"Alternate Breath" Rebreather?

[FritzCat66]

I was having fun discussing various dive technologies with a buddy the other day and I got to thinking about this idea, wondered if you rebreather guys had ever heard of such a thing, or why it wouldn't work.

The idea is basically a one-breath rebreather. Based on the fact that a typical exhaled breath still contains plenty of O2, certainly enough for one more breath, I imagine a "rebreather" with a single nitrox cylinder, a double hose, semi-automatic two-way valves on both intake and exhaust, and a counterlung.

Every two breaths would constitute one cycle:

1) Upon initial inhalation, intake from nitrox cylinder.
2) Upon initial exhalation, exhaust to counterlung.
3) Upon second inhalation, intake from counterlung.
4) Upon second exhalation, exhaust to water.​

In other words, rebreathe just one breath, then take a fresh hit of nitrox.

Wouldn't have much advantage over plain open circuit, just an increase in gas efficiency. But seems like that increase could potentially be significant. Buoyancy would be kinda wonky, as it would vary every other breath. But it would also be very simple - no sorb, no diluent, etc., and seems like it could also be reasonably compact.

Of course the above procedure is simplified and there would need to be lots of checks and balances to make sure, for example, that the counterlung gets completely flushed each time, that the valve doesn't get stuck so the diver is unwittingly rebreathing one breath over and over, etc. One obvious problem would be if the diver descended much between the first and second breaths, there wouldn't be sufficient volume in the counterlung for another breath - going up would be the opposite, but seems like that could be solved fairly simply with an OPV.

But I was wondering if anything like this has ever been tried, and/or why it couldn't possibly work.

 

[munselln8]

Interesting thought. A test prototype could be as simple as taking a delfated balloon with you and check out the "wonkyness" of bouyancy, also descending and ascending after having breathed in the balloon to guage the increase/decrease of available air. Also, a small balloon would ensure sufficient pressure to ensure evacuation of the "second" breath to avoid accumulation of excess air but might burst on ascent (not to mention increasing bouyancy). Weird, but interesting....have fun
Good diving...

 

[Cave Diver]

I was having fun discussing various dive technologies with a buddy the other day and I got to thinking about this idea, wondered if you rebreather guys had ever heard of such a thing, or why it wouldn't work.

The idea is basically a one-breath rebreather. Based on the fact that a typical exhaled breath still contains plenty of O2, certainly enough for one more breath, I imagine a "rebreather" with a single nitrox cylinder, a double hose, semi-automatic two-way valves on both intake and exhaust, and a counterlung.

Every two breaths would constitute one cycle:

1) Upon initial inhalation, intake from nitrox cylinder.
2) Upon initial exhalation, exhaust to counterlung.
3) Upon second inhalation, intake from counterlung.
4) Upon second exhalation, exhaust to water.​

In other words, rebreathe just one breath, then take a fresh hit of nitrox.

Wouldn't have much advantage over plain open circuit, just an increase in gas efficiency. But seems like that increase could potentially be significant. Buoyancy would be kinda wonky, as it would vary every other breath. But it would also be very simple - no sorb, no diluent, etc., and seems like it could also be reasonably compact.

Of course the above procedure is simplified and there would need to be lots of checks and balances to make sure, for example, that the counterlung gets completely flushed each time, that the valve doesn't get stuck so the diver is unwittingly rebreathing one breath over and over, etc. One obvious problem would be if the diver descended much between the first and second breaths, there wouldn't be sufficient volume in the counterlung for another breath - going up would be the opposite, but seems like that could be solved fairly simply with an OPV.

But I was wondering if anything like this has ever been tried, and/or why it couldn't possibly work.

What you're describing is very similar to SCR (semi closed rebreather)

A SCR, such as the popular Drager Dolphin is a Nitrox Rebreather. A typical 50% Nitrox supply will flow into the loop at a preset rate of 7.4 liters per minute. This rate is approximately 20% greater than the oxygen you will be metabolizing, so you will be venting this 20%, which is why it’s called Semi Closed Circuit. Your flow rate will need to be increased, as you lower the Nitrox supply gas percentage. This is accomplished with preset orifices. Your Nitrox gas oxygen percentage, that you will be breathing is dependent on the supply Oxygen percentage, the flow rate, and your metabolic rate. This is called your Inspired Oxygen percentage, FiO2, and can be anywhere from 5% to 25% less than your supply oxygen percentage.
A SCR for all practical purposes, is like diving a Nitrox gas, is self maintaining, and other than monitoring, does not require any action from the diver or electronics.

Semi Closed Rebreather (SCR) and Closed Circuit Rebreather (CCR)

Semi-closed, on the other hand, implies that most of the gas is reused and only a small amount is exhausted to the surroundings. This allows the use of Nitrox mixtures for the system.

The semi-closed circuit rebreather (SCR) can be manufactured without the use of batteries or electronic components in a very reliable system. It's only moving parts are the check valves in the mouthpiece and the demand valve override for deep inhalations. It can be simple, useful, and provide many of the benefits divers seek in rebreathers. With the use of Nitrox mixtures, the benefits of EAN use are retained with the added benefits of a properly designed SCR which includes:

* Quiet, reduced bubble operation
* Extended bottom time (due to efficient use of gas)
* Lighter, more comfortable diving systems
* All the physiological benefits of EAN (Nitrox)

http://www.thescubaclub.com/rebreathers.html

 

[Amphiprion]

Without a scrubber I would worry about CO2 buildup.

 

[FritzCat66]

I'm aware of SCR's; they do look to be close to what I'm describing except:

1) looks like they use scrubbers - this would eliminate that, along with the potential for cocktail, etc.

2) since it seems the SCR just puts all the gas in the same "pool", there's still risk of having insufficient O2 if the injection rate isn't right, whereas with this it's either-or: one breath from the tank, one from the counterlung - the gas "pools" are kept separate, so no injection rate to monitor.

But yeah, looking at the schematics for some SCR's, we're talking pretty close. Like OC, you would still have to set the mix for max depth, unlike CCR (although like an SCR your average PPO2 on this system would be somewhat less than what's in your cylinder).

Without a scrubber I would worry about CO2 buildup.

Yes, because every other breath you're getting a little CO2, so if that last dose hasn't cleared, you're steadily increasing your internal CO2. I wonder how many breaths on clean gas (no CO2) it takes to clear one breath's worth of exhaled CO2? If it were a fixed number, the system could be adjusted to compensate, with proportional loss in efficiency: that is, if it took three breaths to clear the CO2 from one rebreathed breath, then you could set the ratio 3:1, three hits of pure nitrox, one hit from the counterlung.

Or add a scrubber, which brings us back to an SCR.

Ah well, already too complicated! Silly idea to begin with.

 

[Crush]

Without a scrubber I would worry about CO2 buildup.

+1

Your breathing reflex is triggered by increased CO2 levels, not O2. In fact, your body's way of dealing with low O2 levels is to make you pass out. On land this is OK as the change in the elevation of your head relative to your heart (when you fall to the ground) will help maintain oxygenation of your brain.

Back to CO2. Without a CO2 scrubber you will be increasing the amount of CO2 in your body with this type of rebreather. While this may give you a headache it will also trigger your breathing reflex. I suspect that you will find that your respiration rate will increase markedly, offsetting in whole or in part the benefit of your rebreather. That being said, I have never tried this - it is just my hypothesis.

 

[Amphiprion]

Might want to speak with an expert regarding CO2 retention. I understand what you are saying about a ratio, but I'm not sure if that is the case or not. If you are mechanically inclined you could build such a device and couch dive it. If you don't get a CO2 hit, you could try walking or jogging around the block with it to see about increased CO2 levels.

Don't even think about trying it out in a pool unless you have a buddy trained in CPR watching you the whole time.

 

[netmage]

So... empty counterlung with an ADV affixed to a gas supply...

Breathe in - bottoms out the ADV - fills 'lung'.
Breathe Out, Breathe In, Breathe Out thru Nose...
Breathe in - bottoming out the ADV - filling w/ fresh gas
Breathe Out, Breathe In, Breathe Out thru Nose...

-repeat-

Is that what you mean?

 

[FritzCat66]

A little different from what I was originally thinking since your technique basically always has the diver sucking off the lung instead of alternating between the lung and gas, and forcing the diver to exhaust to atmosphere instead of via some automatic system.

Not that it matters, because it still suffers the same CO2 buildup problem.

If CO2 levels in the loop could be quickly and reliably monitored and automatically flushed & refilled with more clean gas it might work.

But still, now we're getting very close to a standard semi-closed design, just without a scrubber. Not much point.

 

[netmage]

A little different from what I was originally thinking since your technique basically always has the diver sucking off the lung instead of alternating between the lung and gas, and forcing the diver to exhaust to atmosphere instead of via some automatic system.

Not that it matters, because it still suffers the same CO2 buildup problem.

If CO2 levels in the loop could be quickly and reliably monitored and automatically flushed & refilled with more clean gas it might work.

But still, now we're getting very close to a standard semi-closed design, just without a scrubber. Not much point.

An ADV = Automatic Diluent Valve, is basically a regulator hooked up to the counterlung... it IS the fresh gas supply...

Whatever CO2 is remaining inside the 'counterlung' is immediately diluted once the ADV fires and fills it back up with fresh gas...

The concern would be dead air space, such as found in Full Face Mask rigs that become eddys for CO2 to build up...

What I described is essentially what we refer to as going 'Open Loop' as a bailout option in CCR diving...