@halocline the argument for helium below 100ft that
@PfcAJ has basically nothing to do with ppO2.
@Boston Breakwater saying something off topic is not where that post went sideways.
We have 2.5 variables that weigh into MOD for any particular gas
First and foremost is ppO2 which is heavily emphasized in nitrox courses due to oxygen toxicity and is unfortunately the only variable that is really taught in most courses. That ppO2 limit is personal preference and can go anywhere from 1.0 to 2.0. 1.6 is the accept MOD that is used for marking bottles. In that case, Air has a MOD of 220ft.
Once we start getting into trimix classes, narcosis starts becoming very important, and when we start to talk about deep air it is obviously very important. A combined ppO2+ppN2 less than 4 is my personal preference. Research is saying that O2 is just as narcotic to us as N2 and where best mix for trimix used to only set N2 to some limit, I think it is much more prudent to set the combined partial pressure to 4. That happens to be 100ft, and that is the point where symptoms of hubris, dark narc, memory issues, and overall processing power/situational awareness become an issue. Again, you can set that limit to whatever you want, but the agencies normal recommendation is to set ppN2 to a max of 4.0, or set a combined ppO2+ppN2 to 4.0. In this case the MOD of air would be 132fsw or 99fws.
The half variable. I say half because it is really a subset of the narcosis variable, and that is gas density. Why does gas density matter? When you are trying to breathe, and it gets harder to breathe, you have a natural tendency to start breathing shallow which leads to CO2 retention in the lungs. CO2 is 20x as narcotic as nitrogen and can lead to all sorts of issues. CO2 is a scary effing gas and downright deadly. This is part of the reason that many divers consider DPV's a required piece of safety equipment when diving to trimix depths in order to limit the production of CO2 by minimizing exertion. It's the reason that many well known divers have died, most notably David Shaw, and I firmly believe is the real reason that most people get narc'd, not nitrogen. Unlike N2 though, it doesn't clear immediately after you get to a shallower depth, and it has lingering effects *my least favorite is a horrific headache at the base of your skull that lasts for over a day*. Based on that, the MOD for air is something that you have to choose for yourself. For me? It is variable. I'm much less reluctant to dive air to 180ft on a lazy drift dive in a bathing suit than I am to do a 180ft cave dive or even to go that deep in our local lakes which are cold and dark. I stand with AJ that below 100ft I want to have helium, though I don't consider it a hard and fast rule. With CCR now I have a lot more helium in a lot sooner than I would on OC and it's noticeably better.
I'll posit these questions to you
@Boston Breakwater
Why are you curious about this subject when the industry has said that you shouldn't breathe air deeper than 100-130ft depending on the agency?
If you're curious because there is some sort of dive you want to do and are thinking about doing it on deep air, where is the dive, how deep is it, and what are you wanting to do on said dive?