> As a nitrox certified diver I can offer that most common nitrox mixes do not exceed 34% You're right. I have never practically seen a Nitrox mixture with 40%

> As a nitrox certified diver I can offer that most common nitrox mixes do not exceed 34% You're right. I have never practically seen a Nitrox mixture with 40% oxygen. 28% would be far more typical. Mostly I dived with mixtures around 32%. For wild and wonderful reasons, however, my SSI Nitrox certification allows mixtures of up to 40%. This would allow a maximum operating depth of 25 meters (30 meters if you stretch the safety margin), which is not very much when you go - for example - wreck diving at a lot of places.