Underwater Oxygen Generator

[Excelsior]

You know how submarines use a process of electrolysis, etc, process called electrolysis splits the water molecules into H2 and O. Oxygen can?t exist alone so it bonds with another oxygen molecule to make O2. The H2 is discharged overboard and the O2 is released into the sub.

Could they make a smaller version of this for personal use underwater? Like a backpack?

 

[Evadman]

It would have to be extreamly large to generate enough O2 for consumption. Also the power needed would be large as well.

 

[pm]

A quick search on Google produced that it takes approximately ~7kwh hour to generate one cubic meter of oxygen from water. Electrolysis of oxygen from water is extremely efficient (what you get out is very close to what you put in) but it is a very energy intensive to break the H-O bond.

To put this amount of energy in perspective, a typical R/C racing NiCd battery pack is 10V and 1Ah. That's a 10wh pack and it weighs about 200 grams. Leaving aside details of maximum efficiency electrolysis voltages for a moment, that's would mean that you'd require a 700 packs to generate one cubic meter of oxygen - or about 140kg worth of batteries. In addition, it's worth noting that pure oxygen is pretty poisonous at underwater pressures and also that long-bottom times will result in pretty serious problems with nitrogen in the bloodsteam. Fuel cells might eventually make it feasible to do something like this though - with a little engineering work to solve the various problems..

In the meantime, chemical rebreathers (CO2 scrubbers) are a currently available viable alternative to staying under water for quite a while.

Personally after a single tank dive to about 30 ft (usually a little over an hour for me), I'm pretty much ready for an above-water break, anyway. I'm not sure that I'd be ready to spend much more time than that under water. On those rare times that I go on deeper dives, longer times would be nice, but that would introduce slow/long ascents to avoid the bends.

 

[RossGr]

pm gave you the numbers, keep in mind that a nuc sub has a reactor at its heart this provides all the power it needs plus some. That is the reason these boats can stay underwater for months at a time. IF it were not for the crew they only need to come up for every year or two for refueling.

 

[f95toli]

You can get equipment for personal use that extend the time you can spend under water significantly. The idea is to remove the CO2 (which is toxic) from the air you breath out, then you can recycle the air (since you do not consume all the oxygen in the air you breathe in).
This technique has been used in sub-marines for many decades but I think it is quite new for personal use (I guess the military has used for some time).

 

[bwanaaa]

I thought there was a metal (platinum? zinc?)that when added to hcl resulted in the release of h2,o2,and cl2. Then you just have to get riid of the h2 and cl2.

But another way to carry a lot of o2 under water would be as a liquid. LOX has been used in rockets for a long time. The technology for producing it in mass quantities,managing it, and delivering it already exists. Instead of a scuba tank that supplies you for 30 mins, you could carry a purse that supplies you for the day.

But the japanese alreadyy have the artificial gill down to the size of a backpack.

 

[Mark R]

Liquid oxygen doesn't strike me a as very practical - sure, liquid oxygen is more compact than compressed air (approximately by a factor 5), but there are problems with making it safe to use.

Pure oxygen is highly toxic at pressures used underwater, you have to dilute it - preferably with helium.

Clearly a liquid oxygen/helium mixture is not possible, so in order to maintain a safe concentration of oxygen you have to use a rebreather system, with CO2 scrubber in order to prevent loss of diluent.

You then have the problem of cryogenicity - ingress of heat will rapidly evaporate the oxygen which then has to be vented; dewar flasks are bulky, and relatively fragile, probably more so than a conventional cylinder.

 

[pm]

For the curious, here is a link to the Inspiration rebreather - from the reading that I did this was described as the most impressive of the high-end consumer rebreathers on the market. It's good for about a 6 hour dive. I was surprised to note that the dive time appears to be independent of depth (as opposed to pressurized tanks where deeper dives result in much shorter dive times). I guess that makes sense, but it's a strange concept to me.

Filled to 200 bar the Inspiration's 3 litre oxygen cylinder contains 600 litres of oxygen. Therefore, at the average consumption rate of 1 lpm, theoretically, each fill should last 600 minutes or 10 hours. In practice some gas is lost during mask clearing and extra O2 is added during ascents. Nevertheless, it is realistic to achieve at least 6 hours diving from one 3 litre cylinder of oxygen.

 

[rgwalt]

I would think that dive time would vary with depth for any type of compressed O2 scuba gear...

Ryan

 

[RossGr]

I think the difference with this rebreather is that it is a CLOSED system. With conventional tanks, you take a breath, extract a certain precentage of O2, the rest is exhaled and wasted. With this system, the O2 not used in a breath is returned to the system for later use. It is the release of air at depth that creates the majority of the waste.

One way to look at it may be that the body requires a fixed NUMBER of O2 molecules this number is indepentent of the depth, if you do not throw away air with every breath the tank will supply the same number of molecules at any depth.

Note that the rebreather seems to have a 50-60m limit. For recreational diveing this should not be a restriction. 125' was a sweet spot in my diving experiance.

I was spoiled, diving in Guantanamo Cuba in 1970, upon return to the PNW I dove in the Pacific 1 time, have never been back.

 

[sao123]

Can someone explaine why pure liquid O2 (oxygen) is toxic?

 

[pm]

It's not even that pure liquid oxygen is toxic. At high pressures high concentrations of oxygen are toxic.

Quoting from a New Jersey dive website (that I found with Google using "oxygen scuba toxic" as the search terms).

Human lungs are designed to extract the oxygen we need from air - a mixture of roughly 21% oxygen and 79% nitrogen, at a pressure of one atmosphere ( about 14.7 psia.) As you dive deeper and longer while breathing air, the increased pressure causes ever-greater amounts of both gases to dissolve in your blood and tissues. One would expect that eventually such elevated concentrations would become troublesome, and indeed that is the case. As it turns out, nitrogen, with its greater concentration in the air, is the first gas to become a problem during a dive to recreational depths ( <130 ft. )

Then there's some discussion of enriched oxygen tanks which allow longer bottom time by reducing the amount of nitrogen in favor of additional oxygen (like 40% oxygen and 60% nitrogen). Then it continues with by discussing the downside of enriched oxygen mixtures:

This is because the depth limits of enriched oxygen mixtures are generally based upon oxygen toxicity effects, while the depth limits for air are based on the probability of nitrogen narcosis. Nitrogen narcosis is a well known effect that deep divers experience while breathing air at depths beyond 140-160 ft. Its onset is gradual and the effects, comparable to drunkenness, are usually manageable by experienced divers. Oxygen toxicity, on the other hand, strikes swiftly and without warning, and is usually fatal underwater - convulsions, leading to loss of consciousness and drowning, although it takes more than one or two breaths to bring it on.

Here's another link:

The occurrence and type of oxygen toxicity correlate with the O2 concentration, the ambient pressure, the length of time supplemental O2 is inhaled, and the diver's level of activity. Since air contains 21% oxygen, the amount of oxygen inhaled at sea level is .21 atm. O2; this amount is safe to breathe forever. From clinical experience it appears that patients can breathe .40 atm O2 indefinitely, and possibly up to .60 atm O2 for weeks at a time (equivalent to 40% O2 and 60% O2 at sea level, respectively), without apparent oxygen toxicity.

In healthy subjects, 100% oxygen at atmospheric pressure (1 atm. O2) causes chest discomfort, pain and cough after only a few hours. If inhaled continuously over 24 hours, 1 atm. O2 can lead to lung congestion (pulmonary edema) and, if continued, death. Obviously, doctors try not to use high concentrations of oxygen unless absolutely necessary. Patients who require 100% oxygen because of heart or lung disease are critically ill and will almost always be cared for in a hospital intensive care unit.

Although potentially toxic, 1 atm. O2 does not cause seizures. However, when 100% oxygen is delivered at pressures two or more times sea level pressure, the first toxic manifestation can be a seizure. A seizure is a sudden electrical discharge from the brain that causes uncontrolled muscle movement. If seizures occur under water the diver will likely be unable to breathe through the regulator and will drown (if rescue is not immediate).

 

[rgwalt]

Originally posted by: RossGr
I think the difference with this rebreather is that it is a CLOSED system. With conventional tanks, you take a breath, extract a certain precentage of O2, the rest is exhaled and wasted. With this system, the O2 not used in a breath is returned to the system for later use. It is the release of air at depth that creates the majority of the waste.

One way to look at it may be that the body requires a fixed NUMBER of O2 molecules this number is indepentent of the depth, if you do not throw away air with every breath the tank will supply the same number of molecules at any depth.

Note that the rebreather seems to have a 50-60m limit. For recreational diveing this should not be a restriction. 125' was a sweet spot in my diving experiance.

I was spoiled, diving in Guantanamo Cuba in 1970, upon return to the PNW I dove in the Pacific 1 time, have never been back.

Ah, I didn't think about the fact that it is a closed system. Makes sense now.

Ryan

 

[Mark R]

Can someone explaine why pure liquid O2 (oxygen) is toxic?

As has already been said, it doesn't have to be liquid - simply breathing pure oxygen gas is toxic.

There are 2 main sites of action: 1) the lungs, and 2) the brain

1) Oxygen is a powerful oxidising agent, and will react with many chemicals found within the body, to form extremely unstable 'free radicals' and other oxidising compounds (e.g. hydrogen peroxide). The lungs are naturally protected against free radicals, the tissues produce catalytic enzymes which destroy these dangerous chemicals.

However, when breathing pure oxygen (or a lower concentration of oxygen at higher pressure), free radicals are produced faster than they can be removed. They accumulate and cause injury to the tissue, potentially leading to respiratory failure and pulmonary oedema (fluid on the lungs).

2) Oxygen reacts with 'neurotransmitters' in the brain. The function of the brain is carefully regulated by chemical messages sent from cell to cell. Excessive oxygen destroys these transmitters, and regulation of electrical activity is lost. The result can be loss of consciousness and seizures (epileptic fits).

Brain effects are of fairly quick onset, but require very high levels of oxygen (pure oxygen at a depth of 100 feet). Lung effects are slower but require less oxygen (pure oxygen at atmospheric pressure (or 50% oxygen at 30 feet) could begin to cause injury after 6 hours).

 

[KilroySmith]

It's still not clear why liquid Oxygen can't be used diving. My Grandfather-in-law visited last year, he needs oxygen continuously. We had a service deliver a bulk oxygen tank, and he brought his carryable, day-size tank. Neither one used compressed oxygen - both stored liquid oxygen, both were essentially stainless-steel thermoses. The McDonalds near my house doesn't use bottles of compressed CO2 to carbonate their soft drinks like they used to when I was a kid; they've got one big tank (6' Tall, 2' circum) filled with liquid CO2, another stainless steel thermos. Thus the technology exists in the consumer world to safely deliver, transfer, store, and use liquid gases.

I would imagine that one could use a tank of liquid for Scuba by filling the tank with a mixture of liquid gases (though you may not be able to control the gas composition, as each element would vaporize at a different temperature), by having seperate compartments for each liquid and mixing after vaporization (perhaps a problem maintaining gas composition in the face of equipment failures), by mixing in an available gas (would water vapor reduce oxygen toxicity?), or some other means.

I can only imagine the small size tank that would result from combining an hours worth of liquid O2/N2, and a rebreather setup. Propane-bottle sized SCUBA outfit, anyone?

/frank

 

[NogginBoink]

Basically, oxygen at partial pressures over 1 atmosphere is toxic.

It's the partial pressure of O2 that makes it toxic, I belive.

 

[NogginBoink]

Originally posted by: pm
A quick search on Google produced that it takes approximately ~7kwh hour to generate one cubic meter of oxygen from water. Electrolysis of oxygen from water is extremely efficient (what you get out is very close to what you put in) but it is a very energy intensive to break the H-O bond.

To put this amount of energy in perspective, a typical R/C racing NiCd battery pack is 10V and 1Ah. That's a 10wh pack and it weighs about 200 grams. Leaving aside details of maximum efficiency electrolysis voltages for a moment, that's would mean that you'd require a 700 packs to generate one cubic meter of oxygen - or about 140kg worth of batteries. In addition, it's worth noting that pure oxygen is pretty poisonous at underwater pressures and also that long-bottom times will result in pretty serious problems with nitrogen in the bloodsteam. Fuel cells might eventually make it feasible to do something like this though - with a little engineering work to solve the various problems..

In the meantime, chemical rebreathers (CO2 scrubbers) are a currently available viable alternative to staying under water for quite a while.

Personally after a single tank dive to about 30 ft (usually a little over an hour for me), I'm pretty much ready for an above-water break, anyway. I'm not sure that I'd be ready to spend much more time than that under water. On those rare times that I go on deeper dives, longer times would be nice, but that would introduce slow/long ascents to avoid the bends.

That would be silly-- take O2 and H2 as fuel for the fuel cells, combine 'em to make water and power, and use that power to turn water into O2 and H2.

Why not just use the O2 you're taking with you, and breathe that?

 

[Excelsior]

I had actually forgotten about this thread, but Im glad it wasnt left behind. Ive learned quite a bit so far.

 

[pm]

That would be silly-- take O2 and H2 as fuel for the fuel cells, combine 'em to make water and power, and use that power to turn water into O2 and H2.

Well, I didn't have a hydrogen fuel cell in mind when I wrote that. I was thinking more along the lines of methanol and benzene cells. Hydrogen fuel cells are pretty much at the prototype stage and there are still plenty of engineering challenges to them whereas methanol ones are currently available. But now that you mention it, that is a good point. I wonder if using a methanol fuel cell that consumes oxygen could produce more oxygen though electrolysis than it consumes. You're probably right though... it was a bit silly.