Why deep diver take oxygen mixed with helium instead of pure oxygen while going deep in sea for respiration?

Answer:

To breathe underwater a diver must be supplied air at a pressure equal to that of the water surrounding the diver.

However there is an upper limit of oxygen pressure above which the oxygen becomes biochemically toxic.

Therefore, it is necessary to include something to dilute the oxygen in a diver's breathing gas. To satisfy normal breathing requirements at high pressures, it is necessary to supply between 0.2 and 1.5 atmospheres of oxygen with the rest of the balance made up by a non-toxic diluent (such as nitrogen).

Air of course is a suitable breathing mixture for a diver based primarily on nitrogen as the oxygen diluent.Air is in fact the preferred breathing mixture for all dives to depths of less than about 150-200 fsw (feet of sea water).

However, even at 150 fsw, when breathing air most divers feel the effects of nitrogen narcosis.

Beyond this depth helium is preferred as the diluent and is in fact particularly well suited to the depth range immediately beyond air diving (e.g., 150-250 fsw). Helium does not cause narcosis at these pressures, is relatively inexpensive and is readily available. Moreover helium has a low density and is, therefore, easy to breathe at such pressures.

There are problems with helium though, problems that are seriously amplified as diving depths approach those of the outer continental shelves and beyond.

First there is the problem of communication. Everyone knows what breathing helium will do to your voice. Due principally, it is believed, to changes in the speed of sound in the gas medium, this effect is a sensitive function of depth. Helium speech at sea level is distorted, in a way that seems funny to both the listener and the speaker, but it is completely intelligible. At 200 fsw speech with Helium is still reasonably understandable.

However, as depths increase to the range between 400 and 600 fsw the situation becomes more serious, and to someone trying to get a job done helium speech is no longer considered funny. Speech in this range is totally lost on an untrained ear, though anticipated statements can be understood by a listener familiar with the voice and the situation.

So often, however, a sudden change in the topic of conversation throws everyone off, and it is necessary for the diver to speak slowly, repeat himself and to try to say things a different way. It can be done but it is slow and consequently expensive.

The other problem is that Helium is about 4 times as good at conducting heat as nitrogen. Which when at the chilly depth of 500 fsw means that you are going to get VERY cold.