A few years ago I wrote a post about an Indiegogo campaign to raise money for a gravity powered light. I rather poured scorn on their claims but I notice that they are still very much in business. I continue to have doubts about their ability to produce meaningful amounts of useful light but others seem to think it is working.
There is another interesting page at www.indiegogo.com with a fundraiser for a device called a Triton Rebreather. It is a seriously cool-looking piece of kit. It is battery operated and claims to be able to allow you to swim under water for up to 45 minutes. Having originally claimed that it got all of its oxygen for this from oxygen dissolved in the water they have now relaunched explaining that they combine “artificial gills technology & liquid oxygen technology”. The device will contain small cylinders of liquid oxygen that supplement the oxygen the device can extract from the water.
It is possible to think about this in terms of human requirements for oxygen. At rest a typical adult breathes in about 500 cm3 of air every breath. Of that about 20% is oxygen, most of the rest being nitrogen. Exhaled breath is about 15% oxygen meaning that the lungs have hung onto 5% of the original 500 ml. This means that each breath you absorb about 25 cm3 of oxygen.
The density of oxygen is 0.0014 g/cm3 so we can work out what mass 25 cm3 of oxygen would have.
Mass = Density x Volume
Mass = 0.0014 g/cm3 x 25 cm3
Mass = 0.035 g which is 35 mg
So humans absorb about 35 mg of oxygen with every breath when breathing normally. Now we need to know how much oxygen there is dissolved in water. The figure I found was for well oxygenated (ie probably cold) seawater and it was 6 mg/litre. This means that to supply enough oxygen for a resting human you would need to extract all the dissolved oxygen from 35 mg ÷ 6 mg/litre = 5.8 litres so let’s call it 6 litres of water.
A normal healthy person breathes about 15 times per minute. 15 breaths x 6 litres/breath = 90 litres of seawater every minute assuming that the system was 100% efficient. This seems very unlikely in such a small device. This is why they have now admitted that liquid oxygen is also incorporated into the design. Liquid oxygen has a density of just over 1 g/cm3; about the same as water. If the cylinders holding this liquid oxygen are going to fit into the design shown they will need to be fairly small – lets say about 10 cm in length and 2 cm in diameter. This means that they would hold 10πr2 cubic centimetres of liquid oxygen each, or about 60 cm3 in total.
If the density of liquid oxygen is 1 g/cm3 then this means that the apparatus holds about 60 g (60,000 mg) of oxygen. That is enough oxygen to keep you alive for a while. However, we don’t breathe pure oxygen in tiny 25 cm3 breaths at a time. We need 500 cm3 of a mixture that is only 20% oxygen. You could replace 80% of the contents of the cylinder with an inert gas like nitrogen or helium, leaving 12,000 mg of oxygen.
As I explained earlier, our lungs can only extract about ¼ of the oxygen in the air we breathe, so for every 35 g we absorb, we breathe out another 105 g. This means we would eat into the oxygen in 140 g chunks. 12,000 divided by 140 means about 86 breaths. At 15 breaths per minute that is less than 6 minutes under the water. Oh dear.
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Scuba tanks do not contain liquid oxygen but cleaned and compressed air. I found this on the dangers of breathing pure oxygen…
The result is that free oxygen binds to the surface proteins of the lungs, interferes with the operation of the central nervous system and also attacks the retina.
…breathing oxygen at [..] roughly two and a half times normal [concentration] for more than 16 hours can lead to irreversible lung damage and, eventually, death.
On the Indiegogo site, the lead designer’s biography has the following opening sentence…
Jeabyun is a designer from South Korea who breathes science and technology
Well that’s OK then, he doesn’t need oxygen anyway.
I don’t see how it can possibly work. Maybe you can, or maybe you can see problems I have not mentioned above; do offer any thoughts you have in the comments section below.
Questions…
- Some fish are bigger than humans and they cope perfectly well extracting oxygen dissolved in water. What key difference distinguishes a fish’s oxygen requirement from ours?
- Why does cold water hold more dissolved oxygen than warmer water?
- What affect would cold water have on the respiration rate of a diver?
- What structures in lungs extract oxygen from air?
- Suggest a feature they have which helps them to do this efficiently.
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