Technical diving with oc versus ccr gas requirements

[tbone1004]

Could you link the 1st stages via HP ports, and then equalise if required, or connect the CMF to a manifold fed from 2 o2 cyls, 1 isolated until required.

I mean probably? but that's annoying. If I do it I'll just put a splitter on the inlet of the MAV and have both hoses coming up to it. Leave aux bottle off, and turn it on as necessary. You can also use differential IP with the aux bottle at a lower IP and then it would automatically switch over. Would definitely want a check valve or shutoff on the primary side though in case you have to shut off because of something failing on the LP side of the regulator where the aux would just back feed which would be bad.

 

[broncobowsher]

Just plug in the deco mix and manually squirt a little in as needed. Simple way to get your O2. At this point you are not going any farther. Failure = end dive, go home. Staying on the loop will use way less deco (bailout) gas than just breathing it OC. Hopefully you had enough rebreather training to know how to fly one manually. O2 side failure isn't that big of a deal (providing it is 1st stage or anything outside of the body of the rebreather).

That is one of the nice things about a rebreather. There are so many different options to get gas/oxygen you need even with failures. When doing deep dives you only bail out if you really need to bail out. When in doubt, bail out is great for the basic recreational rebreather dives. Once you mix deco in, stay on the loop if possible.

 

[rjack321]

Once you mix deco in, stay on the loop if possible.

Whatever floats your boat, but over and over again people don't BO soon enough and end up dead. From the chairs on the sidelines it always looks manageable - until it's not and it's not always even clear why. Compare with the number of people who bailed out and died (by any cause) is infinitesimally small. And in those cases its almost exclusively medical events where it seems like the diver felt bad, didn't know what the issue was, bailed, died, and we learn post-mortem that they had a heart problem or stroke.

Number of people trying to stay on the loop with a seemingly "manageable" problem and die = hundreds
Number of people who BO and die = a rare handful

The conclusion that its better to stay on the loop until forced to bail as a last resort is not supported by historical data.

 

[GF99/99]

KISS Sidekick with O2 sphere. I can swing it forward and see the SPG, but plugging in offboard O2 from a second O2 bottle is super sketchy with a CMF. Open needle valve isn't an issue since you can open it up all the way to clear it, but if you have a CMF I wouldn't attempt it, even with QC4's which have very small water ingress. If that was a "must" then I would put a second O2 MAV on there so you had a "dumb" mav on the spare bottle that doesn't have a leak on it.
If I was going to put redundant O2 on it, it would be a drysuit bottle that was plumbed in from the beginning of the dive but with the valve off and strapped to the CCR. With the Meg on a rack, since I use the 3l as inflation, a drysuit bottle rigged as normal would go into the O2 MAV on the lung.

Could you link the 1st stages via HP ports, and then equalise if required, or connect the CMF to a manifold fed from 2 o2 cyls, 1 isolated until required.

I mean probably? but that's annoying. If I do it I'll just put a splitter on the inlet of the MAV and have both hoses coming up to it. Leave aux bottle off, and turn it on as necessary. You can also use differential IP with the aux bottle at a lower IP and then it would automatically switch over. Would definitely want a check valve or shutoff on the primary side though in case you have to shut off because of something failing on the LP side of the regulator where the aux would just back feed which would be bad.

This is always an interesting topic and I think there are many trains of thought on this. I am not saying my way is correct its just what works for me but if I was doing anything sub 150m I would bring back up O2 do I need it probably not but there is no good argument not to bring it, better to play it safe than sorry. Like tbone described I strap it to the unit (so I have primary O2 3l and back up O2 3l, then on the other side I have 3l dil and 3l inflate) and have it plugged into my O2 side MAV (my primary O2 lpi is just sitting there next to it). If I have a loss of primary O2 then I just manually fly off the MAV being supplied by the backup O2. JonG1 reason why I would not tie the O2's together is this defeats the purpose. The reason for the backup O2 if you have a loss of primary O2. I think we all know that if you start a dive with full O2 your not going to metabolize an entire 3l during your dive so the reason why you would have a loss of O2 is if you had some kind of failure on your O2 side, chances are this would be the 1st stage or hose coming off it. If you had you backup O2 plumbed into your primary 1st stage your shooting your self in the foot. Maybe by thought process is wrong on this but that is just how I view it. I like to have independent redundancy.

 

[GF99/99]

Just plug in the deco mix and manually squirt a little in as needed. Simple way to get your O2. At this point you are not going any farther. Failure = end dive, go home. Staying on the loop will use way less deco (bailout) gas than just breathing it OC. Hopefully you had enough rebreather training to know how to fly one manually. O2 side failure isn't that big of a deal (providing it is 1st stage or anything outside of the body of the rebreather).

That is one of the nice things about a rebreather. There are so many different options to get gas/oxygen you need even with failures. When doing deep dives you only bail out if you really need to bail out. When in doubt, bail out is great for the basic recreational rebreather dives. Once you mix deco in, stay on the loop if possible.

Whatever floats your boat, but over and over again people don't BO soon enough and end up dead. From the chairs on the sidelines it always looks manageable - until it's not and it's not always even clear why. Compare with the number of people who bailed out and died (by any cause) is infinitesimally small. And in those cases its almost exclusively medical events where it seems like the diver felt bad, didn't know what the issue was, bailed, died, and we learn post-mortem that they had a heart problem or stroke.

Number of people trying to stay on the loop with a seemingly "manageable" problem and die = hundreds
Number of people who BO and die = a rare handful

The conclusion that its better to stay on the loop until forced to bail as a last resort is not supported by historical data.

This is another good topic of discussion and once again I am not saying I am correct but its just my thoughts and based off of personal experience. I agree with Broncobowsher loss of O2 is not that big of a deal this is not a Co2 hit, flooded unit major emergency. Yes I agree if in doubt bail out but if your a MOD3 diver I would like to think at this level of diving your able to solve a loss of o2 problem with out bailing out. Like I said there is nothing wrong with bailing out and yes if you not comfortable or you don't know what the problem is bail out but if possible its always more efficient to say on the loop. If you loose O2 you still have lots of time to solve the problem its not like you loop goes hypoxic instantly and as a MOD3 diver you should be able to plug in your off board o2 in a reasonable amount of time. Plus at a MOD3 level plugging in off boards should just be second nature. Once again different trains of thought on this so not saying I am right but for anything sub 80ish meters I dive using dil switches so plugging in off boards is just a normal part of diving for me.

 

[RainPilot]

Hardly worth the trouble and expense of a CCR and associated training - no? Unless kensuf's criteria are used

For square profiles, there can be a limit to the amount of bottom time gained for OC vs CCR, if you are carrying appropriate bailout. Where it REALLY gets nice, is when you do TTS planning for multilevel dives. An example:

We have a wall-like dive site here, bottom is around 80m, runs all the way up to the surface. If I plan an OC dive there, I need to plan gases etc for, basically, a 60m square profile, knowing that if there's nothing to see at 60 then I will be gaining a lot of gas as I ascend early but that really doesn't help in the water for replanning, unless I have done a LOT of preplanning for every possible combination.

On CCR, I plan the dives and gases for the 60 square profile, and get to a max TTS that I can accommodate on my bailouts. Now, during the dive, if I end up stopping at 40m to watch a mola mola getting cleaned, as long as I remain inside my TTS, I can be a LOT more flexible with my dynamic planning on the dive. So the 60m for 25 min dive may end up becoming a 40m for 40 min then up to 30m for 10 min then 20m to watch the hammerheads for a while then up and out, deco done on the swim as required. That dive will give me a LOT more bottom time than if I were to do it on OC.

 

[RainPilot]

Whatever floats your boat, but over and over again people don't BO soon enough and end up dead. From the chairs on the sidelines it always looks manageable - until it's not and it's not always even clear why. Compare with the number of people who bailed out and died (by any cause) is infinitesimally small. And in those cases its almost exclusively medical events where it seems like the diver felt bad, didn't know what the issue was, bailed, died, and we learn post-mortem that they had a heart problem or stroke.

Number of people trying to stay on the loop with a seemingly "manageable" problem and die = hundreds
Number of people who BO and die = a rare handful

The conclusion that its better to stay on the loop until forced to bail as a last resort is not supported by historical data.

I really don't like the CCR training mindset of "stay on the loop as long as you can" when BO is almost always a better idea. I get that there are dives where carrying sufficient BO is problematic but that shouldn't be the standard that all training devolves to. I get the SCR thing, thats useful for sure, but some of the rest of it (feathering O2 etc etc) really just seems to be adding workload and possible failure modes that don't seem necessary to me.

I feel that too many divers use that training and mindset to rationalise carrying insufficient BO and that's worrying.

 

[JonG1]

I've just remembered the HP to HP port idea was more as a work around for exceeding the point at which a CMF would fail to operate with a blocked 1st stage.

The linked 1st stage if not blocked could be used manually below that depth.

OT though.

 

[JonG1]

On a related note, is anyone using steel bailout and deco in SM in salt water.

Common in ohe, mainly to inc. vol of gas carried (12l 232b as opposed to 11l 200b).

The mix tin, is neutral til used obv. A nitrox 80 is heavy until used ..obv.

It's common enough to rack steel on GUE style rigs, if you accept that you aren't going to be handing off as if they were BM is there a cogent reason not to?

 

[KentB]

This is part of the reason I now own a booster. The other part is the road trip time to get fills. Going to a shop, an hour away, drop it off, drive another hour home, drive another hour to pick it up, and another hour home. 4 hours of drive time to get fills. And have to plan that around when they are open, when the filler is present, and when I have time as well.

that’s basically the sole reason why I owned my own compressor by about dive 30