Theoretical question, inert gas saturation

[TicDiver]

Thought experiment:

For example, if I do a dive (20 m, 60 minutes) and breathe (20% O2 / 80% N2) and (20% O2 / 80% He) alternately for 10 minutes.

Do I then have less decompression obligation than if I breathe (20% O2 / 40% He / 40% N2) for 60 minutes at 20 m?

I hope I have expressed myself reasonably clearly.

 

[Whitrzac]

Yes and no.
The permeability of the gasses are different and offgass at different rates

The book "deco for divers" goes into a decent amount of detail on it

 

[inquis]

With Buhlmann ZHL-16C-GF65/80,
Switching gases: surfGF=147% before ascent, TTS=27 min on 20/0.

[Edit: TTS=40 min if you continue to switch every 10 mins (starting with 20/0), though the switch schedule plays a huge role. In this case, the 10-min schedule has you switching to 20/80 right before you cleared, needlessly prolonging things.]

Single mix: surfGf=132% before ascent, TTS=39 min on 20/40.

 

[Dsix36]

I would think that if you did the alternating in exactly halves, then it would basically be the same as breathing your second gas option anyway, so it should be the same.

 

[inquis]

I would think that if you did the alternating in exactly halves, then it would basically be the same as breathing your second gas option anyway, so it should be the same.

Yep, pretty much. But "decoing" on 20/80 is nearly useless, which is where the practical TTS difference comes in.

 

[Dsix36]

Yep, pretty much. But "decoing" on 20/80 is nearly useless, which is where the practical TTS difference comes in.

I was thinking in terms of ideal situations where the switching would be steady and consistent in very short time frames (like every minute) all the way through deco and to the surface. I know that nobody will really do switches like that though.

 

[Dsix36]

I missed the 10 minute part in the OP, so that kinda kills my thought on the matter.

 

[inquis]

I missed the 10 minute part in the OP, so that kinda kills my thought on the matter.

Your take is a better thought experiment, though. The peculiarities of the 10 minute switch schedule and GF choice complicate the situation.

 

[dmaziuk]

Plug it into subsurface and see.

You may want to leave helium at home

 

[dylanfromwinnipeg]

I ran some numbers and the 20/40/40 looks like a more efficient gas for this dive. I used 100% oxy for the descent (just to avoid the descent affecting the numbers - obv you'd never do that for real).

On 20/40/40 that's a NDL dive, you incur a tiny ceiling but it goes away as you ascend.

Switching back and forth you'll def have deco obligations, with about a 3m ceiling at the end of bottom time.

Note: (I did the calcs with a GF of 100/100).