Question Bailout gas configs for tech/deep chestmount rebreathers

[flymolo]

Correct. This doesn’t work as well if you lose both rebreather and 10/70

Who teaches that?

 

[Cio]

Not MOD 3 but reading this got me to thinking, why everyone is suggesting 50 and 100 as deco mixes, I know those are common gases, but doesn't seem idela for such adeep dive. I started playing with multideco. ...

Deep drive gas/ BO, seems unnecessary to me and adds complexity.

330 ft, 20 min bottom time, 70/70 GF (using multideco bailout dive plan)

BM doubles (80s) with 10/70
al80 of 50
al 80 of 100
193 min runtime

I thought shif the O2 up and start earlier
BM doubles (80s) with 10/70
al80 of 40
al 80 of 80
175 runtime, less brutal on the lungs

using deep bailout mix
BM doubles 15/55 plus deep drive gas
al80 of 50
al 80 of 100
160 minute runtime, 111% CNS

BM doubles 15/55 plus deep drive gas
al80 of 40
al 80 of 80
153 min runtime.

10/70
al80 of 32
al 80 of 80
runtime of 156 min


15/55
al80 of 32
al 80 of 80
runtime 146 min. (shortest time but requires deep BO cylinder.)

With better BO mixes, the difference between deep bail out and no deep bailout is 10 min, seems like a worthy compromise if it eliminates a cylinder.


Like I said, not MOD 3, so maybe I am missing something, but seems like with optimized BO deco mixes, the difference is not worth the added cylinder.

 

[rjack321]

Like I said, not MOD 3, so maybe I am missing something, but seems like with optimized BO deco mixes, the difference is not worth the added cylinder.

You need to cover a lost or insufficient deco gas, 50% and 100% can cover for each other on shallow stops while things like 32 and 40% are terrible at covering for O2 or 80%. 10/70 BO kinda dive pretty much always requires a 4th gas. 12/65 same. 15/15 can sometimes be a 2 deco gas BO but its marginal if your runtime is more than about 100mins due to lack of volume

 

[kierentec]

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.

 

[justinthedeeps]

Not MOD 3 but reading this got me to thinking, why everyone is suggesting 50 and 100 as deco mixes, I know those are common gases, but doesn't seem idela for such adeep dive. I started playing with multideco. ...

Seems like everyone doing a deep dive should at least be taking a look at gas planning principles like this. In addition to increased discipline, control, skills etc, this is a foundational part of a Mod3 course--isn't it?

I know I said it before, but at your first real deco stops, like anything over a couple minutes, we should certainly consider having a deco gas appropriately planned for that depth. 50% nitrox begins to fall short.

Haven't there already been some infamous bad accidents and ICD hits with deep divers switching to 50% nitrox directly from a high helium mix? Neglecting an intermediate deeper deco gas with some helium in it?

 

[mr_v]

Not MOD 3 but reading this got me to thinking, why everyone is suggesting 50 and 100 as deco mixes, I know those are common gases, but doesn't seem idela for such adeep dive. I started playing with multideco. ...

With better BO mixes, the difference between deep bail out and no deep bailout is 10 min, seems like a worthy compromise if it eliminates a cylinder.

Like I said, not MOD 3, so maybe I am missing something, but seems like with optimized BO deco mixes, the difference is not worth the added cylinder.

I'll address this in reverse order.

10 minute savings is negligible, as 100m dive is likely to be a 5+ hour endeavor if you account for travel time and all the prep work. If you must save 10 minutes, work faster ;-)

Most CC divers who dive to 100m have various bailout mixes on hand. There is little to no overhead in getting an extra tank.

It is easy to dilute 50 and 100 mixes, so they work well for deco and operational considerations.

Finally, MultiDeco may give you theoretical numbers that will fail in practice. Deep BO gives you options and buys time when nothing else will. Do you ever want to be in a position where you wished you had more baiout? I sure do not.

 

[justinthedeeps]

I'll address this in reverse order.

10 minute savings is negligible, as 100m dive is likely to be a 5+ hour endeavor if you account for travel time and all the prep work. If you must save 10 minutes, work faster ;-)

Most CC divers who dive to 100m have various bailout mixes on hand. There is little to no overhead in getting an extra tank.

It is easy to dilute 50 and 100 mixes, so they work well for deco and operational considerations.

Finally, MultiDeco may give you theoretical numbers that will fail in practice. Deep BO gives you options and buys time when nothing else will. Do you ever want to be in a position where you wished you had more baiout? I sure do not.

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.

 

[mr_v]

I question why you think it necessary to dismiss someone's careful attempt to further explore these matters.

Do not simply disregard the important of very carefully analyzing your gas plan.
I doubt the best deep divers are taking that approach.

Shame on you for discouraging careful thought on this! Or maybe I misunderstand you, and it is just 'wording'

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 you are not considering that carefully, this does not sound like a qualified Mod3 diver.

Please re-read my post. I am not dismissing anyone's plans - I am pointing out the fact that a saving 10 minutes is negligible in the grand schema of things. If we are about time savings, there are other ways to do things.

I have always encouraged critical thinking; hence, my reply. As I appreciate everyone's time, I do not want to write a ten page novel - simple facts should do. The fact is that the proposed dive plans may not have enough room for error for me and given everything else in my disposal, like additional gases ready to go, I'd rather opt for something else.

Finally, I am totally lost on your last point. I haven't stated a thing about 50% gas being the only deco gas.

 

[justinthedeeps]

Please re-read my post. I am not dismissing anyone's plans - I am pointing out the fact that a saving 10 minutes is negligible in the grand schema of things. If we are about time savings, there are other ways to do things.

I have always encouraged critical thinking; hence, my reply. As I appreciate everyone's time, I do not want to write a ten page novel - simple facts should do. The fact is that the proposed dive plans may not have enough room for error for me and given everything else in my disposal, like additional gases ready to go, I'd rather opt for something else.

Finally, I am totally lost on your last point. I haven't stated a thing about 50% gas being the only deco gas.

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.

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

 

[rjack321]

Haven't there already been some infamous bad accidents and ICD hits with deep divers switching to 50% nitrox directly from a high helium mix? Neglecting an intermediate deeper deco gas with some helium in it?

I'd love you to find an example. This is a non-existent phenomenon outside of diving bell kinds of conditions. 1000s of folks have switched from 15/55 to 50% with zero "IBCD" cause it doesn't exist. As noted above if you are on a 12/65 or more dive you 99.9% of the time need a deeper gas due to volume concerns.