Overbreathing the scrubber

[Cave Diver]

I'm new to the rebreather stuff just beginning to learn about it now. Isn't the amount of CO2 you are exhaling proportional to the amount of O2 you are metabolizing. Theoretically couldn't you calculate the amount of CO2 being exhaled from the rate of O2 being injected back into the loop?

In theory, yes. The problem with theory is it doesn't always bear out in reality. Some people produce much more CO2 than others, for reasons which aren't entirely understood. Using calculations based off of O2 consumption might put you in the ballpark, but there would still be a lot of room for error.

Then you get into scrubber efficiency. You could have two identical units packed by different people that yield vastly different efficiencies.

Then... Well, you get the idea.

 

[Dsix36]

Most of us rebreather divers are awaiting the introduction of a sensor to monitor CO2 in the breathing loop. Technology is right around the corner, I hope!

 

[jimdiverman]

I'm new to the rebreather stuff just beginning to learn about it now. Isn't the amount of CO2 you are exhaling proportional to the amount of O2 you are metabolizing. Theoretically couldn't you calculate the amount of CO2 being exhaled from the rate of O2 being injected back into the loop?

Cheeky,

It is my understanding that the amount of O2 metabolized is roughly equivalent to the amount of CO2 expired. The problem is that even if the O2 is being added back into the loop at the rate that CO2 is being expired, if the CO2 is not being scrubbed, a dangerous condition (CO2 buildup) still exists in the loop. The idea is that we really need to have a direct measurement of CO2 in the loop to be sure of that level and initiate a purge of the loop and/or bailout to OC.

 

[wedivebc]

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 had a CO2 incident this year in a cave in Mexico. I was completely unaware of any problem. After the 3rd time my buddy asked me if I was OK he yelled at me to bail out. It was not until I got on open circuit bailout that I realized what had happened. The fresh gas made me feel like a fog had lifted immediately. I watched the video later of what behavior I had exhibited that caused him concern and I was amazed at how unstable I was drifting up and down and wandering away from the line. I felt no air hunger, headache of any classic symtoms yet my scubber had shown a lot of purple indication in the expended scrubber material. I had used medical grade sorb in Mexico because the good stuff is so expensive. Many lessons learned that day.

Oh yeah one more thing, Cave_Divers posts are spot-on

 

[Cave Diver]

I had a CO2 incident this year in a cave in Mexico. I was completely unaware of any problem. After the 3rd time my buddy asked me if I was OK he yelled at me to bail out. It was not until I got on open circuit bailout that I realized what had happened. The fresh gas made me feel like a fog had lifted immediately.

If that doesn't show the importance of good buddy skills, I don't know what does.

Questions: Was your buddy OC or CCR? If CCR, was he using the same sorb? Is there a thread about this "coulda been an incident" somewhere already?

 

[Meng_Tze]

...

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

All things being equal, you would probably be right.

All divers with RB's are taught to monitor PO2 at all times. If an issue arises with that they would logically address that. At least one hopes so. The only thing you do not always see or feel sneak up on you is Hypercapnia. But being astutely aware of increased breathing is a first indication something is wrong and should be checked.

 

[Kevua]

I actually asked this question to a few on a different forum and this was pretty much the replies that I got:

"not likely, however, quick changes in depth can affect your PPO2, so you need to watch that."

 

[wedivebc]

If that doesn't show the importance of good buddy skills, I don't know what does.

Questions: Was your buddy OC or CCR? If CCR, was he using the same sorb? Is there a thread about this "coulda been an incident" somewhere already?

I agree about the importance of a buddy. There was a near miss in Nanaimo on a CCR recently (Optima had mushroom valve failure) buddy was able to drag victim back to the boat where oxygen was administered. His lips and earlobes were blue and was in bad shape. He started to black out on descent and at 20ft realized something was wrong and had the presence of mind to hit his wing inflator.
My near miss was at Chac Mool, we were about 1000ft in exiting from the "fang" when I bailed. Both my buddies were on megs as well. He was using sodasorb I think which he packed from home. I had borrowed a radial scrubber which I may not have packed too well, also the sorb was quite large grained but like I said, lots of purple showing after the dive. Funny thing is I did not breathe the OC gas excessively nor did I have any confusion issues when I got on OC. My buddy said I calmly put my setpoint low (so gas doesn't vent from the RB I am no longer breathing off) and checked my OC SPG and swam home. 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.

 

[PerroneFord]

EXCELLENT thread, and goes directly to how important it is not only to have a good buddy, but one who understands what a diver in apparent distress looks like on a RB.

I was very worried about my CO2 production from some indicators I had seen previously, so I was very curious in class to see what the scrubber would tell. We breathed the HELL out of the unit to put it nearly into a worst case scenario. We did loop floods in the pool, etc. Basically everything you DON'T want to do on a RB dive. And we tested and tested and tested.

I had a much better feeling about CCR after that class, and a healthy respect for how quickly things can go sideways.

 

[Tienuts]

I had a CO2 incident this year in a cave in Mexico. I was completely unaware of any problem.

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.