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In my textbook it said that helium is used in oxygen tanks (diving apparatus) by deep sea divers.

When I searched the internet for the reason it said that :

A mix of helium and oxygen is used in diving apparatus by deep sea divers, due to low solubility of helium in blood even at high pressure.

My doubts:

  • Why to mix helium when we can just fill the tanks with pure oxygen?

  • Why can't other inert gas be used in place of helium?

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As TAR86 said pure oxygen is problematic.

A little something from the Wikipedia page on oxygen toxicity

Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lungs and eyes. Oxygen toxicity is a concern for underwater divers, those on high concentrations of supplemental oxygen (particularly premature babies), and those undergoing hyperbaric oxygen therapy.

Turns out nitrogen is also toxic at high pressure see nitrogen narcosis.

Helium is used primarily because it's cheaper as compared to other alternatives and is not narcotic.

Other gases are too heavy and they diffuse into the bloodstream.

For more information see this PDF on exotic diving gases.

Fun Fact
Argon is used but can you guess why?

Used to inflate scuba suits, helps in insulating them.

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    $\begingroup$ It is not that helium is cheaper than anything. It is just that there are no alternatives. All other gases start dissolving in blood, causing weird effects. $\endgroup$
    – Ivan Neretin
    Commented Feb 27, 2018 at 9:55
  • $\begingroup$ Indeed, helium is not 'cheap' and only getting more expensive. $\endgroup$
    – Jon Custer
    Commented Feb 27, 2018 at 15:00
  • $\begingroup$ @JonCuster cheaper than neon at least(which is the only other somewhat safe option). $\endgroup$
    – Avyansh Katiyar
    Commented Feb 27, 2018 at 15:28
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    $\begingroup$ Yeah, the price (and supply) of neon has also gotten out of hand, but at least it can be distilled out of air. Helium comes from (a small number of) natural gas fields, and is the only gas that is worthwhile to ship internationally. $\endgroup$
    – Jon Custer
    Commented Feb 27, 2018 at 15:31

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