Question When do we speak of technical diving ?

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No amount of training and equipment can overcome the physiological effects of breathing a gas that is too dense. Well, maybe some athletes have increased their ability to off-gas, I have no clue about that, but that's not the training we're talking about here.

Can it be done? Of course, people do it all the time, and have gone way past those limits. Will training minimize the risk? Of course. But there is a risk, and based on the research of Gavin Anthony and Simon Mitchell linked by @Imla, I consider this risk to be too high and completely unnecessary. So, I stand by my statement: I think diving below 40m on air is reckless.
What you said is generally true, but it must be remembered the effort done 50 years ago by some Italian inventors about overcoming the limits posed by gas density by developing special regulators which can "assist" human breathing.
At the time (1960-1970), "coral hunters" working in Sardinia, catching red coral trees for the jewelry industry, were considered the top-notch of scuba diving. These solo divers did routinely exceed 100 m depth in air, but, of course, the fatality rate was large.
One of them was Raimondo Bucher (already famous for other previous activities, such as the very first free diving depth records), who analysed the cause of several accidents and concluded that the major risk factor was CO2 poisoning, induced by the large WOB due to air density.
So he actively searched for a mitigation of this risk factor, developing the very first regulator operating "on offer" instead of "on demand".
Bucher patented his idea and attempted to license it to one of the major manufacturers of the time, Pirelli. But in the end the Bucher "offer" regulator never reached mass production, albeit several prototypes were built and tested with success.
More than 10 years later, another inventor, Luigi Ferraro, already known for the work he had done at Cressi (the Pinocchio Mask, the Rondine Fins, etc.), left the company and, with his friend Jacques Cousteau, founded his new company, called Technisub and based in Genoa.
Ferraro was well aware of the ideas of Bucher, but he preferred to develop his own "offer" regulator, called the Inject. This went in production and was commercialised worldwide, thanks to the affiliation of Technisub with La Spirotechnique by J. Cousteau, and hence with Aqualung.
These "offer" regulators had a big drawback, their "injecting" behaviour did cause a significant increase of air consumption. So, despite some commercial success, in the end the market went back to "demand" regulators.
This was definitely triggered by the success of the Scubapro Pilot, which, whilst remaining a "demand" regulator, thanks to its pilot valve, was minimising both inspiration and expiration effort even at great depth.
In conclusion, the idea that equipment cannot overcome the problems due to high density of air at large depth must be partially corrected: proper equipment can, at least partially, overcome these problems. However, this equipment in nowadays not anymore commercially available, and the drawbacks (air consumption, mechanical complexity) were not judged enough for keeping them in production.
 
diving below 40m on air is reckless.

OK...
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Good argument. Your selective quoting left out, probably intentionally, the words "I consider". It is my risk assessment, based on leading research.

I gather from these forums that you do some pretty serious diving, and you obviously have way more experience than I do, so maybe you can execute dives to below 40m on air with a risk that is acceptable to you. That's your prerogative. I concede that there are very proficient and experienced divers, maybe you are one of them, that could execute dives exceeding 40m on air with less risk than some divers have entering the water, but I still think it's a completely unnecessary increase in risk.

When your lungs are not working efficiently, sure you can try to decrease the load on them by minimizing effort, but why not have the margin to be able to increase effort if necessary? You can probably also deal with your cognitive functions being decreased, but why not have a clear head? Again, your prerogative. And I'm sure you have lots of experience telling you it's fine, for you. I still think even you would be better off with a bit more leeway.

And more importantly, I think that ridiculing the risk of diving below 40m on air is even more reckless than actually doing it, considering that you might influence divers with way less experience than you that could easily underestimate the risk.
 
... So, I stand by my statement: I think diving below 40m on air is reckless. ...
One person's "recklessness", or "foolhardiness", is not everyone's.

The GCI deep air dive I referred to in post #142 above took place the day before my wedding day. The day after the dive, my GF and I were married on the beach (Cayman Diving Lodge, East End, GCI, destroyed by Hurricane Ivan a few years afterward). If we had believed that the planned deep air dive was "reckless" or "foolhardy", then I wouldn't have done it. And she wouldn't have let me--especially since we had already spent considerable $$$ for the wedding before the dive boat had left the pier!

Of course, every informed diver makes his/her own risk assessments, himself/herself.

rx7diver
 
As someone who will be taking ANDP this year and Trimex in a year or two, can I substitute Helium or some other mixture in leiu of N2 for the Extended Dive class? YES I realize Helium is expensive and is difficult to source. Will TDI let you substitute?

OR

After ANDP can I skip extended range and go to Trimex? No rush to get certified, just concern about breathing N2 at depths deeper than 140ft. that is part of the extended range course. From reading these threads I'm starting to get a bit paranoid.
 
As someone who will be taking ANDP this year and Trimex in a year or two, can I substitute Helium or some other mixture in leiu of N2 for the Extended Dive class? YES I realize Helium is expensive and is difficult to source. Will TDI let you substitute?

OR

After ANDP can I skip extended range and go to Trimex? No rush to get certified, just concern about breathing N2 at depths deeper than 140ft. that is part of the extended range course. From reading these threads I'm starting to get a bit paranoid.
Yes, you can skip extended range and go straight to Trimix. In fact that is the orthodox progression.

Don't let the up-thread doom and gloom dissuade you. Air is safe to dive deeper than recreational depths IF you are not facing adversity, obstruction or any other challenge. If I'm in tropical water with no overhead and no penetration, with a DPV (so WOB is less of an issue) with an experienced buddy, I'll happily dive 50ish metres on air. But if it's cold, or in a cave, or in a wreck, or in a training situation, I'll be adding helium to the mix much, much shallower.
 
As someone who will be taking ANDP this year and Trimex in a year or two, can I substitute Helium or some other mixture in leiu of N2 for the Extended Dive class? YES I realize Helium is expensive and is difficult to source. Will TDI let you substitute?

OR

After ANDP can I skip extended range and go to Trimex? No rush to get certified, just concern about breathing N2 at depths deeper than 140ft. that is part of the extended range course. From reading these threads I'm starting to get a bit paranoid.
You can substitute deco procedures for Helitrox, and then do trimix.
 
As someone who will be taking ANDP this year and Trimex in a year or two, can I substitute Helium or some other mixture in leiu of N2 for the Extended Dive class? YES I realize Helium is expensive and is difficult to source. Will TDI let you substitute?

OR

After ANDP can I skip extended range and go to Trimex? No rush to get certified, just concern about breathing N2 at depths deeper than 140ft. that is part of the extended range course. From reading these threads I'm starting to get a bit paranoid.
Expanding on @VikingDives' post. I completed TDI AN/Helitrox last year. Helitrox course = DP + limited trimix. Helitrox course difference from DP: extra $ + 2-3 hrs trimix classroom + 1-2 trimix dives. Helitrox cert limits: max 35% He, 45m/150 ft max depth, 1 deco cylinder - it's a half step to normoxic Trimix cert.

If you have the time and $ and you're thinking of going OC trimix or CCR trimix in the future, Helitrox cert may be worth it instead of DP. It unlocks trimix earlier which can mitigate narcosis and gas density issues for deeper than 30m/100 ft dives - which might be helpful for dives with lots of task loading at depth, like Advanced Wreck course. And from Helitrox cert you can either progress to OC Trimix or CCR Helitrox.
 
... If you have the time and $ and you're thinking of going OC trimix or CCR trimix in the future, Helitrox cert may be worth it instead of DP. It unlocks trimix earlier which can mitigate narcosis and gas density issues for deeper than 30m/100 ft dives - which might be helpful for dives with lots of task loading at depth, like Advanced Wreck course. And from Helitrox cert you can either progress to OC Trimix or CCR Helitrox.

Or simply fill your empty doubles 1/3 full (say) of He, and top off with banked EAN36. Analyze the contents, and compute the real He content. Calculate your MOD using your measured FO2. Fill your EAN36 [sic] deco bottle and your O2 deco bottle. Use your desktop software to generate some appropriate deco tables for these gases and your cylinder capacities and your personal working RMV. And go diving.

J/K!

No one would ever seriously recommend or consider doing this--even to a diver who is an experienced Technical Nitrox or Decompression Procedures or Advanced Deep Air diver.

Still, the simplicity is beautiful, no? If it works??!!

rx7diver
 
Or simply fill your empty doubles 1/3 full (say) of He, and top off with banked EAN36. Analyze the contents, and compute the real He content. Calculate your MOD using your measured FO2. Fill your EAN36 [sic] deco bottle and your O2 deco bottle. Use your desktop software to generate some appropriate deco tables for these gases and your cylinder capacities and your personal working RMV. And go diving.

J/K!

No one would ever seriously recommend or consider doing this--even to a diver who is an experienced Technical Nitrox or Decompression Procedures or Advanced Deep Air diver.

Still, the simplicity is beautiful, no? If it works??!!

rx7diver
🤔 I understand sarcasm and wit, but not comprehending how this is relevant to TDI Helitrox certification and further progression through technical training courses
 
https://www.shearwater.com/products/teric/

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