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You can scuba dive without air tanks if you bring enough battery power along to electrolyse water into a breathable 2:1 mixture of hydrogen and oxygen. That's been done, so
  [vote for,

This Idea is a _rebreather_ for such a system. It takes advantage of the lightness of hydrogen to concentrate the CO2 (which isn't so light) making simple water scrubbing feasible. It recycles the otherwise-wasted exhaled oxygen (we only intake a small percentage from a breath before the body's CO2 alarm goes off forcing us to take another breath).

An ambient pressure scrubbing tank continuously holds say 10 breaths worth of exhalation being processed. The hydrogen will float to the top of the tank, thus (and this is the important bit) increasing the partial pressure of the CO2, at the bottom, threefold.

The bottom third of the tank is continuously sprayed with local water which dissolves most of the CO2. The remaining gas mix is then fed back into the system to form the bulk of the next breath.

Add something fiddly to keep it operationally upright.

Now we only need 1-2kg of batteries to stay down an hour as compared to 5kg in an open (non-recycling) system. No need for heavy pressure tanks or bulky soda lime, and just plug it into the wall to recharge.

As a bonus your voice is even squeakier than it is on Helium.

Average human uses .87kg O2 per day.
3.685kWh to crack 1l water into hydrogen and oxygen
Humans exhale about 1kg of CO2 per day.
1L of water @ 20-30C can hold 1.5g of CO2
Lithium ion battery holds about .72MJ/kg.

FlyingToaster, Aug 13 2012

Hydrox - Hydrogen, Oxygen mix for deep dives http://en.wikipedia...%28breathing_gas%29
No Smoking [FlyingToaster, Aug 13 2012]


       //An ambient pressure scrubbing tank continuously holds say 10 breaths worth of exhalation being processed (more, at depth).//   

       A breath is a breath, regardless of ambient pressure. At depth, you might be able to fit a greater mass of air into the tank, but your breaths will also take in a greater mass of air as well. So an ambient pressure tank that can hold ten breaths worth of air at the surface will hold ten breaths worth at any depth. But that only works if you're somehow adding additional air into the tank as you go deeper. Otherwise, an ambient pressure tank would hold /fewer/ breaths as you go deeper, not more. This is already the case with standard SCUBA equipment; a tank that lasts you an hour at the surface will only last a fraction of that at depth.   

       Therein lies the problem. As you go deeper, you'd need to generate more and more gas just to be able to breathe, since the volume of each breath remains the same, but the mass of the air increases with depth to maintain ambient pressure. So at 10 meters you'd need to split twice as much water, at 20 meters you'd need to split three times as much, and so on. So at the sort of depths where it would make sense to use exotic (and dangerous) mixes like hydrox, say a couple hundred meters, the energy consumption required would quickly render this system impractical.
ytk, Aug 13 2012

       [ytk] that's not right.   

       I assume pp. is the yardstick the body uses to measure CO2 ?, otherwise at double the pressure you'd just take half as many breaths. Therefore while you need more bulk of breathing-gas, the diver isn't taking any more mols of O2 out and putting CO2 in, than at any other depth.   

       This idea is for a rebreather not an open-system where you'd be wasting all that breathing-gas.   

       While you'd have to split more water during the descent, simply to keep the scrubbing tank pressure up, that would be made up for by *not* having to split as much water during the _ascent_ since the stuff available in the tank is already done.   

       So, unless you spend the entire dive bobbing straight up and down really fast, that's not an issue.   

       (however "more efficient at depth" redacted from post)   

       //somehow adding additional air into the tank//
Not a problem: during descent the electrolyser directly contributes to filling the rebreathing tank, during ascent the electrolyser is turned off and the diver breaths the extra: more hydrogen is vented but most of the oxygen can be saved and used.
FlyingToaster, Aug 13 2012


       How deep is this intended to go ? To begin with, best to assume no more than a standard recreational dive set.   

       So, neglecting decompression, oxygen narcosis etc, the unit has to (a) remove H20, CO and CO2, and (b) resupply O2.   

       You have the advantage that the surrounding water is a very effective heatsink for condensing the water from the expired gas.   

       Now ...   

       It's going to be hard to electrolyse the recovered (condensed) water as it will be deionised, so an electrolyte will need to be added. Then the hydrogen needs to be rejected ... not hard, a suitable porous medium which will pass H2 but not other molecules is feasable - consider the zeolites used in low-rate oxygen generators.   

       But said recovered water is going to be saturated with CO2 (H2CO3). Electrolysis is going to produce an interesting mix of products.   

       Since the hydrogen is being rejected from the system, make-up water is needed. This will need to be deionised (probably by reverse osmosis) as the electrolysis of sea water directly will release chlorine ... unless you carry a bag of DI water as part of the kit, which makes much more sense.   

       A small cylinder of compressed air is carried simply to maintain pressure equilibrium between the breathing circuit and the environment.   

       The gas quality monitoring system is going to need to be very accurate and reliable.   

       The atmosphere processing systems on nuclear subs are big, complex, consume a lot of energy, and are multiply redundant. Squeezing all that into a backpack is something of a challenge.
8th of 7, Aug 13 2012

       Hmm... I think you've misunderstood...   

       Water that the diver is swimming through is taken and electrolysed into hydrogen and oxygen which is recombined into "hydrox", a breathing gas. (The H2 is kept as a dilutant because breathing pure oxygen causes oxygen narcosis.)   

       The new hydrox is then mixed with recycled hydrox from the scrubber, and the diver breathes in the combination.   

       What the diver breathes out has had some of the oxygen replaced with carbon-dioxide, which we need to get rid of before it can be rebreathed.   

       Inside the scrubber, hydrogen floats to the top leaving CO2 and O2 at the bottom. This increases the local partial-pressure of CO2. Water (also from ambient) is sprayed into the CO2/O2 mix, dissolving the CO2; the resulting weak soda-water just flows out the drain back into the ocean/lake. There's say 10 breaths worth of gas in the scrubber at any given time, so there's enough time, as it flows through to not only dissolve the CO2 but to neutralize any CO as well as reduce any humidity from the diver's breath to ambient temperature's.   

       Some hydrogen may be allowed to bubble out before the scrubbed hydrox is mixed in with the fresh, to maintain whatever the optimum ratio of H2:O2 is.   

       "rinse and repeat".   

       The mixture should be good until 300m at which point hydrogen narcosis starts kicking in and you need to add some helium or methane.   

       Regarding pressure differential, the easiest way would be to run the hydrolyser at a higher rate during descent, directly into the scrubber. Once the pressure is balanced of course the hydrolyser goes back to its normal production rate. Conversely during ascent the hydrolyser can be run at a reduced rate, using up gas from the scrubber.   

       Regarding electrolytes: to simplifty the post it's assumed that the lake has enough DS to enable conduction, and that the amount of Chlorine produced from ocean water isn't enough to injure the diver. Either way both cases have simple solutions: add a small amount of electrolyte, or osmose out enough salt.
FlyingToaster, Aug 14 2012


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