Question Bailout gas configs for tech/deep chestmount rebreathers

[justinthedeeps]

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.

Good to know, thank you! I thought it might get awkward with the manifold and stuff.

I remember hearing CCR1/2 divers grumble about it ~5 years ago, and then also go on 20 metre dives with 10/70 or gods knows what in the onboards because that was what they had.

 

[rjack321]

Good to know, thank you! I thought it might get awkward with the manifold and stuff.

I remember hearing CCR1/2 divers grumble about it ~5 years ago, and then also go on 20 metre dives with 10/70 or gods knows what in the onboards because that was what they had.

The Lola manifolds are easy although during the pandemic lp50s were like gold because their popularity exploded. Not just for the JJ, but units like the sidewinder were diving them extensively so they got scooped up fast and prices were high. There are more 50s around now though so that issue is mostly gone. They are still a popular tank though, rarely on sale, and OC divers coming into CCR diving rarely already own them (unless they are OC sump divers)

It certainly happens that you have the wrong dil/out. And its a big pain to top them off too cause a itty bitty CCR sized booster isn't going to cut it. But on the other hand it's not really fun to have 4 different 3L and a giant collection of al80s of BO in the garage either. It's generally easier to never use the al80 BOs and rotate through using 3L dil bottle on a conventional BM CCR. That practicality is one reason why BM CCRs are generally a better first unit than SM or chest mount units requiring new tanks for dil/out and large boosters to get topped off.

 

[lostsheep]

It’s very possible I misinterpreted @kierentec ‘s post. Reading it again it seems he might just be objecting to the single dilout which I am not a fan of myself.

I just don’t see a chest mount unit as inherently a square peg in a round hole.

 

[flymolo]

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.

this is exactly what i do, it solves a lot of the logistical problems. if you use the kent tooling brackets like the fathom orignally came with it's easier to swap the tanks

 

[Cio]

Why discourage careful thought on this? Or maybe I misunderstand.

50% can indeed be insufficient as first deco gas, for very deep or long dives. How much deco are you doing before reaching that 50%? On what mix? If we are not considering that carefully, it does not sound like qualified Mod3 practices.

OK, so, again, not MOD 3 but I look at this in two parts. Deep BO and DECO BO.

Back mounted doubles seem to take care of deep in my opinion, two regs, access to plenty of gas.. Double 80s may not make sense, maybe double 100s or 120s depending on what your up to.....

You say add a deep BO because it won't hurt, but...

DECO bailout, so far the consensus is, 2 deco BOs 50, 100, 40 , 80, whatever. If you lose one of those, you are done. You don't have enough deep bailout, even if your carrying 2 x intermediate and 1 deep BO, and you do not have enough deco gas in you remaining 1 x 80 deco bailout. You need to breathe down everything you have, and hope to get the least bent you can but it would appear to be guaranteed DCS

If you are going to add that extra bottle, why not add an additional deco bailout? If you think about possible failures, it seems more likely you may need and additional the deco gas, rather than deep gas, no? (Lets assume we aren't switching regs underwater)

DEEP can be compensated by the size of the twinset ( 2 x 120s is monster) . The twinset can survive a failure and still satisfy the deep requirement .
Deco cannot survive a failure so why not stack that deck?

 

[flymolo]

OK, so, again, not MOD 3 but I look at this in two parts. Deep BO and DECO BO.

Back mounted doubles seem to take care of deep in my opinion, two regs, access to plenty of gas.. Double 80s may not make sense, maybe double 100s or 120s depending on what your up to.....

You say add a deep BO because it won't hurt, but...

DECO bailout, so far the consensus is, 2 deco BOs 50, 100, 40 , 80, whatever. If you lose one of those, you are done. You don't have enough deep bailout, even if your carrying 2 x intermediate and 1 deep BO, and you do not have enough deco gas in you remaining 1 x 80 deco bailout. You need to breathe down everything you have, and hope to get the least bent you can but it would appear to be guaranteed DCS

If you are going to add that extra bottle, why not add an additional deco bailout? If you think about possible failures, it seems more likely you may need and additional the deco gas, rather than deep gas, no? (Lets assume we aren't switching regs underwater)

DEEP can be compensated by the size of the twinset ( 2 x 120s is monster) . The twinset can survive a failure and still satisfy the deep requirement .
Deco cannot survive a failure so why not stack that deck?

Deep BO can be done in a set of LP50s and deco for a mod3 dive would likely be something like 100%, 50%, and an intermediate gas like 21/35 or even 35/25 in some cases. Carry 2 deco stages on the left and the third on a leash and rotate on the way up.

For lost deco gas planning you want a team mate so you can share theirs. It’s unlikely you’d both be bailed out on the same time.

I guess you could go bigger on the backgas tanks but in the ocean your runtimes would (should?) probably not necessitate that

 

[justinthedeeps]

@Cio I think you are winning in this thread, for being the only one who thought it interesting enough to look at a dive planner. I'm pretty sure that is Day 1 on the Mod3 course.

I am also glad people bring up which gases are the most important ones for redundancy.

I think it is a little surprising to see so many talking about 100+ metre dives with 20 minute bottom times, who don't seem to have even considered whether there is anything more suitable than the Standard Brawndo of a twinset, a 50% and a 100%. The deco mutilator!

The "Standard Gases" may indeed simplify logistics and unite teams, but I don't think that is sufficient reason to spend an extra 30(+) minutes on a critical bailout ascent because nobody considered anything else in the plan. Just going by your numbers from before...

A deeper deco mix with 32% oxygen in it, and perhaps 20% helium, is clearly far more suitable that 15/55 or 21/35 for all of the deco stops between 40 and 21 metres. I am very surprised that nobody is mentioning or accepting this as another standard deco gas (ok @flymolo just did).

80% may well also be just as good as the 100%, especially at shallowest stops including 3 metres/10 feet, because it has no helium in it. Any gas without helium in it provides the maximum decompression gradient for helium. Nothing says stay at 10 metres on 80%. You keep moving up, and the helium keeps leaving. Not to mention the lower CNS%, for anyone who still believes in it.

You don't realize this stuff if you don't open a planner and try things!

 

[flymolo]

@Cio I think you are winning in this thread, for being the only one who thought it interesting enough to look at a dive planner. I'm pretty sure that is Day 1 on the Mod3 course.

I am also glad people bring up which gases are the most important ones for redundancy.

I think it is a little surprising to see so many talking about 100+ metre dives with 20 minute bottom times, who don't seem to have even considered whether there is anything more suitable than the Standard Brawndo of a twinset, a 50% and a 100%. The deco mutilator!

The "Standard Gases" may indeed simplify logistics and unite teams, but I don't think that is sufficient reason to spend an extra 30(+) minutes on a critical bailout ascent because nobody considered anything else in the plan. Just going by your numbers from before...

A deeper deco mix with 32% oxygen in it, and perhaps 20% helium, is clearly far more suitable that 15/55 or 21/35 for all of the deco stops between 40 and 21 metres. I am very surprised that nobody is mentioning or accepting this as another standard deco gas (ok @flymolo just did).

80% may well also be just as good as the 100%, especially at 3 metres/10 feet, because it has no helium in it. Any gas without helium in it provides the maximum decompression gradient for helium. Nothing says stay at 10 metres on 80%. You keep moving moving up, and the helium keeps leaving.

You don't realize this stuff if you don't open a planner and try things!

You should have a look at the standard gas table for deco mixes

https://indepthmag.com/wp-content/uploads/2021/05/Deco_mix_chart-1-1024x357.png

 

[justinthedeeps]

You should have a look at the standard gas table for deco mixes

https://indepthmag.com/wp-content/uploads/2021/05/Deco_mix_chart-1-1024x357.png

I agree, but you are now the only other person who has mentioned it. We're on page 8 now.

With a 35/25 (or 32/20) to switch to at ~40 metres, you definitely do not need a twinset of 15/55 to get there.

Or if you do, please show me a dive plan where you do need that.

80% is also missing, the advantages of which both Cio and I have pointed out. What is the GUE rebuttal?

 

[flymolo]

I agree, but you are now the only other person who has mentioned it. We're on page 8 now.

With a 35/25 (or 32/20) to switch to at ~40 metres, you definitely do not need a twinset of 15/55 to get there.

Or if you do, please show me a dive plan where you do need that.

What size twinset are we talking about? My configuration uses a set of back mounted lp50s. What pressure I fill them to is another story…