Question Bailout gas configs for tech/deep chestmount rebreathers

[lostsheep]

I know better than to jump into this thread because I know too well how the discussion goes, but oh well, here we go…

I’ve said it many times, if you are using a single deep bailout as your dil, you are effectively diving without deep bailout, just a big dil bottle, and is an example of trying to jam a square peg (sidemount or chest mount rebreathers) into a round hole (deep diving). 5 or 6 years ago, we wouldn’t have been having this discussion, because everyone knew it was dumb. Now with the explosion of sidemount and chestmount units… we’re forgetting the basics because we’re trying to sell these units as the best thing ever.“Oh but I can just race up quick and switch to my next gas” is a horrible mindset. You never really know how long it will take you to ascend when dealing with a failure. Team separation, buoyancy issues, etc aside, what if you’re inside a wreck or should really make it back to the up line before ascending?

Carry sufficient bailout gas to get yourself from max depth to the next gas. Drive the unit with a SEPARATE gas source.

Why would a chest mounted unit with backmounted doubles as dilout be functionally different from a backmounted unit in a heavy configuration?

Serious question, I’m not being flippant.

 

[mr_v]

Hi sorry yeah I toned down my response. It sounded like discouragement on first read.

It seems like one of the main points of @Cio's post was choosing deco gases other than just 50% and 100%. I see now that you said "can be diluted easily." To what? Perhaps 40% and 80%, as Cio explored? Is there agreement here, or dismissal? I can't tell.

@Cio asked why these gasses were popular and I provided my unconventional reasoning. Besides being good deco gases, they're also easy to dilute if you need more gas in the field. Of course, being good deco gases does not make them the best deco gases for every scenario.

The other part was to compare configs of deeper gases. To which you said "it's fine we just bring them all." I am not sure how that helps

Simple answer: you have more options. If you're diving deep on CC regularly, there is a good chance that you already have all these gasses, and bringing an extra tank or two does should not require significant effort. Having more tanks in the field also allows you to alter plans if necessary.

My overall take on this entire thread is this: why would you limit yourself if you have multiple options? There are several threads on SB that allude to bringing only the min gas amount and, frankly, that scares me. I have yet to see a diver say "I wish I did not have all that gas with me."

P.S. My bias is overhead diving.

 

[flymolo]

Why would a chest mounted unit with backmounted doubles as dilout be functionally different from a backmounted unit in a heavy configuration?

Serious question, I’m not being flippant.

It's not different, but unless I'm mistaken what's been proposed in this thread is a using a single al80(?) for dilout. Using a small set of backmounted doubles for dilout seems like a far more reasonable approach, and then all your deco gas bailouts could be carried the same way as you would on OC, and your dilout supply would have redundancy

 

[justinthedeeps]

@Cio asked why these gasses were popular and I provided my unconventional reasoning. Besides being good deco gases, they're also easy to dilute if you need more gas in the field. Of course, being good deco gases does not make them the best deco gases for every scenario.



Simple answer: you have more options. If you're diving deep on CC regularly, there is a good chance that you already have all these gasses, and bringing an extra tank or two does should not require significant effort. Having more tanks in the field also allows you to alter plans if necessary.

My overall take on this entire thread is this: why would you limit yourself if you have multiple options? There are several threads on SB that allude to bringing only the min gas amount and, frankly, that scares me. I have yet to see a diver say "I wish I did not have all that gas with me."

P.S. My bias is overhead diving.

Ok thanks yeah I see now. All great points.

Cylinders are like bathtub toys once floating. And we have DPVs now.
But at some point is there a limit? Otherwise, why are people using dual rebreathers instead?

I don't think Cio's point exactly was just to skimp on gas though.

I think we are identifying potential reasons why typical twinsets and Standard Gases are not necessarily optimal for a CCR gas planning perspective, and the response is understandably well we just do it this way. But it is lite on analysis.

 

[acxc]

What I took from this thread, and the piece in Indepth, was that the single 80 of deep gas was attempting to solve the problem of having something that would have a pp02 low enough to match desired setpoint at maximum depth (which isn't as ideal for a full bailout), and then maintaining a much higher quantiy of gas that is adequate to bailout to at depth (maybe just not the VERY bottom) and get to your first deco stops in your backmount cylinders. I started using something similar last season, and found it to be a pretty seamless way to transition between 150' dives and 200+ within the same trips where I didn't have the convenience of adjusting my backgas for every profile.

 

[rjack321]

It's not different, but unless I'm mistaken what's been proposed in this thread is a using a single al80(?) for dilout. Using a small set of backmounted doubles for dilout seems like a far more reasonable approach, and then all your deco gas bailouts could be carried the same way as you would on OC, and your dilout supply would have redundancy

For the folks diving a sidekick, flex and other "big tube on one side" CCRs its fairly common to drive the unit dil off a 2L up to an AL40 tucked underneath. Under the opposite side is a big single BO going up to lp120 size sometimes using an H or Y valve. The remainder of the BOs including deco gases are stages dropped from a leash. But nobody is claiming these units are for ocean diving (wisely so because they are terrible choices for a deep ocean dive)

 

[justinthedeeps]

Why would a chest mounted unit with backmounted doubles as dilout be functionally different from a backmounted unit in a heavy configuration?

Serious question, I’m not being flippant.

That's what I was asking too. I thought one of original posts was saying this. You are right, and it sounds fine.

Except then *another* deepest cylinder was added to the equation, which didn't make sense to me. Why would you need that if you already had a twinset of bottom gas?

Strangely I got "sadness" reactions to that question, and answers like: "it's good and I don't have time to explain"

It turns out, if I understand correctly, that it was actually a twinset of non-bottom gas. So that's different.

It obviously still works, and the argument is like:
1) well we have this twinset of Mod2 gas already, and we want to go deeper. Instead of making up another twinset of Mod3 bottom gas, let's just tack on a stage of deeper gas, and well it's good to have lots of gas anyway for the mid-phase.
2) we have to get up to our 50% mix at ~20 metres, and it could takes a lot of this mid-depth gas to do that, especially in a bad situation.

So those are actually good points.
But is that optimal? It seems there are multiple nerdy(?) details why it is not.

But beyond that, in spite of replies to the contrary, deep chestmount/sidemount CCR diving is [evidently] still a developing area when it comes to bailout strategies. So that's what this thread is about

 

[justinthedeeps]

the convenience of adjusting my backgas for every profile.

I think here you really nail one major known weakness to manifolded twinset approaches for CCR diving.

Once you fill that twinset with a huge amount of a certain spendy special gas, it really shapes how you look at dive plans. Because you are now stuck with it.

So that is why we are talking about just clipping on other stages of things as a patch. It is operationally convenient if you are a twinset BO diver. But it seems like this is creating theoretically suboptimal situations from a gas planning perspective. There is also now that you no longer have redundancy in your deepest gas. There could be yet more reasons.

This is particularly relevant for the GUE/DIR twin onboard twinset style approach, because you can't just go switch to a different twinset. GUE I think solves that with their "25 dives at the same depth" kind of approach to training. And it works great if you have just one particular depth objective, like an expedition to one important wreck. But as soon as your are considering dives with different max depths, it gets awkward.

Now it seems this has become the case also for deep chestmount diving?

I even brought up the idea of ok, maybe lose the manifold. Now you can put whatever gases you need on your back. This was also dismissed. Of course, this is not seen as convenient either. We already filled these twinsets with $500 of trimix!

 

[rjack321]

That's what I was asking too. I thought one of original posts was saying this. You are right, and it sounds fine.

It's an incredibly convoluted way compared to just diving a BM unit in the first place.

It obviously still works, and the argument is like:
1) well we have this twinset of Mod2 gas already, and we want to go deeper. Instead of making up another twinset of Mod3 bottom bas, let's just tack on a stage of deeper gas, and well it's good to have lots of gas anyway for the mid-phase.
2) we have to get up to our 50% mix at ~20 metres, and it could takes a lot of this mid-depth gas to do that, especially in a bad situation.

This is a big nopes. You never want to have the gas on your back be unbreathable at depth. Whether that is OC, dil or BO isnt really important, relying on the one stage that you slapped on your side is bad news. If you are doing a 100m dive then plan for it properly with another set of those (redundant) mini doubles on your back or dump the mix in them and refill or something. Slapping one stage on your side to get you deeper is a giant no.

 

[rjack321]

This is particularly relevant for the GUE/DIR twin onboard twinset style approach, because you can't just go switch to a different twinset.

Most folks diving the JJ this way have multiple sets of lp50s (eg a 21/35 and a 15/55 set). Its just the cost of diving this way and not really any different than me having 8 different 80s of BO in the garage.