So, what's in the hang tank?

I was diving in Subic Bay in the Philippines at a pretty good dive centre and we were undertaking no-stop diving activities at deeper recreational depths. As a safety precaution, they suspended a hang tank at five metres for low on air situations - great stuff for recreational diving operations.

In their hang tank was a nitrox blend of 31% oxygen and entry-level divers with no nitrox certification were undertaking dives - was there an issue here? I would suggest the answer is no as a blend of EAN31 has a partial pressure of 0.15 ATA at 5 metres - that's pretty conservative. Referencing a CNS table, a partial pressure in the 0.6 range has a maximum CNS limit of 720 minutes. Generally most recreational divers undertake a safety stop for 3 to 5 minutes, that's not a huge uptake of CNS oxygen toxicity so at this point, who really cares?