Limitations of TDI helitrox?

[Pearlman]

Yeah sadly TDI has never really recognized that neither the helium nor the depth are the loaded gun, it's the deco gas and carrying two of them and/or a stage + deco gas with minimal training is asking for trouble

Genuine question - What course and agency would you say covers (or is the entry-level) with adequate/required training levels to handle multiple stage bottles?

 

[Wibble]

As if adding more helium was dangerous or something. Some of this TDI nonsense would be hilarious if it didn't lead to people getting confused on the basics and having to unlearn misinformation later.
(No argument with limiting the number of deco bottles, which is where the real risk of a gas switching error lies.)

Thinking back to when I took the ANDP+H course, the 20% helium limit is fine for 45m/150ft and wouldn't lead to big changes in the dive profile. In other words, was a good course for the initial steps into diving at that depth where many wrecks lay around the UK coast (as in many thousands of them including Scapa). It was almost sold as the perfect course for UK diving.

However... why restrict yourself to the limit of being able to *safely* complete the dive on backgas alone where your deco cylinder isn't available because it failed some way (under the one failure rule). Effectively this means 30 mins on the bottom with either 30-ish mins of deco on 80% or 80-ish mins of backgas deco.

What you really want is to have two deco gases so you've got one to deco out on should the other fail. This then gives you 50-ish mins on the bottom with deco on either/both 50% and 80%, popping up at 2 hours. For that you then need the Normoxic Trimix course...

 

[crofrog]

150ft (45m) gas densities

Mix	Gas density	END
21/20	5.98 g/L	115ft (35m)
21/30	5.38 g/L	95ft (29m)
21/35	5.09 g/L	85ft (26m)

For a 150ft (45m) 30 minute dive with no deco gas using multi deco you get the following TDTs

Mix	TDT
21/20	59min
21/30	64min
21/35	70min

At a .5 cuft/min (14l/min) rmv this extra 11 minutes translates to 9cuft (264l) of additional back gas or 137psi (11bar) in double 100s. That shouldn’t be making a difference in your dive planning.

I’m not sure I see a reason to accept a gas that is denser than the ideal limit proposed by Anthony and Mitchell and that has more narcotic potential.


Does TDI specify limits to gas density and end?

 

[LFMarm]

Does TDI specify limits to gas density and end?

Yes for CCR mixed gas. Not sure about other certifications.

 

[chapman_164]

Does TDI truly limit AN/DP or AN/DP+H to a single deco cylinder? In AN/DP (not +H, but basically the same course just without helium) I was trained with multiple deco cylinders. The course standards don't restrict the number of deco cylinders in any way and seem to leave the door open for this as "Decompression mix cylinder(s)" (plural allowed but not required) is in the list of Required Equipment. My understanding is that TDI allows leeway in how courses are taught and as long as instructors meet the minimum requirements without breaking any standards. Since my instructors went beyond the minimum requirements (while still staying within the confines outlined in the standards) I feel comfortable diving with stages and multiple deco mixes as these "diving activities approximate those of training". Granted, Trimix specifically requires multiple deco cylinders (or stage + deco), but I'm glad I had some great instructors who included that work at the AN/DP level.

 

[kierentec]

The TDI standards are left ambiguous on purpose. There’s nothing expressly limiting you to one deco/stage bottle, but there’s nothing saying you are trained for it. Some instructors teach additional stages/deco gasses because there’s nothing in the standards saying they can’t. I don’t think it’s necessary or appropriate for 45m/150ft open water tech dives, but some instructors think it adds value to their course. TDI doesn’t care. They always have the standard that post certification dives need to be in similar conditions to training conducted, which they can say is environmental, equipment, etc. So if you were only trained with one deco gas and you yourself accidentally switching to O2 at 70ft thinking it was your 50%, they can dodge any liability. That’s what their standards are really for and why they are written how they are.

The 35% limitation is there because it was a step better than the 20% than the previous standard. Which took years of begging to get them to increase. The rationale behind 20% was because you could use the air deco tables, then it took almost a decade to convince the powers that be that nobody uses tables anymore, and then even more begging and pleading to get them to read current research on gas density, CO2 and narcosis and let us use the appropriate gasses for deep diving.

 

[Nick_Radov]

It’s an arbitrary limit on the card to prevent you from getting a richer mix to dive too deep, I think. It’s just ANDP with a bit less narcosis.

TDI has an element of self sufficiency, you have one deco stage, if it’s gone for some reason, you generally plan to clear on your back gas. A bit tricky to clear with a lot of helium in back gas and no teammates around.

So in other words the TDI maximum He limit is yet another convoluted and illogical "solution" to problems which shouldn't exist in the first place. And are you telling me they still believe in that old "helium penalty" nonsense about deco?

 

[flymolo]

Thinking back to when I took the ANDP+H course, the 20% helium limit is fine for 45m/150ft and wouldn't lead to big changes in the dive profile. In other words, was a good course for the initial steps into diving at that depth where many wrecks lay around the UK coast (as in many thousands of them including Scapa). It was almost sold as the perfect course for UK diving.

However... why restrict yourself to the limit of being able to *safely* complete the dive on backgas alone where your deco cylinder isn't available because it failed some way (under the one failure rule). Effectively this means 30 mins on the bottom with either 30-ish mins of deco on 80% or 80-ish mins of backgas deco.

What you really want is to have two deco gases so you've got one to deco out on should the other fail. This then gives you 50-ish mins on the bottom with deco on either/both 50% and 80%, popping up at 2 hours. For that you then need the Normoxic Trimix course...

so carry two deco gasses for a 150 foot dive and do all the switches solo without verification all to avoid diving with a teammate...gotcha

 

[Nick_Radov]

Genuine question - What course and agency would you say covers (or is the entry-level) with adequate/required training levels to handle multiple stage bottles?

I don't understand the question. Handling multiple stage bottles greatly increases complexity and the risk of switching to the wrong gas. This isn't entry level. Regardless of agency, a diver should have significant experience with a single stage before taking a course that involves multiple stages. The GUE Tech 2 course covers this well so that would be one option for those who want to go that route.

 

[flymolo]

I don't understand the question. Handling multiple stage bottles greatly increases complexity and the risk of switching to the wrong gas. This isn't entry level. Regardless of agency, a diver should have significant experience with a single stage before taking a course that involves multiple stages. The GUE Tech 2 course covers this well so that would be one option for those who want to go that route.

or normoxic trimix or a stage class although that is typically taught in caves