Did Learning Rebreather Improve you SAC rate?

[J-Vo]

@JackZwykle

Well, your OC SAC matters because it's the basis for bailout planning. I'm not aware of anyone trying to calculate there CCR SAC.

Have you adjusted your bailout planning SAC based on your new increased OC SAC? I don't plan to change mine based on my lower numbers, but it does make me more confident in my calculations.

 

[Pavao]

No changes for me, been about 7 years, but that’s likely due to the fact I still did a ton of OC dives after CCR training, can’t beat the simplicity of OC and that’s what I do when it makes more sense.

 

[JackZwykle]

@JackZwykle

Well, your OC SAC matters because it's the basis for bailout planning. I'm not aware of anyone trying to calculate there CCR SAC.

Have you adjusted your bailout planning SAC based on your new increased OC SAC? I don't plan to change mine based on my lower numbers, but it does make me more confident in my calculations.

I plan for those high numbers plus 50%, but I recognise that I may be doing an overkill. Still - no one died from finishing the dive with too much gas.

And I wanted to point out that higher acquired SAC comes from CCR breathing patterns which I recognise as positive for safety and comfort on CCR. Just a side effect.

 

[1fspeed]

Definitely got much, MUCH worse (I don't want to admit how much exactly ).
I need to make a conscious effort to limit the SAC when say training BO or things get silly.

However - I am happy about it and I make sure my reflexes are "SAC high" on CCR.
From speaking to divers and instructors ten times more knowledgeable than I will even be - they think a lot of unexplained fatal accidents where very experienced divers, in perfect conditions and without any apparent reason just "freaked out" - were caused by CO2 induced "dark narc".
To the best of my knowledge CO2 retainer induced dark narc hasn't been researched much, but two things verifiably help with reducing CO2 risks - mix density and... breathing pattern.
When kept within reason of course - breathing deep and a lot is simply safer (and more relaxed, no feeling of restriction).
You can put pure heliox in your dil but it won't help you much if you breathe too slow and/or shallow to remove all CO2 in time.
There's another aspect of it - where sofno starts losing efficiency due to loss of temperature in very cold waters if hot gas and CO2 is not delivered to it fast enough. Again more pondering than being able to point to research papers.

From personal experience I can recall a very weird dive which we had just after certification. Water temp was 6-8C, outside temp -2C (after a walk to the water the scrubber was probably quite cold even with prebreathe), we were chilling shallow around 15m hanging by a wall and relaxing.
First and last time in my life I suddenly had a hard to explain bout of stress and feeling of danger, without any reason at all.
It's hard to explain, not full "out of control" panic, but really, really bad "knot in the stomach, hair raising on the neck" dread.
I also clearly recall the feeling of "needing to get out of here".
I managed to control myself and it passed. In retrospective my best guess (after just being crazy ) is that when relaxing I started breathing around 12-14 SAC and maybe shallow as it was true bliss to finally relax on CCR, which cooled the sofno past the efficiency limit, didn't vent the lungs enough and caused blood CO2 buildup.
From other situations I know I'm a retainer sadly.
Stress panting removed it when dread hit me and allowed me to control the situation.
Luckily there was no deco, but if it continued at big deph/deco I dread what could have happened if it started building up more and more.

Apologies for a long rambling, but I thought that other responses while completely valid missed the point that on CCR the OC obsession with lowering the SAC is not only pointless, but may kill you.

Breathe away compadres!

Had a very similar experience early this summer in Lake Superior. Water temp was 5°C, during a slow, poking around kinda dive. Scrubber had one previous 60min dive on it (3hr scrubber) when I suddenly felt a sense of panic and just wanted to get the hell out of the water. Bailed to OC and felt fine in a matter of minutes. Post dive inspection of the unit revealed nothing...

I'm am new to CCR and at that time had maybe 50 hours on my unit. Wondered if I went too far in keeping loop volume to a minimum...

 

[JackZwykle]

Had a very similar experience early this summer in Lake Superior. Water temp was 5°C, during a slow, poking around kinda dive. Scrubber had one previous 60min dive on it (3hr scrubber) when I suddenly felt a sense of panic and just wanted to get the hell out of the water. Bailed to OC and felt fine in a matter of minutes. Post dive inspection of the unit revealed nothing...

I'm am new to CCR and at that time had maybe 50 hours on my unit. Wondered if I went too far in keeping loop volume to a minimum...

I will try to answer with my limited knowledge, please correct me if I'm wrong.
1. I think the scrubber was "overloaded" or you were breathing too shallow. Not spent scrubber.
2. At these temperatures it's best to always have fresh, not reused.
3. You can't really go too far with minimal loop - ADV will kick in. But you can definitely breathe too shallow. My wife discovered that she naturally breathes fast but shallow and it created a problem on our early MOD1. She had to start breathing much deeper or she felt suffocating in 5 mins. When you breathe shallow you essentially keep shaking the dead space gas and your lungs exchange may drop to almost nonexistent. The CO2 rich gas just keeps jumping up and down in your airways, mouth and BOV/DSV.
4. At those temperatures and with preused scrubber you run the risk of overloading the material, which would be compounded by slow / shallow breathing. It is where the path of gas it has to react is shortened because of spent layer and the temperature lowers the chemical reactions times to such level that some CO2 can "fly past" and back into the loop. If you add slow breathing to it you remove the two sources of heat helping the efficiency - breath warmth (lesser) and CO2 for reaction (bigger). Even colder and slower scrubber, even more reaction path shortening - and you start cooking with CO2.

I find the ubiquitous "3 hour scrubber" label somewhat dangerous for beginner CCR divers.
It's a 3kg scrubber. NOT 3 hours scrubber. It has been tested for a very specific depth, temperature and CO2 input, a combination that you are often VERY far from and in both ways.
Talking to experienced divers and with my limited experience - a 3kg axial can last you 7-8 hours in tropical waters, shallow and zero movement, while I just provided my story about a suspected cold and overloaded scrubber, you did yours and I've heard a very experienced diver having to bail out after 45 mins (very cold dive, working super hard against the current, deep on trimix) on his fresh scrubber. He realised what happened and managed to get back on the loop. (Yes you can recover most of overloaded scrubber situations - if it's overloaded and not a bypass).

I plan to do some homework with assistance in the near future and actually experience oncoming hypercapnia symptoms sitting in the room.
Listening to the recent Dr Harris' lecture it looks like a very good idea.


Stay safe and deep breaths!

 

[inquis]

you run the risk of overloading the material, which would be compounded by slow / shallow breathing

Actually the opposite. Overloading the scrubber is a danger with fast, hard breathing -- as found when working hard. The dwell time is greatly reduced in such cases. Slow, shallow breathing promotes hypercapnia via CO2 retention in the lungs. Density can also contribute to retention, and trimix is useful as a means to combat that. (It's not just for narcosis.)

 

[1fspeed]

I will try to answer with my limited knowledge, please correct me if I'm wrong.
1. I think the scrubber was "overloaded" or you were breathing too shallow. Not spent scrubber.
2. At these temperatures it's best to always have fresh, not reused.
3. You can't really go too far with minimal loop - ADV will kick in. But you can definitely breathe too shallow. My wife discovered that she naturally breathes fast but shallow and it created a problem on our early MOD1. She had to start breathing much deeper or she felt suffocating in 5 mins. When you breathe shallow you essentially keep shaking the dead space gas and your lungs exchange may drop to almost nonexistent. The CO2 rich gas just keeps jumping up and down in your airways, mouth and BOV/DSV.
4. At those temperatures and with preused scrubber you run the risk of overloading the material, which would be compounded by slow / shallow breathing. It is where the path of gas it has to react is shortened because of spent layer and the temperature lowers the chemical reactions times to such level that some CO2 can "fly past" and back into the loop. If you add slow breathing to it you remove the two sources of heat helping the efficiency - breath warmth (lesser) and CO2 for reaction (bigger). Even colder and slower scrubber, even more reaction path shortening - and you start cooking with CO2.

I find the ubiquitous "3 hour scrubber" label somewhat dangerous for beginner CCR divers.
It's a 3kg scrubber. NOT 3 hours scrubber. It has been tested for a very specific depth, temperature and CO2 input, a combination that you are often VERY far from and in both ways.
Talking to experienced divers and with my limited experience - a 3kg axial can last you 7-8 hours in tropical waters, shallow and zero movement, while I just provided my story about a suspected cold and overloaded scrubber, you did yours and I've heard a very experienced diver having to bail out after 45 mins (very cold dive, working super hard against the current, deep on trimix) on his fresh scrubber. He realised what happened and managed to get back on the loop. (Yes you can recover most of overloaded scrubber situations - if it's overloaded and not a bypass).

I plan to do some homework with assistance in the near future and actually experience oncoming hypercapnia symptoms sitting in the room.
Listening to the recent Dr Harris' lecture it looks like a very good idea.


Stay safe and deep breaths!

Thanks for the detailed reply.
1) Agree. scrubber was not spent.
2) Good advice for sure.
3) My ADV is off most of the time. Shore diver, gradual descents. When I do use it, I turn it off at depth.

Taking deeper breathes does seem like good advice, particular in the cold.

So many variables when it comes to scrubber duration... From what I understand, even resting CO2 production can vary dramatically from person to person.

A "hypercapnia party" . Honestly, I've considered doing the same thing but so far have chickened out. If you do that experiment I'd be very interested in how it goes. Although, I also understand that peoples tolerance/reaction to hypercapnia also varies dramatically. Argh! Too many variables.

Thanks again

 

[JackZwykle]

Actually the opposite. Overloading the scrubber is a danger with fast, hard breathing -- as found when working hard. The dwell time is greatly reduced in such cases. Slow, shallow breathing promotes hypercapnia via CO2 retention in the lungs. Density can also contribute to retention, and trimix is useful as a means to combat that. (It's not just for narcosis.)

Yes, of course. I meant that shallow, limited breathing cools the scrubber, after which it either overloads by itself (spent layer, too cold) or you overload it when you suddenly need to make an effort.
Of course hard breathing can also overload the scrubber (when sprinting), but I've found that if it has been hot to start with and mix density is below 5g/l (which I always try to keep) - I haven't managed to overload it with effort itself.

But yes, a fair correction, the main issue is CO2 retention in the lungs (and in consequence in the blood).

 

[AJ]

So many variables when it comes to scrubber duration... From what I understand, even resting CO2 production can vary dramatically from person to person.

A "hypercapnia party" . Honestly, I've considered doing the same thing but so far have chickened out. If you do that experiment I'd be very interested in how it goes. Although, I also understand that peoples tolerance/reaction to hypercapnia also varies dramatically. Argh! Too many variables.

I learned the hard way too. Scrubber life is theoretical, not an absolute truth. When doing a demanding dive, always use a fresh scrubber. Also replace scrubber content after a month max. Even when you're not using it, the filling deteriorates anyway.

What I do these days is:
1. Replace before a demanding dive (deco/cave/mine), does not matter if or how much the scrubber is used;
2. Replace after 2 dives of max. 1 hour per dive;
3. Replace after on month.

It's a lesson every ccr diver learns sooner or later I guess. You can only hope the lesson will be learned in a benign enviroment.

 

[JackZwykle]

I learned the hard way too. Scrubber life is theoretical, not an absolute truth. When doing a demanding dive, always use a fresh scrubber. Also replace scrubber content after a month max. Even when you're not using it, the filling deteriorates anyway.

What I do these days is:
1. Replace before a demanding dive (deco/cave/mine), does not matter if or how much the scrubber is used;
2. Replace after 2 dives of max. 1 hour per dive;
3. Replace after on month.

It's a lesson every ccr diver learns sooner or later I guess. You can only hope the lesson will be learned in a benign enviroment.

Interesting, me and my wife use the exact same rules.
We've calculated that typically what's MAYBE left in the scrubber is a beer or two per person in terms of money... just not worth our lives.

Anyway, we went quite off the subject of SAC rate, so I shut up.