Limitations of TDI helitrox?

[justinthedeeps]

What @Nick_Radov do you specifically disagree on?
As my post there brings up several thoughts that could be discussed, and is fairly open to many points in this thread, including yours

 

[justinthedeeps]

How about you @rjack321? What do you disagree on?

 

[justinthedeeps]

Are we now debating 20% He vs. 35% He for a basic 45 metre dive?

They both work, with relatively little apparent differences in risk or divability. TDI says use your brain, dive your plan.
The 20/20 is completely sufficient for a basic course in the tropics

You spend more on the 21/35, get a bit less narc'ed and a bit less density...
I personally prefer that for coldwater diving on a CCR

But I would also readily accept 20/20--or even air--and 'dive within your limits' accordingly. Especially on open circuit!

As others have said, TDI teaches the principles, and then trusts people to make informed and reasonable decisions about their gas mixes and plans, based on what they trained for what the circumstances are. Amen to that

The 55 (or 60) metres on air thing can go away, totally doable but yuck
You could do a 15 metre dive on 10/70 or 50/50 if you really wanted to, I guess ... not recommended by anyone though

Question still open: who believes (or does not believe) that having more helium in the gas mix is a greater risk (or is not a greater risk) for DCS/DCI?

 

[justinthedeeps]

Counterpoint to the 'do what you want' thinking: when we have lost skilled divers on weird mixes like 10/50 who are clearly not respecting helium, and even lying to computers about the mix, some boundaries do need to exist in the training

 

[Wibble]

I don't understand backgas deco as a contingency when the classes are taught to a buddy team. you can share deco gas, can't you?

You're then relying on a buddy; you should be self-sufficient on all but the most extreme dives (where the entire plan is based upon teem diving -- this is completely outside of ANDP or normoxic)

Self-sufficiency is absolutely the most critical mindset for all technical diving. Reliance upon others is dangerous and leads to poorer skills.


Back to ANDP+H... that course is perfect for diving in the seas around the UK on the enormous number of wrecks we have around that 45m/150ft range. Because of the single deco stage you *must* plan--and run your dive--for deco on backgas. Buddy separation is a very common in waters with current, wrecks and frequent poor visibility 2m/6ft. Divers MUST be self-sufficient for executing the dive and resolving issues by themselves.

Agreed, for the first dozen or so post-course dives, then diving closely with others is good to ensure the skills are properly performed.

 

[justinthedeeps]

Because of the single deco stage you *must* plan--and run your dive--for deco on backgas.

Is that for open circuit? What is your mix, backgas capacity and max bottom time allowed on your typical 45 metre dive when you run the OC plan without a deco gas?

Is this why some of these basic tech OC courses are running multiple deco cylinders per diver, even though it is unnecessary in the standards?
Is it unofficially a 'self-reliant' tech/deco course?
Or is it to extend bottom times while shaving a few minutes off the deco?
Possibly an upsell? Instructor giving a taste of the next level?

 

[Wibble]

Let’s go back to first principles…

A recreational diver is progressing towards more challenging diving which requires decompression training. This requires quite different equipment from the NDL diving where the surface is always an option. There's various "intro to tech" courses including GUE Fundimentals where basic skills for running redundant kit is taught.

Advanced Nitrox and Decompression Procedures are taught by all agencies, not just TDI. This course will be the initial level of decompression diving restricted to depth, gas types and frequently duration and length of decompression. This allows the diver to develop their skills and gain experience within relatively safe constraints.

My ANDP+H was taught as a single cylinder decompression gas. This meant all planning was for that ONE failure, i.e. no deco gas (yes, loads of backgas shutdown drills, etc.). Thus your planning must allow you to return to the surface on backgas alone.

"One failure" is the principle of most dive planning. If you've no deco gas, you must have a redundant supply (manifold shutdowns, independent doubles, sidemount...). If deco diving then you must plan for one of the gases to fail.

It is interesting to see that my TDI ANDP+H certification card doesn't mention how many deco cylinders you can have:

For more decompression, both deeper and longer, there's the Extended Range/Normoxic Trimix course that teaches two deco cylinder techniques where you have to switch between deco gases on ascent. This is a more advanced course and relies upon your previous experience with ANDP for safe switching (NoTox) and being stable in the water holding your stops. The standards required for that course are higher than the ANDP course.

Again, the cert card doesn't mention how many deco cylinders you can have; just the max depth and min oxygen content.


But my first principles point stands; diving is a progression, you need to be competent at each stage of your progression and gaining experience before moving on to the next level. ANDP+H is a very good course and works exceedingly well for the types of diving found around the UK coast. Admittedly with the cost of Helium, Open Circuit trimix isn't really viable any longer. However, you still need to be competent with open circuit should you venture into the rebreather realm. Arguably, this means that ANDP is a vital progression even down to rebreather diving.