1/2 sized standard units (e.g. JJ-CCR, P2, etc)

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One of the things that would concern me about a smaller scrubber is bed depth and dwell time. These things could become problematic at deeper depths.
The ISC pathfinder with its 4.5-5lb scrubber has a 200ft depth rating. It's been used deeper but I'm pretty sure those are warmer waters than the 4c test conditions. The diameter is definitely smaller which means the bed depth is less since its a radial.

I don't think the 4lb radial mini scrubber in the Meg has a depth rating but it has the same bed depth as the larger 5.5 and 8lb radial, its just very short.
 
A Revo uses two 1.35kg/3lb scrubbers. These are monitored using their temperature monitoring (RMS) which can properly assess when the reaction 'front' reaches the second scrubber. It's surprising how long you can dive on a single scrubber; I've never yet managed to dive long enough to start consuming the second one. As a result you throw the first used scrubber and swap the old second scrubber to be the new 'first' scrubber and refill the consumed scrubber to become the new second scrubber.

I've (in my limited time on the Revo) only ever replaced one 1.35/3lb scrubber at a time for every dive -- max dive time circa 3h. With that low cost there's no point in pushing it.

One of the principles of having two scrubbers in series is reliability. Should the first scrubber break through or be consumed, the second one's there to save your dive/life. The monitoring will pick this up and alert you. The key to this is mixing the gas between the scrubbers; this means the CO2 from a breakthough would be mixed across the whole face of the second scrubber and not concentrated on a small area.

One wonders with good monitoring if those scrubbers could be reduced in size. However, reducing size would probably mean a higher WOB (if narrower) or a shorter scrubber life (if shallower). Maybe that's why there's no half sized rebreathers.
 
A Revo uses two 1.35kg/3lb scrubbers. These are monitored using their temperature monitoring (RMS) which can properly assess when the reaction 'front' reaches the second scrubber. It's surprising how long you can dive on a single scrubber; I've never yet managed to dive long enough to start consuming the second one. As a result you throw the first used scrubber and swap the old second scrubber to be the new 'first' scrubber and refill the consumed scrubber to become the new second scrubber.

I've (in my limited time on the Revo) only ever replaced one 1.35/3lb scrubber at a time for every dive -- max dive time circa 3h. With that low cost there's no point in pushing it.

One of the principles of having two scrubbers in series is reliability. Should the first scrubber break through or be consumed, the second one's there to save your dive/life. The monitoring will pick this up and alert you. The key to this is mixing the gas between the scrubbers; this means the CO2 from a breakthough would be mixed across the whole face of the second scrubber and not concentrated on a small area.

One wonders with good monitoring if those scrubbers could be reduced in size. However, reducing size would probably mean a higher WOB (if narrower) or a shorter scrubber life (if shallower). Maybe that's why there's no half sized rebreathers.
Calling RMS good is a bit of a stretch isn't it?
 
Calling RMS good is a bit of a stretch isn't it?

I think it works very well. Am I missing something?

The main point is unequivocally knowing when the reaction front has reached the second scrubber; prior to then you can rely on the electronics; thereafter it's the 45min limit.
 
I forgot the Drager Ray. No longer made but backmounted in a jacket BC like harness with over the shoulder CLs. The scrubber is about a 3.5lb axial. Rated to 40m (due to limited mass of sorb and dwell time) and about 90mins IIRC. It's an SCR but fairly easy to convert to mCCR if you can find one.
 
Calling RMS good is a bit of a stretch isn't it?

Yours hasn't failed yet, give it a few more dives.

The rEvo RMS is great and works very well for tracking scrubber usage. Having dived a rEvo with RMS for 8 years i have had more than my fair share of hardware failures, in my experience these have been resolved.
 
I guess the idea is if you're going lite, it's in situations where the diving is super easy and bailing out is perfectly fine; you might even just finish your whole dive that way without ever "aborting." Warm tropical shore and boat diving come to mind.

I'm not sure why all of the recent answers to that are chest and side mount units? Unless that's the only way to shave weight?
 

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