Slight clarification: the Apple Watch Ultra has a water resistance rating of 100m. The Oceanic+ app has a 130ft/40m limit because that's the recreational diving limit.
[1]: https://support.apple.com/kb/SP879?viewlocale=en_IN&locale=e....
[1]: https://support.apple.com/kb/SP879?viewlocale=en_IN&locale=e....
A) oxygen partial pressure with compressed air reaches 1.05atm, with 1.2-1.4atm posing a serious risk of oxygen-induced seizures (extremely dangerous)
B) nitrogen partial pressure becomes high enough that it becomes narcotic (every gas gets you high at enough pressure, even noble gasses).
Narcosis is an issue with nitrogen and oxygen (as well as most other gasses). Helium is the exception. It is a noble gas and has no real narcotic effect; rather the opposite. Other noble gasses such as argon are much more narcotic than nitrogen and aren't used as diving breathing gasses (outside of a few limited experiments).
The other problem with nitrogen and oxygen is that they are relatively dense and so at greater depths they cause increased work of breathing, which in turn causes CO2 accumulation. CO2 is itself highly narcotic, and has other negative physiological effects. So for deeper dives we add progressively more helium to the mix.
Well that's definitely better - I wonder what happens when you go under 130 feet, though. A dedicated dive computer would at least provide accurate information, rather than throwing you to the wolves because you exceeded some legal limit.
Also on the plus side, some of the existing air pressure transmitters use bluetooth, so if they could get that working, they'd actually be one of the cheapest air-integrated computers on the market.
(edit: they do not use bluetooth, they use low-frequency radio ~38khz)
I think I'll stick with my Shearwater Peregrine though, pros and cons considered.
Also, in the case of lack of evidence either way, assuming it would shit the bed is _definitely_ the right approach in diving.
I think your concerns are valid and merited for your very advanced level of diving, but at the same time the vast, vast, vast majority of divers aren't going anywhere near 130 feet. If I were going on a standard "simple" dive to <60 feet, I think I'd be pretty comfortable with just the apple watch ultra, a dumb watch + cheap depth guage and a written back-up plan.
1: https://www.dcrainmaker.com/2022/11/diving-with-the-apple-wa...
Also, I never dive past recreational limits, I'm not a technical diver by any means. I would simply have philosophical concerns about a piece of equipment that could provide accurate information beyond recreational limits, but simply doesn't for "legal" reasons. It probably makes sense to people - "oh they don't want to be liable for giving advice past recreational limits", but other companies seem to have navigated this legal issue and choose to provide accurate information regardless of this arbitrary limit.
As another commenter pointed out, I have no idea what it does past 130 feet, but the disclaimer that it _cannot_ be used past 130 feet, rather than one that says you _shouldn't_ use it past 130 feet, is somewhat concerning to me, even if I don't approach those limits.