Air, which was first used in diving tanks, contains roughly 20% oxygen, and 80% nitrogen.
The trouble with nitrogen in this situation is that nitrogen is a fairly heavy gas, and is soluble in blood at high pressure. Long term use of nitrogen can cause a strange sense of euphoria, or well being called nitrogen narcosis. This is a bit like being drunk, and makes the diver unable to assess dangers. Just as working drunk on the surface is not a good idea, working drunk at the bottom of the sea can be extremely hazardous.
Divers working at depth would have nitrogen dissolved into their blood, and it becomes necessary to decompress (surface) very slowly, to allow the nitrogen to come back out of the blood to be breathed out. Divers who come up too quickly have a huge risk of the nitrogen being released from the blood too quickly and forming bubbles in the bloodstream. This causes blockages, as the bubbles, although tiny, cannot pass through the fine capillaries. The condition is called the Bends, is very painful, and life threatening.
One way to avoid all these problems is to avoid the nitrogen. Divers who work at depth or for long periods use a mixture of 20% Oxygen and 80% Helium. Some recreational divers use a slightly cheaper mix of Helium, Oxygen and air. It still has nitrogen, but less than air.
Helium is used for a number of reasons - It is light, cheap, and does not dissolve in blood the same way that nitrogen does. Being inert it cannot be toxic to the diver or corrosive to equipment.
The trouble with nitrogen in this situation is that nitrogen is a fairly heavy gas, and is soluble in blood at high pressure. Long term use of nitrogen can cause a strange sense of euphoria, or well being called nitrogen narcosis. This is a bit like being drunk, and makes the diver unable to assess dangers. Just as working drunk on the surface is not a good idea, working drunk at the bottom of the sea can be extremely hazardous.
Divers working at depth would have nitrogen dissolved into their blood, and it becomes necessary to decompress (surface) very slowly, to allow the nitrogen to come back out of the blood to be breathed out. Divers who come up too quickly have a huge risk of the nitrogen being released from the blood too quickly and forming bubbles in the bloodstream. This causes blockages, as the bubbles, although tiny, cannot pass through the fine capillaries. The condition is called the Bends, is very painful, and life threatening.
One way to avoid all these problems is to avoid the nitrogen. Divers who work at depth or for long periods use a mixture of 20% Oxygen and 80% Helium. Some recreational divers use a slightly cheaper mix of Helium, Oxygen and air. It still has nitrogen, but less than air.
Helium is used for a number of reasons - It is light, cheap, and does not dissolve in blood the same way that nitrogen does. Being inert it cannot be toxic to the diver or corrosive to equipment.
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Air is about 78% Nitrogen. The high pressure of deep diving causes Nitrogen to dissolve into the blood causing Nitrogen narcosis which can severely disorient a diver and cause him to lose consciousness. If the diver surfaces too rapidly, the lower pressure allows the Nitrogen to form bubbles in the blood which results in a potentially fatal condition called "The Bends". Using pure oxygen is not possible as it becomes toxic at high concentrations under pressure. Helium is cheap, non-toxic, does not dissolve into the bloodstream and allows the Oxygen to be diluted to safe levels. It prevents all of the Nitrogen problems associated with using air for deep dives.