Why do you need training for normoxic trimix?

[Merlino]

Hi all!

Please let me preface my post by saying I have no intent to start diving helium without the proper training.

With that out of the way, I'm curious if anyone can explain to me why it is exactly we need training to dive normoxic trimix to say, recreational depths/NDL? As an example, diving 30/30 to say 40 meters? I've tried searching the forum and googling but haven't found the answer.

If there is a thread I missed which clearly answers the question, please do not hesitate to point it out to me

Thanks!

 

[tbone1004]

CYOA from the legal beagles mostly. Any of the good instructors will combine what is called ART or advanced recreational trimix, or variations on that theme with advanced nitrox/deco procedures to answer the recreational depths point.

On the NDL point, the helium changes NDL's considerably and many people that dive trimix *myself included* consider depths below 100ft to be too deep to bother with NDL diving. Helium adds a considerable deco penalty with most computers that allow trimix, and many more don't have an option for it. The dive planning therefor becomes much more involved and the class is in there to make sure you have the ability to plan those dives properly and they usually allow 10-15minutes of backgas decompression to make the dive worth doing

 

[Merlino]

CYOA from the legal beagles mostly.

I was afraid that was at least part of the answer...

On the NDL point, the helium changes NDL's considerably and many people that dive trimix *myself included* consider depths below 100ft to be too deep to bother with NDL diving. Helium adds a considerable deco penalty with most computers that allow trimix, and many more don't have an option for it. The dive planning therefor becomes much more involved and the class is in there to make sure you have the ability to plan those dives properly and they usually allow 10-15minutes of backgas decompression to make the dive worth doing

Thanks! I was under the impression that general thoughts on helium penalties had changed? But if computers are still incorporating the helium penalty into their algorithms then that doesn't really matter, I guess..

So from a practical point of view: you'll want to get into deco to have a dive that's long enough to be interesting, and to be able to do deco you need training. Correct?

 

[AJ]

Thanks! I was under the impression that general thoughts on helium penalties had changed? But if computers are still incorporating the helium penalty into their algorithms then that doesn't really matter, I guess..

Knowledge is progressing, but that does not mean computer manufacturers do change their algorithims on the fly. Porbably they will wait until there's consensus about the helium penalty.

So from a practical point of view: you'll want to get into deco to have a dive that's long enough to be interesting, and to be able to do deco you need training. Correct?

That's it in a nutshell, yes. Deco diving introduces more danger, therefore you want more training to dive safe.

 

[seeker242]

So from a practical point of view: you'll want to get into deco to have a dive that's long enough to be interesting, and to be able to do deco you need training. Correct?

Yes, but not necessarily Trimix per say. There can still be very interesting deco dives in recreational depths, that can be done on just nitrox.

 

[tbone1004]

@Merlino
Current opinion is here, but yes the computers still have it in there, so it will stay until there is more evidence out there.
Eliminating The Helium Penalty - Shearwater Research

Practically yes, you'll want to be able to do deco.

If we look at your example of 30/30 to 40m, which has a fairly high ppO2 IMO, and we look at it with a gradient factor of 50/80 which is fairly aggressive, you only have 4 minutes of NDL diving in deco planner. You get up to 8 minutes with a 1 minute stop at 20ft *which we can count as essentially NDL diving*, and up to 14 minutes with 4 minutes of stops at 20ft, but they are mandatory stops. If we allow decompression diving, you can go up to 20mins bottom time with 14 minutes of backgas deco. If we add O2 as a deco gas, we can get 30 mins of bottom time with 15 minutes of deco. If we look at the last dive of 30 minutes. If we remove the helium and dive EAN30 but keep the O2, total deco time only drops to 13 minutes. On backgas, it is 26 minutes of deco on EAN30, and 33mins on 30/30 which shows the helium penalty being put in on that planner.

That's a significant difference in bottom time if you allow for decompression, even backgas, and makes the dive much more worthwhile

 

[Wookie]

No reason I can come up with. It's like diving, only with helium in the mix.

Unless you are one of the folks who want to know everything about decompression theory, who cares? Follow your computer/tables/whatever deco schedule that suits your fancy and go for it.

Now, I can see the need for Advanced nitrox and deco. Although I didn't take any of them. I was basically signed off by a IANTD board member. I did some dives with him and bought him a trip to Cozumel and paid for his gas while there.

--BUT. No charter operator will take you out without the correct cards for the dive.

 

[CWK]

You might want to know about isobaric counterdiffusion if you want to go into deco.

 

[rjack321]

You might want to know about isobaric counterdiffusion if you want to go into deco.

Except its totally irrelevant for any modern OC tech diving. And certainly not important for normoxic trimix depths which usually considered to extend to ~60-70m. Unless you are switching to air way beyond recommended ENDs, like at 90m or something, IBCD is a giant nothingburger. People sure like to bring it up online though.

 

[Wookie]

Except its totally irrelevant for any modern OC tech diving. And certainly not important for normoxic trimix depths which usually considered to extend to ~60-70m. Unless you are switching to air way beyond recommended ENDs, like at 90m or something, IBCD is a giant nothingburger. People sure like to bring it up online though.

I’ve actually seen the results. I like to get off helium quite deep, like 130 feet onto 32%, and 80% at 15 feet. I watched my buddy go blank for an entire stop. She switched to rebreather shortly after.