SCI-TECH & AGRI

Underwater breathing using dissolved air in the water

OBSERVING FISH: An Israeli inventor, Alon Bodner, turned to fish for inspiration. Fish use the dissolved air in the water to breathe.

OBSERVING FISH: An Israeli inventor, Alon Bodner, turned to fish for inspiration. Fish use the dissolved air in the water to breathe.  

K.S. RAJGOPAL

There are a number of limitations to existing underwater breathing equipment

IF THE idea of breathing underwater without cumbersome compressed air tanks has been the dream of science fiction writers for many years, An Israeli inventor has developed a device which will enable just that.There are a number of limitations to the existing compressed air tank underwater breathing method. The first is the amount of time a diver can stay under water, which is dependent on the compressed air tank capacity. Another limitation is the dependence on compressed air refuelling facilities near the dive site which are costly to operate and are used to compress air into the tanks which might be dangerous if not handled properly. Finally, when these tanks are in use, they empty out and change the balance of the diver.

Chemical separation

Engineers have tried to overcome these limitations for many years now. Nuclear submarines and the International Space Station use systems that generate oxygen from water by electrolysis, which is chemical separation of oxygen from water. These systems require very large amounts of energy to operate. Divers cannot even consider carrying such large machines, not to mention supplying them with energy.To overcome these limitations, an Israeli inventor, Alon Bodner, turned to fish for inspiration. Fish do not perform chemical separation of oxygen from water. Instead they use the dissolved air that exists in the water in order to breathe.In the ocean, the wind, waves and under water currents help spread small amounts of air inside the water. Studies have shown that at a depth of 200 metres below the sea surface, there is still about 1.5 per cent of dissolved air. Though it does not sound like much, it is enough to allow fish to breathe. Bodner's invention, based on an idea he conceived in 2000 and developed into a laboratory model in 2005 and a real prototype constructed in March 2007, will use these relatively small amounts of air to supply oxygen to scuba divers. According to Bodner he received funding for his project through a grant from The Israeli Ministry of Industry and Commerce in 2002.The system developed by him uses a well-known physical law called Henry's Law, which describes gas absorption in liquids. This law states that the amount of gas that can be dissolved in a liquid body is proportional to the pressure on the liquid body. The law works both ways; lowering the pressure will release gas out of the water. In Bodner's invention, this is done by a centrifuge which rotates rapidly creating low pressure in a chamber containing sea water.

Rechargeable batteries

The system is powered by rechargeable batteries. Calculations have shown that a one kilo Lithium battery can provide a diver with about an hour of diving time. The prototype weighs 15kgs, which is much less than a conventional scuba tank and was found to successfully separate air from water.Elaborating on the principle on which the invention is based, Bodner says that in the open system, a diver can consume about 25 litres of air per minute at the surface.Assuming that there is about 2 per cent of dissolved air in the water at that level, the calculations show that the water flow requirement is 1,250 litres per minute.

More air

As one goes deeper, more air is required. At 10 metres depth the air and water flow requirement is doubled and that is a lot.In closed systems, the air is re-circulated and returned to the diver after the carbon dioxide is removed. For this case we calculate the oxygen consumption rate and not the air consumption rate as before. The body requires the same amount of oxygen at all depths.

Oxygen consumption

For moderate effort, humans consume about 25 litres-per-minute (LPM) of air at atmospheric pressure. The air in the atmosphere contains about 21 per cent oxygen, but we use only about 4 per cent with every breath. So, the air we exhale contains about 17 per cent oxygen and 4 per cent carbon dioxide. In other words we consume about 4 per cent of 25 LPM, which is 1 LPM of oxygen. Assuming that there is half per cent of dissolved oxygen in the water, the resulting water flow requirement is only 200 litres per minute, enabling the design of a compact machine.

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