1 Counter lung vs 2?

[lostsheep]

no effect on gas velocity. Think about it, your lungs are moving gas at some rate based on your inhale/exhale cycle so the velocity of the gas is determined by your lungs. What does happen is what I had mentioned above where in a split lung design ~1/2 of the gas goes through the scrubber on the exhale cycle *being pushed through* and the other half goes through on the inhale cycle *being pulled through*. This means the scrubber is being used for half as long, but twice as often. This doubles dwell time for the gas that is in the scrubber during the "pause" between inhale/exhale cycles if you ever actually take a pause in your breathing cycle. Now because most people inhale and exhale at different speeds, half of that lung volume will be going through the scrubber at a different rate than the other which COULD increase dwell time, but it depends on if your inhale cycle is faster or slower and where the single counterlung is in the loop *most single lung units are on the exhale side, but the Sidewinder is in the middle and is a weird one because of that*. So if you have a really fast inhale and a really slow exhale then the dual lungs would conceivably have more dwell time at least on the exhale cycle and you get whatever gas is "paused" inside the scrubber which on a radial with a massive reaction front probably helps a little bit, but we are definitely in splitting hairs territory. Dual lungs work on most units because it's convenient, don't discount a single lung scrubber JUST because it is a single lung and automatically assume all dual lungs are going to be more efficient.
Also get out of your head that breathing rates are going to have any realistic probability of hypercapnia, if you get hypercapnic on a modern rebreather and you didn't use the scrubber beyond its rated capacity and didn't screw up the packing then you overbreathed your body. These things are tested at breathing rates far higher than most humans can sustain without passing out.
Just for reference MOST humans breathe somewhere around 20 lpm/0.7cfm and these were all tested to 80 or ~2.8cfm. You can't sustain that breathing rate for any length of time without passing out.
Taken from this link A Comparison of CE Test Data for two Closed Circuit Rebreathers – Joseph's Diving Log

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Like this post with one big caveat; velocity and volume flow rate are not synonymous.

Velocity can not be constant around the loop as the cross sectional area of the loop varies , e.g., hoses , counter lungs or open area of the scrubber.

 

[Akimbo]

Also get out of your head that breathing rates are going to have any realistic probability of hypercapnia, if you get hypercapnic on a modern rebreather and you didn't use the scrubber beyond its rated capacity and didn't screw up the packing then you overbreathed your body.

Inside the rig anyway. Shallow breathing can cause hypercapnia, just like it can and does on open circuit. It is a little worse on rebreathers because the dead air space in the mouthpiece is larger, especially with a BOV. But the same is true with FFMs and commercial helmets.

Back to your point. Hypercapnia due to canister and bag design hasn't been a problem in any production rebreathers I have seen since the European WWII-era single hose pure O2 designs were replaced.

I knew one retired SEAL that could over-breathe the Navy's Mark 16 and LARU rebreathers, but he had over a 9 Liter lung capacity. He complained that going into combat breathing mode would max out the rigs.

 

[Geo7]

no effect on gas velocity. Think about it, your lungs are moving gas at some rate based on your inhale/exhale cycle so the velocity of the gas is determined by your lungs. What does happen is what I had mentioned above where in a split lung design ~1/2 of the gas goes through the scrubber on the exhale cycle *being pushed through* and the other half goes through on the inhale cycle *being pulled through*. This means the scrubber is being used for half as long, but twice as often. This doubles dwell time for the gas that is in the scrubber during the "pause" between inhale/exhale cycles if you ever actually take a pause in your breathing cycle. Now because most people inhale and exhale at different speeds, half of that lung volume will be going through the scrubber at a different rate than the other which COULD increase dwell time, but it depends on if your inhale cycle is faster or slower and where the single counterlung is in the loop *most single lung units are on the exhale side, but the Sidewinder is in the middle and is a weird one because of that*. So if you have a really fast inhale and a really slow exhale then the dual lungs would conceivably have more dwell time at least on the exhale cycle and you get whatever gas is "paused" inside the scrubber which on a radial with a massive reaction front probably helps a little bit, but we are definitely in splitting hairs territory. Dual lungs work on most units because it's convenient, don't discount a single lung scrubber JUST because it is a single lung and automatically assume all dual lungs are going to be more efficient.
Also get out of your head that breathing rates are going to have any realistic probability of hypercapnia, if you get hypercapnic on a modern rebreather and you didn't use the scrubber beyond its rated capacity and didn't screw up the packing then you overbreathed your body. These things are tested at breathing rates far higher than most humans can sustain without passing out.
Just for reference MOST humans breathe somewhere around 20 lpm/0.7cfm and these were all tested to 80 or ~2.8cfm. You can't sustain that breathing rate for any length of time without passing out.
Taken from this link A Comparison of CE Test Data for two Closed Circuit Rebreathers – Joseph's Diving Log

View attachment 721715

Excellent, thanks! Indeed I have "overbreathed my body" on a Meg at 130 feet once where my breathing rate did not increase enough to match my work rate (bad habit learned in swimming and jogging). A flush and pause cured it immediately.

 

[Germie]

Are there any rebreathers made in the past 20 years that use 1 counterlung?

Yes, for sure. My Sidekick for example. And I like it, it is a really good sidemount ccr, small, versatile, etc. If you want to use 2 lungs in the sidekick design, the unit will become bigger. So that is not an option for the small sidekick.

 

[Akimbo]

Here is my view of the tradeoffs: An exhalation counterlung has value for water separation, and an inhalation bag has the slight advantage of isolating small volumes of caustic fluid, if you know about the leak. Split counterlungs also makes balancing the rig easier — for backmount and over-the-shoulder bags anyway.

The total volume of the bags sets the maximum tidal volume limit for the rig. One large exhalation backmount bag (before the canister) makes water drains harder to manage and is more limiting to arrange components close to your lungs.

The main downside I see of slit bags is the courigaged hose and fittings count goes up, which increases potential leaks and pieces to clean. It is hard to argue with one bag on a front mount if all you are doing is swimming a compass course (combat swimmers).