Overbreathing the scrubber

[Cave Diver]

I agree about the importance of a buddy. There was a near miss in Nanaimo on a It didn't seem like a big deal and thats why I didn't post it before but in light of recent concerns over CO2 issues I felt Lynne's thread was a good time to bring it up.

I'm glad you did. It's great information and underscores the need for an attentive buddy that understands signs and symptoms and knows how to react.

It's funny, me and my primary dive buddy were discussing some of this type thing today and how the switch to Megs has made us more choosy about who we'd dive with in regards to their abilities to identify CCR related problems.

 

[wedivebc]

Goes to show how CO2 affects people differently.

I had a CO2 incident (during my CCR class ironically.) I knew immediately something was wrong, but didn't know what. I was on a wreck I had dove many times before, with benign conditions - but I had this overwhelming urge to bolt. I remember the mantra - if you think you need to bail then you need to bail, so I did. The panic subsided very quickly once I got on open circuit, although others on the boat told me I was ghost-white when I climbed back on board, and I had a headache which lasted me 3 days.

Any idea what caused it?

 

[Dive-aholic]

Great information! Dave/wedivebc, thanks for relating your Chac Mool story. I think I'll wait until the CO2 monitors are developed before I move on to a RB.

 

[swamp diver]

Thanks. I had a feeling that it would have to be possible to draw gas through the scrubber too fast to allow the reaction that removes the CO2. One would think the result would be high anxiety, but I remember watching a video of someone getting CO2 toxic on a RB, and he didn't go to bailout or even signal a buddy. I wonder if it just creeps up too slowly for the air hunger to register.

I have this awful feeling that CO2 is the culprit in a number of otherwise inexplicable scuba deaths, on rebreathers and otherwise.

I think this may be the excellent video and analysis you were referring too. The diver appears to be unconscious at the end, but had little awareness of the problem beforehand, or the need to bailout quickly when the CO2 breakthrough started. The cause of the breakthrough is well explained in the video.
CO2: The rebreather incident

 

[Cave Diver]

These points were brought up in another thread. I'm responding to them here since this has been an "informational" thread, rather than sidetracking the other thread.

Once again, my response is NOT speculation.

I know you're speaking generally. But just so no one makes any assumptions, Richard's Meg was not flooded.

That means that he bailed out off the loop?

It would seem to indicate the loop got closed at some point, but when...

Just as an FYI, Megs are pretty resistant to flooding. I recall during training when I forgot to close the loop during a drill and didn't realize it until after the drill was over. During the drill, the loop was floating above my head with the mouthpiece open.

A few tentative breaths upon recovery of the loop told me the breathing hoses were clear of water. Since we were about to exit anyway, I didnt clear the counterlungs.

On post-dive teardown, I found no water in the breathing hose, very little in the counter lungs, and nothing in the cannister other than the moisture associated with the normal operation of the CCR.

IF someone has a bailout valve attached to the mouthpiece AND they use it to bail to open circuit that SHOULD close the loop. One indication of such a switch would be a stream of bubbles just like normal OC...

 

[Cave Diver]

Tangential: can you hook up a BOV so that when you bail out you go onto the bailout bottle rather than the diluent when you turn the switch? Does any rebreather out there do this?

You could plumb the BOV into a bailout bottle, but the ones I've seen require a large bore LP hose. Which means you can't use a quick disconnect on it. Which means if you had to pass your bailout bottle to another diver, you would be unable to do so, because it is plumbed into your loop.

On the other hand, if you use a BOV plumbed into your dil tank, and for some reason you ended up with bad diluent, then switching to a BOV for a sanity breath may not be any help, as you'd still have contaminated gas.

At this point, I've chosen to stick with a bailout bottle and reg seperate from my dil, so I can pass it off, or have a totally different gas source, if needed.

Others have different views.

 

[PerroneFord]

Just as an FYI, Megs are pretty resistant to flooding. I recall during training when I forgot to close the loop during a drill and didn't realize it until after the drill was over. During the drill, the loop was floating above my head with the mouthpiece open.

In our pool sessions on the Meg, we intentionally removed the loop from our mouths, flung the loop around, and then cleared the loop. On disassembly, we were surprised to see how little water made it into the can. Very impressive test.

IF someone has a bailout valve attached to the mouthpiece AND they use it to bail to open circuit that SHOULD close the loop. One indication of such a switch would be a stream of bubbles just like normal OC...

Hence my question about when the loop got closed. Was it the diver who closed it bailing out, or was it closed on recovery to prevent the unit potentially flooding on exit. Answers I hope that will be forthcoming down the road.

 

[Cave Diver]

[hijack]TSandM, sorry about the hijack from your original post. However I felt this was a better place to address some of the questions, even though it's gone from a "scrubber" thread to a "general rebreather practice" thread.[/hijack]

 

[Damselfish]

The real question is, is TSandM thinking of trading in a horse for a rebreather?

 

[wedivebc]

Hence my question about when the loop got closed. Was it the diver who closed it bailing out, or was it closed on recovery to prevent the unit potentially flooding on exit. Answers I hope that will be forthcoming down the road.

I guess your meg instructor didn't cover the function os a BOV. Essentially once you switch to OC bailout on a BOV the loop is closed.