Bad News From Santa Rosa Blue Hole

[KevinNM]

In general (there are exceptions using manufactured disposable scrubber units) it seems that most all rebreathers use essenially same granular soda lime products that are widely used in anesthesiology. Sofnolime by Molecular Products seems very commonly used.

 

[TotDoc]

Does everyone use the exact same scrubber? You tell me. I will say that air flow design is important in engines (Kawasaki is very good at this). Wouldn't the same science/engineering also apply to rebreathers? My first guess would say yes, but my first guess may be wrong.

@KevinNM summed it up well regarding scrubber material. Last I dove my SF2, my buddy was diving a meg. We both loaded up using the same scrubber material. There's not a lot of variability between different scrubber materials, and the scrubber material is absolutely shared from one rebreather type to another.

How air moves through the scrubber is a different story. Largely, this comes down to the basic scrubber design - axial vs radial. Some units have multiple scrubbers, but this does not necessarily mean that more scrubbing takes place...I think the differences between units are more based on work of breathing than on ability to scrub CO2. At some level, rebreather designs are a balancing act to maximize dwell time (contact of breathing gas with the scrubber material, enhancing CO2 removal) while limiting breathing resistance.

Work of breathing is dependent on a lot more than the scrubber design, though - every component in the breathing loop has the potential to alter the work of breathing. In fact, even the location of the components, particularly the location of the counterlungs, has a huge impact on the work of breathing. So does your position in the water column. That means that work of breathing -- even of a particular type of rebreather -- is not static, and can change throughout the dive. Work of breathing doesn't correlate directly with ability to scrub CO2, either. And scrubber efficiency and capacity slowly diminishes throughout a dive, as scrubber material is utilized, making that a dynamic process as well.

As you can see, it's hard to post any info on rebreather function without going off on wild tangents. There are a LOT of factors than can be discussed regarding how a rebreather is designed and the way that design impacts the actual function of the unit. This makes the whole topic difficult to approach when vague questions are asked, or when questions are asked that don't really seem appropriate to rebreather diving. For some, this makes rebreathers kind of like fight club...and we don't talk about fight club, unless you are already in fight club.

 

[wetb4igetinthewater]

@KevinNM summed it up well regarding scrubber material. Last I dove my SF2, my buddy was diving a meg. We both loaded up using the same scrubber material. There's not a lot of variability between different scrubber materials, and the scrubber material is absolutely shared from one rebreather type to another.

How air moves through the scrubber is a different story. Largely, this comes down to the basic scrubber design - axial vs radial. Some units have multiple scrubbers, but this does not necessarily mean that more scrubbing takes place...I think the differences between units are more based on work of breathing than on ability to scrub CO2. At some level, rebreather designs are a balancing act to maximize dwell time (contact of breathing gas with the scrubber material, enhancing CO2 removal) while limiting breathing resistance.

Work of breathing is dependent on a lot more than the scrubber design, though - every component in the breathing loop has the potential to alter the work of breathing. In fact, even the location of the components, particularly the location of the counterlungs, has a huge impact on the work of breathing. So does your position in the water column. That means that work of breathing -- even of a particular type of rebreather -- is not static, and can change throughout the dive. Work of breathing doesn't correlate directly with ability to scrub CO2, either. And scrubber efficiency and capacity slowly diminishes throughout a dive, as scrubber material is utilized, making that a dynamic process as well.

As you can see, it's hard to post any info on rebreather function without going off on wild tangents. There are a LOT of factors than can be discussed regarding how a rebreather is designed and the way that design impacts the actual function of the unit. This makes the whole topic difficult to approach when vague questions are asked, or when questions are asked that don't really seem appropriate to rebreather diving. For some, this makes rebreathers kind of like fight club...and we don't talk about fight club, unless you are already in fight club.

Thanks TotDoc. I have Bonazic's book on order. Your post makes me look forward to reading it even more. Thanks.

 

[JohnnyC]

There's nothing special about the Meg that makes it any more resistant to CO2 bypass or overbreathing the unit. Like all units it has several modes of failure that may or may not exist in other units.

Scrubber design is important in that the balancing act between dwell time (axial) and WOB (radial) is important when it comes to CO2 retention. Keep in mind that neither of these are set in stone, and a well-designed axial can outperform a poorly designed radial from a WOB standpoint, and a well-designed radial can outperform a poorly designed axial from a dwell time/CO2 removal standpoint.

The Meg does well with both their axial and radial designs. It is also very flood tolerant due to it's OTS counter lungs and, depending on can, large water trap in the base. Rebreathers that are not flood tolerant can suffer from WOB issues, caustic cocktail issues, etc. There are rebreathers like the SF2 that are very flood tolerant, and with the internal counterlung, quite suited to small spaces as there is no chance of restricting the CL movement during the breathing cycle as with external CL's. It also means that in certain positions, the WOB is higher than something like a Meg with OTSCL's. It's all a tradeoff.

The fact that the Meg was chosen to be the Mk28 rebreather speaks to excellence in design, but that doesn't mean it's the best or infallible. People have died on Megs. People have died on almost every rebreather manufactured. Getting a military contract is nice, but all it means is that it was the best representative to meet the specification. That includes design features, but also cost, ability to integrate into current supply chains, Berry compliance, availability, etc. The very best rebreather might cost 3 times what a Meg does and require sweat from a virgin's armpits to remove CO2. Point is, there may be a reason why the Meg was chosen, and it might not have anything to do with being the "best" rebreather.

As to chemical CO2 removal, everybody uses soda lime. Most don't want a lithium fire on their back. Again, what might be the "best" at removing CO2, may not be the best in terms of use in this environment. It's important to understand these issues.