Question Bailout gas configs for tech/deep chestmount rebreathers

[J-Vo]

For those using a single dilout bottom gas, have you ever actually lost it on a dive?

I ask because I used to dive that way, but actually having a lost gas situation changed my attitude about it. For me it was the 150' 21/35 scenario described above and I had to get up to plug in my 32%. I never want to have that situation on a mod3 dive, f@$k that.

I now have two 108s of deep mod3 gas. I simply use that for every mix dive. Often for simplicity, I'll just use mix an entire trip NDL dives and all.

 

[justinthedeeps]

Often for simplicity, I'll just use mix an entire trip NDL dives and all.

This seems totally doable as long as the diver and the rebreather never have a lapse in ppO₂, AND never S-drill or bail out to that back gas.
I would be concerned, or very cautious if the dil was a hypoxic mix. I'm not sure Mod3 mixes belong on NDL dives?

Technically 10/70 is 'normoxic' below 10 metres, but I would be a lot more comfortable with my BOV plugged directly into an offboard of actually normoxic mix like an S40 or S80 of air/nitrox. And no open circuit regulators on the hypoxic mix. Why risk a fatal S-drill?

 

[J-Vo]

This seems totally doable as long as the diver and the rebreather never have a lapse in ppO₂.
I would be concerned if the dil was a hypoxic mix though. I'm not sure Mod3 mixes belong on NDL dives

10/70 is only hypoxic above about 15'. Getting from there to the surface isn't a big deal on NDL dives. If there is concern about having to do work on the surface, I'll bring a 40 of O2.

Edit: Air is 0.16 pp at my home, so my idea of hypoxic may differ from yours.

Edit: typo on gas content.

 

[justinthedeeps]

20/70 is only hypoxic above about 15'. Getting from there to the surface isn't a big deal on NDL dives. If there is concern about having to do work on the surface, I'll bring a 40 of O2.

Yeah good, definitely have something else to breathe. I still find the idea of having multiple OC regulators and/or BOVs rigged to a large onboard supply of hypoxic gases (GUE/DIR approach?) to be a little risky for shallow phases.

[Sorry I lengthened that post slightly, same thoughts though]

 

[justinthedeeps]

Another thing that could happen with hypoxic back gas on shallow dives is you futz a little above 10 metres and your ADV blasts a bunch of deadly gas in the loop, game over possibly.

 

[J-Vo]

Another thing that could happen with hypoxic back gas on shallow dives is you futz a little above 10 metres and your ADV blasts a bunch of deadly gas in the loop, game over possibly.

I don't have an ADV, but when I did, I turned it off above 10m. Either way you would have to do a full loop flush to have an issue. It's pretty standard these days to keep your bottom mix plugged in as your dill through the entire deco.

 

[justinthedeeps]

I don't have an ADV, but when I did, I turned it off above 10m. Either way you would have to do a full loop flush to have an issue. It's pretty standard these days to keep your bottom mix plugged in as your dill through the entire deco.

Agreed that some precautions can be involved here, however minor.

One more thing about having a separate offboard hypoxic deep bailout is you can easily close that tank valve (after pressurizing) for your initial entry, plan an S-drill at ~10 metres where you re-open it and check the reg/BOV, and close it once again during deco stops, to eliminate any chances of accidentally breathing it above 10 metres.

Not sure if anyone with the onboard-only hypoxic mix approach is doing anything like this? Otherwise it's just remember not to breathe from the 2+ regs with the bye bye gas in them. Task loading? Complicated surface conditions? Sounds dicey to me.

Edit: maybe the above detail is yet one more reason we're hearing people going for Mod3 dives with 21/35 or 15/55 in their twinsets, at least then your big twinset of travel gas isn't immediately deadly on the surface.

 

[rjack321]

Another thing that could happen with hypoxic back gas on shallow dives is you futz a little above 10 metres and your ADV blasts a bunch of deadly gas in the loop, game over possibly.

You'll be shocked to learn there's a whole universe out there diving 10/50 dil on every dive. With some variety of BO (not combo dil/BO)

ps I'm not in that sphere

 

[jale]

And your profile is thicker with BM bailout and CM rebreather. Silly.

Until GUE officialy uses the Halcyon branded German CM.
Then everybody will start to praise the CM unit and the wheel will be reinvented again

 

[PfcAJ]

Until GUE officialy uses the Halcyon branded German CM.
Then everybody will start to praise the CM unit and the wheel will be reinvented again

GUE’s position doesn’t affect mine. I’ve been critical of GUE for quite some time.