Bail Out Breather options?

[stuartv]

Okay, lots of info there! Thank you all.

So, let me see...

First, as a general approach, I didn't realize that it was an automatic thing that all mCCRs would have constant O2 flow (i.e. CMF or needle valve).

My thinking is (now), for a BOB, why not plug the orifice or block the needle valve, so you can manually inject O2, but there is none just flowing into the loop? My further thinking was this: Fill the loop with O2 before starting the dive. On descent, let the ambient pressure drive the ADV to put dil in the loop. During the descent and bottom portions, that would mean you'd be guaranteed to always have a breathable loop, right? And, with no CMF/needle valve, and not breathing it, the loop would stay breathable and have a constant buoyancy. So, during the descent and bottom portion, you don't have to do anything with it at all. It's just sitting there, breathable and waiting on you to need it.

During ascent, you might have to do two things: Vent the BOB loop and, when getting shallow, inject some O2 to ensure it doesn't go hypoxic (in case you have to bail at 3m, for example). I was also thinking what if you weight yourself adequately and just let the BOB loop expand to max loop volume during the ascent and then burp from its OPV the rest of the way? That way its buoyancy becomes and stays constant. Probably not a good idea, but I'm not experienced enough to really say.

How far off base am I, so far?

As for using an eCCR, I was thinking that would be a no-go because, since you're not breathing on it to push the gas around, the auto-injection just would not work (correctly). However, I did not know about the Liberty's BOB mode, so maybe that would address that issue. However, the Liberty seems non-ideal for a different reason - it weighs twice as much as a Sidekick and it's way more expensive. It seems like a good choice for a primary unit, but not so much as a backup.

Regarding requirements for carrying additional gas and OC in particular, what I was thinking was this:

X as primary, with a BOV and the normal 2 x 3L steels on back for O2 and dil
Sidekick slung on the right
AL40 of O2 (or sphere or whatever) slung on the right and connected to Sidekick
Same AL40 of O2 has a whip on it so that it could be connected to the X's MAV, if needed.
AL80 of dil slung on the left and connected to the Sidekick
Same AL80 connected to the X's BOV
Drysuit inflator bottle mounted and used in whatever fashion it is normally done for the X

Shearwater Petrel controller for X on left arm
NERD monitor for X on X loop
Shearwater Petrel monitor for Sidekick on right arm

So, if you have to bail, you flip the BOV on the X and breathe that only for the time it takes you to deploy the loop from the BOB and switch to the BOB's DSV.

You have big bottles of O2 and dil that are available to plug into the X, if needed. Those same bottles are connected to the BOB, if you have to bail. And you could even plug the O2 in for drysuit inflation if you HAD to, and the O2 or dil into the wing for inflation, if you HAD to.

The drivers for all this (in my mind) are not so much that the 3L cylinders on the X won't have enough gas. It's that one of them could fail - i.e. a tank valve O-ring blowout or 1st stage HP seat or whatever. This config seems like it would offer redundant gas that would allow you to stay on the loop. And a redundant CCR, in case the primary loop flooded and could not be recovered.

What am I missing? Why would you still need any OC? @JohnnyC, I read what you said and it implies that if you go hypercapnic, switching to a different CCR is not a solution. You implied that the only solution is to switch to OC. Am I understanding that correctly? Does having the BOV connected to an 80 full of dil solve that issue?

Also, based on what you said, I could see where maybe the eCCR version of a Sidekick would work. Set it to a low setpoint and pump it full of O2 before starting and the solenoid should never fire during the descent and bottom portions? Then manually inject some O2 at some point during the ascent to keep it from going hypoxic without having the solenoid do it for you and mess with your buoyancy? But, what about the fact that you're not breathing its loop, so the O2 sensors wouldn't necessarily "see" the extra O2?

@kensuf The extra 3L steels are not for exactly that reason. My understanding of his reasoning there is the extra O2 is in case he "loses" the primary O2 cylinder - not that he's worried about consuming it all. And extra 3L for suit and wing is (I think) because 1 x 3L of dil by itself is only just enough for wing and loop. He is testing bigger cylinders for his X. Once those are checked out, I believe he'll then drop that extra 3L. I think that means he'll go back to wing and loop connected to his (bigger) dil and (I assume) his drysuit connected to one of his BO cylinders. Something that has no helium in it. I am not actually sure why he is carrying the extra 3L of O2 instead of just having one of BO cylinders full of O2 and being prepared to plug that into his loop if needed. I'm going to ask him about that.

As I think pretty much all of you know, I am totally new to CCR, so I realize this may all be complete rubbish. That's why I am asking y'all! I apologize if my ignorance makes my questions annoying.

 

[Jeremy Williams]

I think that Hypercapnia starts becoming a bad catch-22 really deep. You don’t want to bail to another rebreather because the work-of-breathing could make the problem worse due to ongoing retention. But you also don’t want to be on OC because you are going to be emptying cylinders too quickly due to the typically high respiratory rate.

 

[kensuf]

That's a lot of stuff in that post to process... I already gave you answers to extra inflation and oxygen problems. DIL's easy enough too, go off-board.

 

[tbone1004]

Like @kensuf said, a lot to process, but here's some interpretations because I have a slow spot right now

@stuartv not all mCCR's have a constant flow orifice, UTD for instance doesn't believe in them or ADV's for that matter so that would be a REAL bad choice for a BoB. If you have a mCCR with a leaky valve, you should have the ability to shut it off, whether a shutoff switch or a needle valve. Loops normally start with full O2, and the ADV or MAV on descent should keep the ppO2 breathable, but you do have to balance it off when you get depth-stable.

Depending on the breather, the lungs may provide considerable buoyancy, a 6l counterlung has 13lbs of positive buoyancy which is not something you want to deal with, sure it makes ascents easier, but that's a lot of buoyancy to counteract on descent so maintaining min-loop is better. On the ascent, if you're breathing hypoxic mixes, it may not stay breathable on ascent.

More on the BoB mode with Liberty. Basically it just shuts the solenoid off regardless of setpoint unless it gets 20% below the dil ppO2, so on ascent it will maintain breathable ppO2. It also stops stack counters and other things associated with not actually breathing it, does something with deco but I can't remember off hand. On normal eCCR's you would just set it to surface ppO2 if you can, i.e. 0.2 on my Meg and it won't touch the solenoid unless you make an ascent on hypoxic mix and get close to the surface.
https://ccrliberty.blob.core.windows.net/static/files/en-manual-liberty-bsm-2.0.pdf

The price of a Liberty vs a Sidekick is a lot closer than you may think. Sidekick+cells+Petrel is every bit of $6500, add a sphere for $2k and you're at the price of the liberty. What you get with the liberty is a fully self contained unit with 2l's of O2 and Dil on it so you don't have to plug it into anything. Weight wise, yeah it's not light, but once you add a sphere and an offboard dil bottle, it's about the same.

The ideal solution is really a rack though, promise. My Meg with a pair of LP50's, and a pair of HP 3l's weighs the same as a set of 104's, albeit physically bigger. Plumbing would be as follows. 50's to BOV/ADV/MAV for dil. Let's you bailout with bottom mix, gives you redundancy depending on how you plug those things in, i.e. bov from left post, MAV plugin from right post kind of thing, or a QC6 off of each post to be able to plug into a block with the BOV on it. The 3l for "dil" becomes suit/wing inflation, and then 3l of O2. Add an AL80 of 50% and you're good to go for normoxic. For hypoxic, depending on what you're doing add O2 and deep deco mixes if necessary. No BoB necessary, and a much more compact kit.

 

[JohnnyC]

Setting aside the potential impossibility of bailing out to another loop while on a wicked CO2 hit, you still may have the problem of overbreathing the scrubber on the BOB. If there's not enough dwell time, because you're pushing so much volume, so quickly through the scrubber, essentially makes it ineffectual. Putting the brakes on a hypercapnia event means controlling your breathing rate, and removing the CO2 from your body. If you're just rebreathing a loop full of CO2 because your scrubber physically can't bind the CO2 fast enough, you're not solving the problem. Hence the need for real OC volume in the first place.

It will go fast, but it may just fix the problem enough to enable going onto the BOB. Even if he just accepts the fact that he's cutting his margins close by only carrying one big bailout, at least it gives him a chance to solve the CO2 problem before accepting that he's screwed. I'm a big fan of BOV's for exactly this reason. Nothing is gonna put the brakes on a CO2 hit faster than fresh gas and getting the CO2 out of your body.

Filling a loop with O2 and relying on the ADV may not drop the PO2 enough on descent because you're not metabolizing any of the O2 on the way down like you normally would if you were on the unit. If your BOB dil is the right FO2 for the target depth, your PO2 will be safe when you get to the bottom. If you have O2 on top of that, there's no guarantee that your loop will be safe at max depth. Since CO2 retention seems to play a role in increasing ones susceptibility to an oxtox, you don't want to bail to something hot, you want a safe PO2.

An eCCR isn't going to inject if the setpoint is lower than loop contents. Your dil FO2 will dictate the PO2 the electronics sees. Leave a low setpoint, or even better a BOB mode like the Liberty, and it's a non-issue. You can always throw a shutoff on the solenoid feed. However, like doing it on an mCCR, you've gotta remember to unblock it. Since most eCCR's have separate feeds it's less of an issue than an mCCR where the leaky valve and MAV often enter the loop through one port.

As far as mCCR's, you can do the UTD thing where there's ONLY manual add of O2. No leaky valve or needle means it's not going to leak O2 into the loop, but in a bail out scenario, you're going to have to really fly the BOB. In a stressful situation, requiring a bunch of intervention may be too much task loading depending on the scenario. It's generally accepted that having nothing but an O2 MAV for addition is kinda sketchy.

I think your friend should be looking at changing his perspective on how he's going about it. A rack with LP50's or LP85's is going to get him much more bailout/dil/inflation gas. He'll have more readily available volume on his person, which means he can take a closer look at what he's carrying offboard. A 40 of O2 offboard feeds the CCR, as well as being available offboard O2 deco gas if required. A 40 and a couple 80's of deep and mid gas saves a full 80 of bailout, makes one of his bottles more manageable, and doesn't require the complexity of another rebreather.

 

[stuartv]

Okay, now you have given me a lot to process. One quick question: You (JC and tbone) have both mentioned a "rack". I don't know what that is. Do you mean having a pair of 50s as a normal backmount twinset and then mount the CCR on the back of that?

 

[JohnnyC]

Two examples. Mounts a set of cylinders either side of the unit, on a dedicated frame. Most stock units bottle mounts aren't stout enough to mount large cylinders. In this example, you can see the bands on the AL40's, and the cambands that hold the unit to the frame.

 

[kensuf]

That's my Fathom racked up for open ocean diving. Twin LP50s on either side with LOLA valves, DIL is fed from the LP50s, fully redundant OC bailout -- literally my doubles regs with the addition of a QC6 hose for the DIL feed. Oxygen is fed from a 2 or 3L bottle (I'm using a 2) mounted on the back of the can.

 

[kensuf]

And here it is on a boat before I jump in for a 200' dive.

 

[stuartv]

That's my Fathom racked up for open ocean diving. Twin LP50s on either side with LOLA valves, DIL is fed from the LP50s, fully redundant OC bailout -- literally my doubles regs with the addition of a QC6 hose for the DIL feed. Oxygen is fed from a 2 or 3L bottle (I'm using a 2) mounted on the back of the can.

Fully redundant OC BO meaning you have a BOV on the loop plus that OC 2nd stage on a bungee necklace? Or meaning the 7' hose to 1 50 and the short hose to the other?