JJ CCR Scrubber Radial? vs Axial

[justinthedeeps]

For relaxed (≤1L/min O2 metabolism) dives it is quite possible to put up to six hours (or more) on a well packed standard JJ axial canister without signs or symptoms of CO2 buildup. No guarantee and not recommended for deep or serious diving that could encounter expected or unexpected exertion. The Sofnolime itself is rated at 150 L CO2/kg, which works out **in theory** to much longer than 3 hours. And this does evidently play out in real use.

A few people reportedly got hits on fresh canisters in under two hours, but additional conditions and circumstances of those incidents are not always clear...

Distributions of actual breakthrough times for real in-water diving are not known. All of the [non-military?] tests have been people pedaling vigorously on stationary bikes in dry labs at 2-3+ MET, which is significantly above most diving exertions.

The theory about depth/pressure/density reducing scrubber time is also probably not adequately measured or tested in compression chambers or actual diving tests either.

BOVs should increase the safety margin for earlier sanity breaths or CO2 hit surprises, but this isn't explicitly tested either.

Suppose you packed a bunch of canisters with specific but randomized/blinded known durations, and instruct in-water divers, on real dives, to flip onto OC gas when they suspect a breakthrough. Then measure:
a) how long it takes for them to decide and execute a bailout, and
b) what the ppm CO2 is when they do so.

Would we expect (a) and (b) to be the same when comparing BOV, vs. necklaced reg, vs. staged reg?

 

[rsingler]

Suppose you packed a bunch of canisters with specific but randomized/blinded known durations, and instruct in-water divers, on real dives, to flip onto OC gas when they suspect a breakthrough. Then measure:
a) how long it takes for them to decide and execute a bailout, and
b) what the ppm CO2 is when they do so.

@Dr Simon Mitchell has pointed out that fully 25% of divers may not sense their rising CO2, and would thus potentially have a misadventure during the experiment. Probably not something that any organization would approve, and something that might expose you to significant personal risk if you organized it.

While I've measured my own CO2 response here
Overshooting NDL and mandatory deco stops
and I applaud your desire to obtain more data than mfr testing gives us, I don't think there's any way to do that, that wouldn't expose the experimenter to liability for advocating longer-than-recommended run times.

I think we're stuck with anecdotes.

 

[justinthedeeps]

Worth cross-posting below, if you don't mind!

Generic reliable onboard CO2 monitors/alarms/vibrations for sub-2% levels are coming?

If [x]/760mmHg is the right math, it looks like a CO2 increase of ~1% CO2 per minute in the empty loop, notable anxiety and CO2 retention beginning at 2-3 minutes at 2-4% re-inhaled CO2, and the dreadful 4X+ hyperventilation soon follows.

Add some minutes and sneaky onsets for failing scrubbers? Might explain part of the variance for how long people stay on failing loops?

How many don't notice or acknowledge 2%-4% CO2? Does a slow onset make it harder to notice?

Distractions and fumbling with gear could make a difference, obviously.

I don't think many people admit how quickly panic can set in for unanticipated breathing issues. It's far less than the minute or two that we get on a planned breath hold.

Well, that was interesting! I guess I'm a "normal responder", lol! That was NOT fun.

I had one of my buddies monitor me while I hooked myself up to an ECG and anesthesia machine with no CO2 absorber in the circuit. I ran only maintenance oxygen (300 ml/min), and hooked myself up to an oximeter as well as ECG. As I rebreathed in an oxygen-filled circuit with no CO2 absorption, my inhaled CO2 rose as I produced it.
I'm a 69 y/o, 67 kg, 5'10" guy with an average physique, in good CV health.
I ran two tests: I lasted 5 minutes in the first run, and 7 minutes in the second before bailing out. The table below shows the second test. My baseline pulse had risen from 51 to 63, and I had a bit of a headache from the first run (which didn't start until about 3 min after I bailed the first time). It took about 8 min to get back to my baseline of 5.1 l/m RMV sitting in a chair between tests.
Table parameters are as follows:
-Time (minute)
-Inspired/exhaled CO2 (mm Hg)
-Minute ventilation in liters/min
-Pulse
-Anxiety on a scale of 0-5, where 5 would be almost panicking

Second run results:
0 - 0/39 - 5.1 - 63 - 0
1 - 6/42 - 6.3 - 69 - 0
2 - 15/42 - 7.1 - 65 - 0
3 - 31/45 - 7.9 - 66 - 1
4 - 35/47 - 9.4 - 67 - 2
5 - 45/51 - 13.9 - 67 - 3
6 - 53/55 - 18.5 - 70 - 3
7 - 55/58 - 22.5 - 74 - 4, with dizziness
Oxygen saturation 100% throughout.

Writing this about a half hour after the tests, I still have a mild headache, but otherwise feel fine.

Noteworthy (and just as they taught in med school) was that with a 50% increase in my end-tidal CO2 (39 to 58), my minute ventilation quadrupled! Now imagine that happening at depth due to exertion, with thick air and a poorly tuned regulator, which, coupled with your skip breathing practice, caused your CO2 retention in the first place. Can you stay calm enough to get yourself both out of difficulty and eliminate your hypercarbia? Do you have enough gas at depth to handle 4x your nominal requirement?

That was interesting!

 

[Dr Simon Mitchell]

How many don't notice or acknowledge 2%-4% CO2? Does a slow onset make it harder to notice?

Hello,

See Figure 4 in the paper attached. By the end of the 5 minute period on the rebreather circuit with no scrubber at all, the mean inspired CO2 was nearly 50 mmHg or around 6.5%. Despite that, 5/20 blinded subjects completed the 5 minute period and thought the circuit was fine. Unfortunately, a significant proportion of individuals are not reliably good at detecting rising inspired CO2 levels.

Simon M

 

[justinthedeeps]

Has there ever been a radial canister with a temperature stick system?

Works on Revo mini axial cans with fairly short path length

Which new units/configs will be fitting new versions of CO2 sensors?

Will End-tidal CO2 Monitoring Become a Reality in Diving?

By Ashley Stewart CO2 is a dangerous gas, which is why monitoring rebreather CO2 levels is important. VR Technology’s Kevin Gurr introduced the first gaseous CO2 sensor in the Sentinel in 2008, followed by AP Diving in 2014. Today, five manufacturers offer CO2 sensors, but all of them measure...

indepthmag.com

 

[justinthedeeps]

Hopefully not derailing from 'axial vs radial' question too much here

Thanks to the [author(s) above incl. @Dr Simon Mitchell ] cool studies like this and this, we have the community/industry standard of ~3 hours for CE standard canisters.


And these illustrate that suboptimal canister contents (or condition) could mean even less than 3 hours.

Central to these studies was accurate monitoring of CO2 coming out of the canisters.

Did subjects sense the up to ~2% breakthroughs here?

By rough estimate, from those figures it looks like it took ~10 minutes for CO2 to go from ~0.5% to 2.0%, a time window when an accurate inhale-side CO2 sensor could be useful.

Does a ~10-20% difference of theoretical axial vs. radial scrubber lifetime matter much, when variation between conditions, divers, exertion, packing, depth, temperature etc could all result in equal or higher variation?

 

[justinthedeeps]

Are there comparative tests of canister performances for extreme exertion?

VO2s in 50 to 80 mL/kg/min could result in ~3 to 5 L/min of CO2, double the 2L/min in the standard studies.

Does it cause episodic measurable breakthroughs on canisters that are still otherwise functional?
Does it become increasingly likely as scrubber duration increases? Does it depend on scrubber size or design?

 

[Wibble]

CO2/hypercapnia detection.

It appears to be quite difficult for a diver to realise they've got the onset of hypercapnia, probably because it's something every human being handles throughout every day: exerting oneself. We're so attuned to breathing we just don't think about it and it requires a conscious effort to even monitor one's own breathing rate.

So, on a CCR, we're working a bit harder than normal and may be stressed. This pushes up our breathing rate which we simply don't recognise this as it is "normal" for a human. Of course we know we're working hard, but the breathing didn't trigger the breathing hard alarm.

Hypercapnia sneaks up when scrubbers are exhausted, over breathed, or some breakthrough issue has occurred. The first signs of this will be elevated breathing, but, as humans, we won't normally notice it until it is enough for the conscious brain to recognise deeper breathing.

There are ways of detecting CO2 in the loop, such as CO2 sensors. However, these are known to be less than reliable due to the loop's humidity.

Would monitoring the velocity of gas flow through the loop help? Maybe this could be monitored by our computers and alert us when a smoothed threshold was reached? Admittedly false alarms caused by exertion could be triggered, but it's really there to alert the conscious brain that something's possibly not right?

Was looking at @rsingler's test above and the breathing rate seemed to be the bigger factor.

While I've measured my own CO2 response here
Overshooting NDL and mandatory deco stops

Measurements taken from the counter lungs?

 

[justinthedeeps]

High level CO2-related stuff (and more) in the RF4 consensus statements. Officially hot topics!

 

[justinthedeeps]

It seems like there is a division in thought between respective measurement challenges, and importance/relevance inspired vs. exhaled CO2 levels. Why not monitor both? Cost and complexity aside