 of your Earth years ago...
-- 8th of 7, Jun 27 2016


/it would have been done/

Yes, yes, everything worth inventing has been invented. But how long did it take to invent a wheel, and then how much longer to put the wheels on suitcases? I suspect that the failure to breathe at depth via long tubes to the surface led our ancestors to give up on the whole endeavor.

Empirics are for people with dirt under their fingernails. Esconce yourself firmly in that armchair, like a good halfbaker, and elucidate from first principles why this scheme would not work.

That said my own nails are none too clean and if I had ready access to a pool I would test this in a heartbeat. I have a big box of 30 gallon bags that would be perfect, and a stack of bricks and some pvc pipe. I should clarify: ready access to a pool that will let me traipse on in carrying a big bag, bricks and pipe.
-- bungston, Jun 28 2016


//If this was practical, it would have been done about 4000 of your Earth years ago...//

Most of the early work on oxygen (as opposed to air) for combustion and respiration was done in the 17th or 18th century, with production of oxygen gas in 1772 by Carl Wilhelm Scheele.
So you were off by a minimum of 3756 of my Earth years.
-- Loris, Jun 28 2016


No knowledge of the existance or otherwise of oxygen is neccessary to empirically test this idea.

The problem of the very large (and depth-dependant) buoyancy of the "bag" isn't adequately addressed.

However, what would work- after a fashion- would be a weighted "diving bell" with pipes (intake& output) to the diver. Both would then be equilibrated with the surrounding water and the diver could breathe with minimal effort. How mmuch the CO2 solvation part would work is debateable.

A "bell" with a volume of 2m3 would be easy to construct. Just add weights, some leather hoses, and a "helmet". The diver would need a weightbelt, and would descend and rise outside the bell.

Decompression might be a problem.
-- 8th of 7, Jun 28 2016


/The problem of the very large (and depth-dependant) buoyancy of the "bag" isn't adequately addressed./

Weights.

/diving bell/ It is hard to tote that around while you are swimming around in the dark commando-style. And by commando-style I mean like a secret agent.
-- bungston, Jun 28 2016



random, halfbakery