Limitations of TDI helitrox?

[flymolo]

My question was quasi rhetorical.

If you will occasionally skip the team but you never skip the check, then clearly one is more important than the other.

I think at least one agency insists on the team check to remind themselves of the edge they have

Good one. I’m not a gue diver unless you count my fundies cert but thanks for playing. I appreciate people who view the world as red vs blue

 

[lostsheep]

Wow good one. I’m not a gue diver unless you count my fundies cert but thanks for playing.

Calm down bro, no one said you were.

Edit to address the edit: pray tell which am I, red or blue?

 

[flymolo]

Calm down bro, no one said you were.

are you sure? then why'd you bring it up?

 

[inquis]

Mitchell was pretty specific in saying that the reasons we are doing extra deco with higher FHe may be incorrect, but the amount of deco seems to be correct.

That was in the context of different depths since Buhlmann is not iso-risk as depth varies. It's effectively a depth-dependent conservatism tweak, leveraging the typical correlation between FHe and max depth.

For a 150ft dive [...] based on current research the increase in decompression time prescribed by the currently algorithms [for 21/79 vs 21/35] is likely incorrect.

As such, this is consistent with the study since the depth is the same. In fact, that's exactly the crux of that study -- they used a Trimix deco profile for a heliox dive at the same depth, and showed that the additional deco from the atypically high FHe was not required.

 

[crofrog]

Unless there has been something that has come out in the last couple years that I missed, I don't think that was the takeaway. Mitchell was pretty specific in saying that the reasons we are doing extra deco with higher FHe may be incorrect, but the amount of deco seems to be correct.

A 150' for 30 min dive with 21/79 gives a run time of 163 minutes
A 150' for 30 min dive with 21/35 gives a run time of 89 minutes
(with GF 40/65)

I'm not sure how you arrived at those numbers. Neither MultiDeco nor Subsurface generates equivalent results, but I think you might be missing the core point of what he wrote.

If profiles with the "Helium Penalty" are showing, say, 30 minutes of total decompression time (TDT), and profiles with fHe set to 0 are showing 25 minutes, then the absence of a helium penalty effectively means divers are following a profile that is less conservative, even if both are calculated using the same GF high value.

For example, using GF 40/85 in Subsurface for a dive with 21/35 to 150 feet for 30 minutes, you get a runtime of 103 minutes. If you remove helium from the mix, the runtime drops to 97 minutes. Adjusting the GF high on the air profile to match the same total decompression time as the trimix profile results in a new GF high of 80%.

In essence, if the helium penalty doesn't exist, the profile you’re diving is effectively more conservative due to the lower GF high required to achieve the same TDT.

 

[crofrog]

That was in the context of different depths since Buhlmann is not iso-risk as depth varies. It's effectively a depth-dependent conservatism tweak, leveraging the typical correlation between FHe and max depth.

Exactly.

As such, this is consistent with the study since the depth is the same. In fact, that's exactly the crux of that study -- they used a Trimix deco profile for a heliox dive at the same depth, and showed that the additional deco from the atypically high FHe was not required.

The fun fact was in the study they had two more cases of DCS with the trimix than with the heliox...

 

[inquis]

The fun fact was in the study they had two more cases of DCS with the trimix than with the heliox...

Yeah, I wonder if any of them expected that to happen!