[jsjohnst]

> You can't breathe the 200m mix at sea level pressure.

What makes you think you can’t (other than it being hypoxic)? You most certainly can breath Heliox, in fact, it has been used medically[0] for almost 100 years, far longer than used for commercial diving.

[0] https://en.m.wikipedia.org/wiki/Heliox

[johncolanduoni]

Heliox doesn’t denote a specific concentration of helium and oxygen; even just within diving the mix varies depending on the depth it is to be used at.

[jsjohnst]

Correct! A common ratio for deep diving is 12/88, but 21/79, 30/70 and 40/60 are also used at less depth.

In fact, Heliox would usually not be used for dives to 200m due to HPNS risk. Usually it would be Trimix for that.

Either way, GP is mostly wrong on the points they made.