Helium for a 50% bailout bottle?

[RyanT]

An 80 at 2200psi or so is lighter in the water than a 40 and still has more gas. Full 40's aren't egregious, but I would rather deal with the extra size

Not me, man. Id much rather have a smaller tank. Less bulk is mo betta

I don't really have a preference either way. Whatever the configuration though, I don't put helium in the 50 mix.

 

[Jack Hammer]

I never felt carrying an 80 of 50% was that big of a deal, annoyance, or inconvenience that I'd want to risk cutting safety margins to save swimming the extra pound. I guess it really depends on the dives people are doing. To each their own.

 

[broncobowsher]

I don't. My bailout is a lot stronger in O2.

But have had discussions regarding it. Comes into play on long deep dives. Takes a little weight off. Can help transition off very rich Helium mixes, Isobaric counter diffusion.

Call it theoretical future issues.

 

[rjack321]

I never felt carrying an 80 of 50% was that big of a deal, annoyance, or inconvenience that I'd want to risk cutting safety margins to save swimming the extra pound. I guess it really depends on the dives people are doing. To each their own.

Years ago people sometimes put helium in their 50% bottle when they had long decos based on the urban story that they "felt better" afterwards. You can eliminate the helium fraction fast once you switch to O2.

What was really happening was they (probably) were doing some sort of oxygen window extended stops on 50% (also all the rage back in the 90s and early 2000s). Going from 10/70 to 35/25 to EAN50 N2 would actually on-gas from the 35/25 all the way through the EAN50 until the 20ft stop. Having helium in the EAN50 avoided a bit of the N2 ongassing. Then they'd get rid of the helium once they switched onto O2. Net they probably had less residual N2 on surfacing. It's probably mostly an artifact of the gases and profiles people were diving back then which was only a scant few years after people discovered they didn't have to switch from trimix to air at 200ft. The "helium is bad" pendulum swung to "helium is great" pretty wildly.

 

[100days-a-year]

Is isobaric counter diffusion even a real threat on dives where 50% is the first deco gas? Pubmed is scarce on info.

 

[rjack321]

Is isobaric counter diffusion even a real threat on dives where 50% is the first deco gas? Pubmed is scarce on info.

By the time you're up to 70ft/21m IBCD is merely academic. Want to switch from 12/65 to 50% @70ft? Or do an "air break" on your trimix at 30ft while you stow the 50% and prepare to switch to O2? Or switch to air dil at a reasonable depth for actually using air dil? Knock yourself out, no big deal, those switches have been done 10s of thousands of times. If you keep your END to <100ft/30m this whole issue is magically a non-issue.

It's a real thing if you're doing trimix to deep air like Exley in days of yore. Or immersed in a trimix atmosphere in a diving bell. But not in a recreational deco context.