Why do scuba divers do not carry only oxygen in their scuba kit or cylinders?
Answers
What you are talking about there is something called Tri-Mix. This is used for deep diving, where the percentage of oxygen can be reduced as well as the percentage of nitrogen.
As you go deeper, the partial pressure of gasses increase. When you exceed certain partial pressures, gasses interact with your body and can either become narcotic or even poisonous. Oxygen becomes poisonous at depths of 70m-80m which means if you breathed pure oxygen, it us unsafe to use it below about 10m.
Nitrogen becomes narcotic at depths from about 30m and in addition, being slow to absorb into body tissue and slow to diffuse out, it can cause decompression sickness. At depths approaching 50m, virtually everyone will be suffering from nitrogen narcosis or 'rapture of the deep'. It can only be mitigated by regular deep diving, a bit like if you drink alcohol regularly you become a bit immune to its effects. For deep commercial diving where divers need to be working and doing jobs like inspecting structures, removing obstructions, welding etc being 'off your face' isn't productive or safe.
So how does helium fit in? Helium is an inert gas. It isn't narcotising and it isn't poisonous even under large partial pressures. You can use it as a 'spacer' gas that reduces the partial pressures of gasses that are dangerous and keeps them in safe limits. Being very light, it also helps reduce breathing effort. As you dive deep, the gas is not only more compressed it becomes heavier per cubic centimeter and more viscous. By using a percentage of helium, it greatly reduces breathing effort.
Helium therefore enables deep diving (greater than 50m) and reduces the effects of oxygen toxicity and nitrogen narcosis. If it was used for shallower diving it could reduce or eliminate decompression sickness but it isn't used because it is so expensive.