CCR Safety v OC safety?

[Mark Chase]

Following on from the above reply to an earlier thread I thought this might make a decent topic on its own:

Originally Posted by Brass Monkey

Thanks Mark, I'll be interested to hear what you make of the box when you start diving it full time. Your views are always informative and well written but I don't think I can go along with the above. Given the multiple options on a Ybod before you even come off the loop I just can't see how OC is safer. Maybe you will think differently when you start using it, then again you may not.


Dave.

You may well be right but OC can only kill you by switching to the wrong gas (which with my rig was as close to imposable as can be). Or by running out of gas. Which again with my rig was not terminal. I can’t think of another OC method of killing my self on a normal dive. (Deco problems and rapid ascent aside). I can’t think of an OC failure that would not be obvious apart possibly from a contaminated gas fill. IMHO OC either works or it doesn’t but maybe others have a different opinion.

At first glance the CCR has several ways of killing you and some of them are subtle and without obvious warning. Too much 02 not enough 02 and the dreaded CO2 scrubber breakthrough, channelling, over packing, settlement of the lime forcing increased WOB and subsequent black out, loop floods, forgetting to switch the gas on…

Did I miss any thing?

The unit will happily allow you to breath right up until without warning you pass out or convulse. The manual tells you to check the handsets once every minuet during the dive. I am sure that after a while you’re going to be doing once every few mins at best. A heads up display is VERY high on my shopping list.

So what do you experienced CCR and OC people think? Is this just newbe paranoia?

ATB

Mark Chase

[Padowan]

As a relative newbie myself, I can agree with your "anxiety" as to the number of failure modes, and the insideous nature of them. They will not make themselves obvious, you have to check for them continuously.

What I have found over the weeks that I've been (quite intensively) diving my unit is that my reliance ont he handsets, and the deathgrip on them that I had when I started so that I could look at them all the time, has dropped. I now "listen" to the unit, I hear and feel almost every injection, and based on (my limited) experience I have a good feel for how long and often it should inject. The handset checks (which I reckon I do every 2-3 minutes now) is a cross check. So being "in tune" with your unit I think is a great way of increasing your safety, as Hyperoxia and Hypoxia should be covered by this - no O2 injection you will be aware of 1) loop volume reduction if you run it minimal 2) not hearing injection. Hyperoxia you would notice 1) bouyancy change caused by excessive injection and 2) you'd hear the gas coming in all the time.

However being "in tune" with your unit will not help you with Hypercapnia - this really is a bit of a random element, but you can reduce the liklihood of this happening by keeping well within the time limits of the Lime and taking care when you pack the scrubber. If the scrubber monitor that they are putting on the Evo is as good as it could be, this IS a nice addition - but I fear that rather than adding safety, it will encourage people to push the scrubbers and ignore the time limitations - not sure what other perspectives people have on this. Lime's relatively cheap - just change it and be done with it, if you ask me.

Loop failure should be preventable by good equipment care and maintenance and a conservative and protective approach to penetration, where you might dislodge or tear a hose.

The BIG safety factor that you do get with a CCR is time. As DrMike wrote in a post a while back, when he got stuck in a deep wreck, he'd probably have been toast if on OC...

In the Red Sea recently I was down at 65m and felt very comfortable staying there, without the constant check, check, check of the bottom time and gas. This, I felt, led to a lower stress level on the dive, which in itself puts you in a better base level position if you do have to deal with a problem.

I take every RB dive seriously - you can't just sling it on and jump in, and because of this, even shallow lake bimbles for skill development have to have the same levels of equipment preparation as a 60m+ open water wreck dive - I think that unless you are the kind of person who can take this attitude, then complacency could potentially set in as the experience increases, and your "benchmark" for a non challenging dive increases - this is where it's likely to bite you I think..

My 2p.

[Brass Monkey]

Perhaps I shouldn’t be replying here as I’m not very experienced at all. Padowan has just about covered it form my point of view.
As usual Mark what you say about the OC set up and some of the potential dangers of CCR is quite right. I suppose my point was a little off the mark, what I wanted to say is. Yes CCR is “potential” more dangerous but you have plenty of options while diving it. OC either works or it doesn’t. If OC fails that’s it, it’s gone, probably pissed away all your gas. OK you can isolate on your twins but that’s still only one other option, then your down to your stages. With CCR you a few options right up to the fully flooded loop. Then your back to your stages same as OC.
The three bogy men of CCR were not really taken into account, I had assumed good CCR practices to minimise such risks. It’s true though that the three bogey men are far more sneaky on CCR than OC thus making it inherently more dangerous.
I think a diver with your knowledge/experience will soon have the box rigged where can breath all of the gas your carrying straight off the loop, giving you even more options.


Dave.

[NotDeadYet]

The biggest danger with rb's is that you have to know you have a problem. With OC it's very apparent, with CC it isn't. You are depebdant, even on manual rebreathers, on electronics, on a chemical reaction, on the integrity of engineering and probably the most sobering of all, two very flimsy small rubber discs. With OC it is designed to stop and provide you with a back-up, CC doesn't have this pleasure. CC tries to give monitoring systems/procedures but these have to work and you have to know how to use them. Maybe because I've never been taught any better (never been taught at all) my view in my fairly limited experience of RBs is "if in doubt bail out".

[BJ]

It's easy to get it wrong (as I have) but you usually have time to put things right again. Apart from a complete flood which could only happen if a corrugated hose got broken, everything else seems down to pilot error. I liken a CCR to flying a helicopter... Superior to driving a car, but would you want everyone you know who drives a car flying a helicopter?

That said, once I had used a Prism Topaz with a head-up display, I never wanted to use an Inspiration again. I am waiting anxiously for the Evolution (or Inspiration with Evolution electonics) with a head-up display.

The Prism normally shows a little green pulsing light (green is good). It will change to blue or red according to ppO2 levels getting too low or too high. When you get a spike or dip you cannot miss the blue or red and it makes you look at the ppO2 display immediately. There are no aubible warnings as these are not allowed in US Navy COTS equipment because they think people might not hear them. The head-up display is not intrusive in the least. It's a comfort!

The PRISM also has auto switching of the set-point so no more diving at 0.7 because you omitted to switch over at depth. (That's what I did with the Inspiration and I'll never forget that!)

That said, I think IMHO that OC bail-out is for those who are not really complete rebreather divers! (That'll cause some shouting!)

http://www.divernet.com/equipment/11...ts.shtml#prism

[Brass Monkey]

Originally Posted by NotDead

"if in doubt bail out".

Sure the book says that but it's a bit of a catch all. It should read "bail out but get back on the loop ASAP" or sort the problem without bailing. That's what I was taught and it makes sense to me.


Dave.

[Mark Chase]

Just completed day two of the training course. Skills, no problem buoyancy control OH MY GOD

Interestingly Andrew nearly killed him self on day one through no fault of his own. Under the expert gaze of the instructor (Paul) and taking three times longer than Paul's suggested time, we assembled the units and carried out a full series of pre dive checks. Then we went to the pool and did getting on for an hour in the water doing skills.

Upon our return to the instructor’s house we striped the units ready for cleaning. When Andrew took the top off of his scrubber we found the main 0 ring kinked and squashed up the side of the spacer ring. This 0 ring seals the scrubber to the scrubber housing. If not in place the spent gas goes up around the scrubber rather than through it and the diver gets a C02 hit. Andrews unit had about a 30mm gap in the continuity of the 0 ring. After 60min of no fining and in a depth of about 1.5m of water Andrew was fine. Had that been a 60m mix dive he would have probably had a CO2 hit.

This from a unit that passed all the predive checks and which was assembled on a bench not on a rocking and rolling boat.

Had that been an incident report of a dead diver I have little doubt it would be put down as diver error. Sloppy assembly of the unit or something like that. In fact I doubt if it is possible to take more care assembling the units than we did that day.

I have to say it gave me pause for thought.


ATB

Mark Chase

[NotDeadYet]

Originally Posted by JJJBBB

That said, I think IMHO that OC bail-out is for those who are not really complete rebreather divers! (That'll cause some shouting!)

Hi John,

each to their own. For me the psychological benefit of switching to a system in an emergency that I know will work and will be safe to breathe instantly outweighs what anyone thinks of my completeness. I'd rather be an alive shit diver than a dead good one. If the loop can be salvaged all well and good but my first stop is my OC reg.

A hell of a lot more than a torn loop hose will flood a RB: punctured counterlung, failed o-ring, punctured ADV diaphragm, torn cable sheath (if using ambient pressure handsets), warped scrubber (happens on Draegers)... I guess one of the benefits of homebuilding is the risk analysis and failure mode studies that you go through.

Cheers,

Stuart

[Drmike]

Originally Posted by NotDeadYet

Hi John,

each to their own. For me the psychological benefit of switching to a system in an emergency that I know will work and will be safe to breathe instantly outweighs what anyone thinks of my completeness. I'd rather be an alive shit diver than a dead good one. If the loop can be salvaged all well and good but my first stop is my OC reg.

A hell of a lot more than a torn loop hose will flood a RB: punctured counterlung, failed o-ring, punctured ADV diaphragm, torn cable sheath (if using ambient pressure handsets), warped scrubber (happens on Draegers)... I guess one of the benefits of homebuilding is the risk analysis and failure mode studies that you go through.

Cheers,

Stuart

I fully agree Stuart. Get to breathable (known safe) gas asap then assess the situation with the ultimate aim if possible to go back to the loop.

However I do it a slightly different way because in the case of a CO2 hit In practice its often not possible to bail off a RB to OC

For 'dodgy gas problems' I practice the faster and more easily accomplished 'if in doubt fire the ADV method'


I had a CO2 hit once where I ended up laying on the sea bed fully aware that I was having a CO2 hit but my arms just wouldn't move. I knew I should bail but it seamed like just too much trouble and the thought of leaving the perceived comfort and safety of the loop was just not an option. I knew all I had to do was grab my reg. I knew I had plenty of OC gas (believe it or not I was only at 25m!!) I remember thinking I should bail to OC but just couldn't be bothered to move my arms at all. Then I thought maybe a flush would be good - but still I just was feeling very incapacitated and just didn't want to be bothered with moving at all. I was laying flat out on the sand with my arms by my side like an idiot just breathing not moving and getting more and more useless by the second. Some how and for some reason I started breathing out every breath thru my nose thus firing the ADV and thus flushing the loop. After a few minutes of this I began to feel better and I sorted myself out. I went back to the loop semiclosed and turned the dive.

I think looking back in my lethargic state any excess movement (flushing, bailing) was just abhorrent to me as was the thought of leaving the perceived safety of the loop. By firing the ADV I didn't have or do either and I think it saved me. Now my first action if I get a CO2 hit is not to bail as I think from what Ive read its not always mentally possible - my first reaction is to breathe out thru my nose and fire the ADV on each breath. It only takes one or two breaths to be on effectively OC anyway.

I personally believe this should be taught in training rather than the bail to OC mantra - as we know that doesn't work.

Once the fresh gas has cleared the head then asap I can bail properly to OC or stay on the loop and manage the situation.

Since this happened a few years ago I am now using a OC/CC switching mouthpiece - but recently I heard a story from one diver who said he was even unable to switch his OC/CC valve when he had an CO2 hit.

People and training agencies that glibly assume its easy to bail off a RB to OC in the case of an CO2 hit are dangerously fooling themselves - there should be better awareness that if the CO2 gets you your probably going to be a pile of useless sh1t and not able to do anything to save yourself - not even to swap a reg.


So my mantra is

if you are starting to doubt - fire the ADV (breathe out thru the nose)
if in doubt (and if able to) bail out to OC

- then

if deep enough or in overhead where gas vol could be an issue try to see if you can go back to the loop' (If your in 15m of water and no deco why risk going back to a loop that could be compromised?)


Cheers
Mike

[Drmike]

Originally Posted by Mark Chase

This 0 ring seals the scrubber to the scrubber housing. If not in place the spent gas goes up around the scrubber rather than through it and the diver gets a C02 hit.

This from a unit that passed all the predive checks and which was assembled on a bench not on a rocking and rolling boat.

That o-ring wont show up in the pre-dive tests - or likely not even in a pre-breathe unless he was breathing the unit for quite a while

In my opinion that canister seal method is a bad design. Its been the cause of many a CO2 hit (I personally know of three divers hit) A captive o-ring would be wiser, (you cant leave it out or get it kinked) I do not know why APD use that method when there are other less error prone ways to effect a seal.

I always assemble my scrubber vertically (never on its side) and take extra care to lower the lid on straight and smoothly so as not to unseat that o-ring. Doing it like this its not possible to kink it.


Cheers
Mike