CO2 buildup in full face masks on rebreathers?

[RedSeaDiver2]

I've always preferred using a rebreather with a full face mask, initially with the Kirby Morgan M48 then the M448 Mod 1, and in more recent times with the rebreather version of the Drager Panorama Nova.

All of those have a long mouthpiece fitted inside the mask - which potentially increases the WOB. AP sell an adaptor which allows their BOV to plug into an OTS Guardian FFM but it doesn't have a mouthpiece fitted and an Inspo with the BOV adaptor + Guardian OTS passes the EN 14313 standard (the EN standard for rebreathers) including the inspired CO2 tests - without a mouthpiece fitted. Lungfish now have their own version called the Alliance which is a Ocean Reef FFM modified for rebreather use and it too does not have a mouthpiece fitted, it is currently undergoing EN testing but their own testing has shown that it will pass the test apparently.

Firstly - has anyone used the Inspo with the Guardian FFM?

Secondly - what is everyone's thoughts on rebreathers + FFM without a mouthpiece?

Thirdly - @Dr Simon Mitchell - what are your thoughts on this?

 

[slimshady]

An adapter is available for the shrimp bov to an ots guardian as well. No mouthpiece though.

 

[Dr Simon Mitchell]

Hello,

The potential hazard associated with an FFM without a mouthpiece is that it can increase the dead space significantly, particularly if the internal seal on the oronasal mask within the FFM is not competent. If that were the case gas from the oronasal 'pocket' can communicate with the face 'pocket' meaning that some of the exhaled gas (containing CO2) can get exhaled into the face pocket and then re-inhaled from there on the next breath. It is analogous to a problem we reported a few months ago with full face snorkel masks - see attached paper or https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10735670/pdf/DHM-53-313.pdf

This is not such a problem with those Kirby Morgan masks because the face pocket is structurally separate from the oral pocket if I recall correctly, but the other masks you mention have an oro-nasal internal seal separating the oronasal pocket from the face pocket. This is fine so long as the seal is competent, but that might not be the case for everyone, especially people with beards.

Simon M

 

[RedSeaDiver2]

Hello,

The potential hazard associated with an FFM without a mouthpiece is that it can increase the dead space significantly, particularly if the internal seal on the oronasal mask within the FFM is not competent. If that were the case gas from the oronasal 'pocket' can communicate with the face 'pocket' meaning that some of the exhaled gas (containing CO2) can get exhaled into the face pocket and then re-inhaled from there on the next breath. It is analogous to a problem we reported a few months ago with full face snorkel masks - see attached paper or https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10735670/pdf/DHM-53-313.pdf

This is not such a problem with those Kirby Morgan masks because the face pocket is structurally separate from the oral pocket if I recall correctly, but the other masks you mention have an oro-nasal internal seal separating the oronasal pocket from the face pocket. This is fine so long as the seal is competent, but that might not be the case for everyone, especially people with beards.

Simon M

Many thanks Simon! I may actually found a solution last night by accident - sometime in the past Kirby Morgan changed the face seal on that m-48 Mod1 mask to something much more conventional - it still creates a seal that separates the mouth "compartment" from the rest of the mask but the face seal is now the more conventional oval that goes right around the face - and with a bit of luck it should seal on my face too! (the old style was a horrible design that didn't fit a lot of people - but they knew that before they released it as I was one of their original testers of the M48 Mod-1 mask and told them that it was bloody horrible and wouldn't fit many people - it seems as though enough people finally told them and they changed the seal more recently to a much better one that is going to fit a wider variety of faces!).

 

[Enevoldsen]

Do you have a picture of the modified m-48 (new face seal) Mine is terrible, but it it only a few years, so i would like to know if i have the old version or the new one…

 

[RedSeaDiver2]

Do you have a picture of the modified m-48 (new face seal) Mine is terrible, but it it only a few years, so i would like to know if i have the old version or the new one…

This is the Kirby Morgan bulletin relating to the change, and you can see the new skirt design in the picture. https://www.kirbymorgan.com/sites/default/files/Bulletin_13_2015.pdf The main physical difference is that the main face seal is now one continuous oval seal around the entire face - and it fits a LOT more people, and then there is the extra seal inside that fits across the bottom of the nose etc to seal the eyes and nose off from the rest of the mask containing the mouth. You do still need to use it with the mouthpiece that is provided with the Kirby Morgan / Hollis pods for the mask.

 

[Still Kicking]

Not rebreather specific, but I recently used a OTS Spectrum and I had a CO2 build up after some exertion. It took me a minute or two before it cleared.

I used to do a lot of work using an AGA on surface supply and never had any CO2 issues, even though I was often huffing and puffing working hard, carrying heavy loads etc.

In hindsight, I figured I should have just hit the purge on my reg and flushed the mask with fresh gas. I'm guessing that's the recommended procedure but it's not mentioned in the manual.

 

[Enevoldsen]

This is the Kirby Morgan bulletin relating to the change, and you can see the new skirt design in the picture. https://www.kirbymorgan.com/sites/default/files/Bulletin_13_2015.pdf The main physical difference is that the main face seal is now one continuous oval seal around the entire face - and it fits a LOT more people, and then there is the extra seal inside that fits across the bottom of the nose etc to seal the eyes and nose off from the rest of the mask containing the mouth. You do still need to use it with the mouthpiece that is provided with the Kirby Morgan / Hollis pods for the mask.

Thanks - I have the old mask then…Dammit….

 

[RedSeaDiver2]

Thanks - I have the old mask then…Dammit….

I kind of thought that might be the case when you said that yours is terrible... - that description fits the old style of mask skirt pretty well - I would add useless, poorly thought out, didn't fit many people and f&%king horrible as well.

 

[Enevoldsen]

I kind of thought that might be the case when you said that yours is terrible... - that description fits the old style of mask skirt pretty well - I would add useless, poorly thought out, didn't fit many people and f&%king horrible as well.

Yes - horrible…. Do you know if the skirt is avaiable as a spare part, so you could change it yourself?

Thanks