Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air.

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Also, what is your "Maximum." depth on air, and what physiological effects did you experience?

Sorry, I missed this part. I think it is fair to say that we (the entire class) were all pretty blasted at 285'/87M -- which was part of the point of the training. Nobody was completely out of control, unable to manage their equipment (deep sea gear), or respond to communications. Tasks were pretty basic like tying simple knots and verbal arithmetic questions. Knots were pretty tough even on deck in these rigs.

full

Obviously this was not Scuba so risk was far more manageable. Very simple work could be accomplished, but not much more. Considerable concentration was essential. The biggest problem for most of us was compromised sensory function:
  • Tunnel vision
  • Numbness-like sensations that interfered with speech and manual dexterity
  • Hearing became distorted
  • Metallic taste sensations
There was definitely euphoria bordering on silly but we could manage the giggles after the first run in a dry chamber.

I know that many new divers believe that diving in 306'/93M of water on air is deadly. By extension, accomplishing complex work, like salvaging a submarine, is impossible. I guess that Chief Stillson and his crew didn't get the memo.

US Navy Experimental Diving Unit
Chief Stillson's assignment coincided with the development of early submarines, and the tragic accidents that accompanied them. Stillson and his divers were sent to salvage the submarine F-4 after she sank off Pearl Harbor Hawaii with all hands in 1915. She was in 306'/93M of water. Divers experienced severe impairment caused by Nitrogen Narcosis which prompted the US Bureau of Mines to suggest Helium-Oxygen as a breathing mixture.
 
The first question is pretty alarming, as the answers can't vary at all.
MOD for air with a max PPO2 of 1.4 is literally set in stone.
How on earth would the answers for that question vary?
Not an instructor I would want to take a course for...
It varies quite a bit. All the factors impacting OxTox apply. This thread talks about it quite a bit:
Oxygen Toxicity Limits & Symptoms

Maximum Operating Depth (M.O.D.) S.C.U.B.A. Diving On Air?
The options and guidelines change over time, data available, and value judgements relating to risk mitigation. For example, the max PPO2 was 2.0 when I was in the US First Class Diving School. That is why certification on air was limited to 285'/87M.


That limit was down from a PPO2 of 3.0 before tests at the Admiralty EDU (Experimental Diving Unit) during WWII.

Cheers.
 
For one who starts to exhibit (post-dive) at 80', I'm impressed...

Thanks, but don't be. Thousands of us got our First Class Diver pin. Fifty 8 hour training days for Second Class Diving School, 90 training days for First Class School, being in your 20s, and career-level motivation makes all the difference.

Would I dive to 285' on air again but on Scuba? ARE YOU OUT OF YOUR MIND? I am confident that it is survivable with proper training and experience. However, it is horribly unproductive and pretty pointless. That's what saturation diving systems are for if you need to get work done and rebreathers with Trimix if you are diving for fun.
 
GUE and UTD are two such agencies. UTD said it from its inception because its founder came from GUE.

Technically none of them since the agencies that say helium starts at 100ft don't breathe air, as EAN32 is their standard gas.

Exactly. The two agencies that talk about trimix shallower than 130 ft are technical dive agencies that don't advocate diving with air ever. And I don't know this, but I would assume they allow 32% nitrox to 110 ft, then advocate helium blends deeper. Am I wrong about that, do they now say that the MOD for 32% is 100 ft? It's not a huge difference, I realize.

But the larger point is that these agencies are not at all the mainstream for diving in general, that would be the recreational agencies. Regardless of what any of us think about PADI, NAUI, SDI, etc....the fact is that millions of successful dives to 130ft on air have been sanctioned by these recreational agencies. So it's not out of line at all to consider the use of helium for all dives deeper than 100ft to be unusually conservative and out of the mainstream, which is what I said was my opinion in my first post. Not that it's 'wrong' or 'foolish' just that for me, it's very conservative. I am perfectly happy diving air to recreational limits (130ft) as have been thousands and thousands and thousands of other divers for decades. They're not all dropping dead.

Technical diving, that's a different story. Those are longer and/or deeper dives or in some other way much more challenging, and in those cases it makes sense to adhere completely with whatever practices the technical training recommends.
 
The two agencies that talk about trimix shallower than 130 ft are technical dive agencies that don't advocate diving with air ever. And I don't know this, but I would assume they allow 32% nitrox to 110 ft, then advocate helium blends deeper. Am I wrong about that, do they now say that the MOD for 32% is 100 ft? It's not a huge difference, I realize.
They do not want an END deeper than 100 feet, and they consider nitrox as narcotic as air, so their limit for EANx 32 is indeed 100 feet.
 
@halocline unless something has changed, I believe it is EAN32 or 30/30 to 100ft, then straight to 21/35. Since we believe that O2 and N2 are comparably narcotic, the use of air or EAN32 to 100ft is irrelevant for this discussion as the END would be the same.

Where the 100ft vs 130ft comes in, and @PfcAJ will have to correct me on this, is the ppO2 limit for bottom mixes at 1.3 due to working dives. That limits EAN32 to have a MOD of 4ata's/100ft.
 
I believe it is EAN32 or 30/30 to 100ft, then straight to 21/35.
When I was with UTD, the gas of choice past 100 feet was 25/25.
 
I just spent 2 months digging out foundations at 50-60m on air as a bottom gas on scuba. Was i narced, yup. Was it anything more then inconvenience... For me not really. I had no trouble orientating in 0 visibility or measuring, recalculating and marking stuff. Biggest thing I noticed is that after doing some delicate sledgehammering it would take me a few seconds more to get my breathing under control, i'm guessing that's due to gas density.
 
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