Question Bailout gas configs for tech/deep chestmount rebreathers

[PfcAJ]

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?

80% is a trash gas.

100% actually has zero inerts so it’s useful not only as deco, but on the surface.

With ccr it’s backup for your o2.

People always get on it at 30’ which is only a 1.5 ppo2.

With oxygen you can get above 20’ if you’re concerned about CNS% (which is fake) and still have max gradient.

Mixing air at high pressures over oxygen isn’t great for an o2 clean perspective. 100% always in the 20ft bottle reduces chances for contaminates (however slight you may feel that to be is debatable).

It’s “quicker” only because it’s an earlier gas switch. The 30’ stop on any gas (50%, nitrox, whatever) is wasted time when you could be on the worlds best deco gas (100%) by just going up 10’. It’s the width of a grocery store aisle, it’s not gunna hurt you. Do a little 30’ time and get on with it.

 

[justinthedeeps]

@PfcAJ thank you, perfect input for this thread

For what it's worth there is only a 100% in my current inventory. So yes 100% is great

BUT I would personally not mind at all using an 80% in my plan, if that's what is available or preferred by team for a dive. And have done so, recently, including for 100 metre dives. And plugged it in for practice of loss of onboard oxygen. It works fine.

 

[PEDiver]

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

Couldn't agree more. But this did make me think - for units like the Fathom Mk3, it seems the common practice is to use the onboard diluent cylinder for wing/suit inflation while using offboard gas to drive the unit (whether that's in the form of manifolded doubles or SM). Since the unit would be run in a dil-out configuration, I'm assuming you get around the issue by being able to use both bailout cylinders to drive the unit in the chance you lose one? Or would you sling an extra AL80 of the same gas as your bailout to drive the unit while keeping the primary bailout cylinders untouched?

Shocking how many people are comfortable with the idea / risk profile of just swimming up to the next gas in the case of a loss of bottom gas with no redundancy. But I guess there's a reason why the Darwin Awards exist.

 

[Whitrzac]

Couldn't agree more. But this did make me think - for units like the Fathom Mk3, it seems the common practice is to use the onboard diluent cylinder for wing/suit inflation while using offboard gas to drive the unit (whether that's in the form of manifolded doubles or SM). Since the unit would be run in a dil-out configuration, I'm assuming you get around the issue by being able to use both bailout cylinders to drive the unit in the chance you lose one? Or would you sling an extra AL80 of the same gas as your bailout to drive the unit while keeping the primary bailout cylinders untouched?

Shocking how many people are comfortable with the idea / risk profile of just swimming up to the next gas in the case of a loss of bottom gas with no redundancy. But I guess there's a reason why the Darwin Awards exist.

Every time one of these threads comes up I ask: Has anyone bailed out, then ran out of gas and died?

To the best of my knowledge, it hasn't happened yet.

There's the article that gets reposted by a cave diver wishing he had more, but he still had at least 1 full tank with a closed valve


Every rb death I've read about they were still on the unit, in the process of bailing out, or had a medical issue

 

[PfcAJ]

Every time one of these threads comes up I ask: Has anyone bailed out, then ran out of gas and died?

To the best of my knowledge, it hasn't happened yet.

There's the article that gets reposted by a cave diver wishing he had more, but he still had at least 1 full tank with a closed valve


Every rb death I've read about they were still on the unit, in the process of bailing out, or had a medical issue

I’m pretty sure Mark Fyvie drowned within a few body lengths of one of his staged bailouts at Ginnie.

Dual fatality at eagles nest ultimately resulted in both divers completely out of gas following the ditching of a CCR.

Just off the top of my head, but I’m fairly certain there are others where more bailout or better positioned bailout could have changed the outcome.

“Die” or “not die” isn’t a good measure for scuba. I’m not looking to just barely skate by when something goes wrong. This **** is for fun. There’s no point to being on the edge.

 

[lostsheep]

Or I dunno just acknowledge that a unit CM isn't the right tool for the 100m ocean dive in the first place?

What is your objection to a CM?

In your opinion, what is the right tool for a 100m ocean dive?

What experience do you have with a CM?

ETA: come to think of it, how many of the CM detractors actually have any time on one?

 

[Cio]

If you bring actual O2 you can cover the loss of CCR O2. 80% you can sorta, but you end up needing to flush the N2 away which lowers your fO2 so its a big messy ascent with likely lots of yo-yoing from all the flushing and adding and flushing. There are ample other reasons to bring O2 BO on 100m a CCR dive. The only people doing 100m dives with air, 32% and 80% BO are fools on the internet - that went out of fashion for good reason 20 years ago.

Fair enough that 100% can replace your O2, I get that's why it is picked. Unfair however, on the 80% not being as good. Using 80% as drive gas in SCR mode is beyond simple and if you cannot do that, you have no business going to 100m. SCR is a legit Bailout mode and is simple in practice, any MOD 3 diver should be skilled in this. With drive gas that rich, in SCR mode, you'll need to blow 1/4 of a lung full out your nose every once in a while, (not very often) there is no flushing required. With 100% you are loosing volume and adding gas. With SCR you exhale and add the same amount back, it is very easy to maintain buoyancy this way.

Regardless, 50 and 100 or 32 and 80.... Pick your gas, it's your dive. I don't care about fashion. I like the idea of getting on a mix earlier, not using 100% for prolonged periods of time and less runtime. Seems like a win win. I do not think either combination is dangerous and maybe MOD 3 training will change my perspective,....

You still do not address my main point why have 3 deep bailout cylinders instead of adding a deco bailout.

1. I agree a single deep bailout with an intermediate gas in doubles is a bad idea (1 is none and all that). Easy solution is to put an H valve on the bottle and problem solved. Similar to above, its your dive so, do whatever you like.
2. when looking at failures, a deco bottle failure leaves the diver without options (assume self-reliance in BO for planning)
3. So, why not stick with your 50 100 but add a 32 (or whatever) in that extra cylinder so your deco BO can still work if there is a failure? or add H valves to the deco bottles?

If you lose your unit and have to BO. Deep bailout BM twins and 2 x deco BO (4 cylinders total) mixes is enough. The suggestion is to add a deep BO, extra won't hurt, right? But, from a recovering from additional failures perspective, the extra gas won't help either, extra deco gas will. I suggest that if you wanted to carry 5 cylinders, upsize your twins for the extra deep BOO gas and put deco gas in that 5th cylinder.

 

[justinthedeeps]

@Cio interesting point on an H valve. Does in theory address the loss of gas issue from a single cylinder. Is anyone doing that?

 

[divingx]

Couldn't agree more. But this did make me think - for units like the Fathom Mk3, it seems the common practice is to use the onboard diluent cylinder for wing/suit inflation while using offboard gas to drive the unit (whether that's in the form of manifolded doubles or SM). Since the unit would be run in a dil-out configuration, I'm assuming you get around the issue by being able to use both bailout cylinders to drive the unit in the chance you lose one? Or would you sling an extra AL80 of the same gas as your bailout to drive the unit while keeping the primary bailout cylinders untouched?

Shocking how many people are comfortable with the idea / risk profile of just swimming up to the next gas in the case of a loss of bottom gas with no redundancy. But I guess there's a reason why the Darwin Awards exist.

You can put QC6 on both sidemount bailouts. In case you lose one tank, just plug in the QC6 from the other tank.

 

[kierentec]

Couldn't agree more. But this did make me think - for units like the Fathom Mk3, it seems the common practice is to use the onboard diluent cylinder for wing/suit inflation while using offboard gas to drive the unit (whether that's in the form of manifolded doubles or SM). Since the unit would be run in a dil-out configuration, I'm assuming you get around the issue by being able to use both bailout cylinders to drive the unit in the chance you lose one? Or would you sling an extra AL80 of the same gas as your bailout to drive the unit while keeping the primary bailout cylinders untouched?

Shocking how many people are comfortable with the idea / risk profile of just swimming up to the next gas in the case of a loss of bottom gas with no redundancy. But I guess there's a reason why the Darwin Awards exist.

The fathom “tech rig” is configured with onboard manifolded bailout/drive gas (either LP50s or AL40s depending on the dive). For shallow dives where you are carrying nitrox on your back, drysuit is driven from the left post , wing is from the right, just like OC. For dives requiring helium, we generally mount an inflation bottle to the can on the left side, mirroring the onboard O2.

The rebreather is driven from the left post via QC6. The MAV splits the dil feed between the dil to the head and the BOV, so whatever gas is plugged into the unit is also plugged into the BOV. Because the back gas is manifolded, if you lose your left post, you can shut down that regulator and switch to your long hose and still have access to all of your back gas. You can then switch to a stage (either bottom gas or a deco gas, depending on where you are and what you need to do) and drive the unit from that and go back on the loop. If you lose your right post, you lose your longhose and power inflator, but the rebreather will continue to function normally.

In cases where you may want to drive the unit on the bottom with a deeper mix than an ideal OC bottom gas (like a 100ft cave dive for example, you would want to drive the unit with 21/35, but have 32% available for OC or SCR), we will generally have 32% on our back and drive the unit from a 21/35 stage.

Gives you a lot of redundancy and simple options that are similar to OC muscle memory.

When diving the fathom in the “cave” configuration, everything is the same except we use side mounted bottom, driving the unit from the left post, longhouse on the right.