Managing loop volume with a full face mask

[Dr Herring]

I’m not a CCR diver yet, I’ve only ever tried the JJ on a try dive with a half mask.

I know that some people use a full face mask with their CCR for the added safety in case of oxygen toxicity or hypoxia. However, I’ve yet to understand how excess loop volume is handled with a full face mask.

Those of you who use a FFM with your CCR, how do you went excess loop volume?

 

[RainPilot]

I use the km48 mask so I can exhale through my nose if needed.

My Poseidon has an OPV on the lung which is also useable.

Most FFM have a purge valve of some kind, if you can press that then you should be good.

Last resort close the DSV and exhale, you should have a vent somewhere.

 

[Dr Herring]

Thanks a lot for your reply. The KM48 seems like a good mask for CCR. Do you use a BOV or just remove the ‘pod’ for bailout? I’ve only had experience with the divator so far of the FFMs that I know of being used with CCR. But I think I’d prefer the 48 for redundancy, rather than adding another failure point in the form of a BOV.

 

[tbone1004]

@Dr Herring you should be using a BOV, when you need to bailout you don't want to have to remove the whole mask if you don't have to.
Why are you wanting the FFM in the first place?

 

[Dr Herring]

@Dr Herring you should be using a BOV, when you need to bailout you don't want to have to remove the whole mask if you don't have to.
Why are you wanting the FFM in the first place?

I’m not sure where I stand on the half mask/FFM on CCR issue yet. I like the comfort of diving with a FFM On OC(and feel comfortable bailing to a half mask), but don’t use it for tech diving. The draw on CCR would be being shielded from the cold water (doing a lot of cold water dives), and the potential benefits in case of oxtox or hypoxia. However, not sure yet that those benefits outweighs the complexity that a FFM on CCR adds as for failure points and bailing out. Another thing I’d worry about using a FFM on CCR is CO2 buildup.

@RainPilot Do you use a mouthpiece in your FFM to lessen that risk?

My thinking about the KM48 is that the bailout protocol would be to remove the pod, and use my bailout reg instead, but maybe there are better ways? What’s your bailout protocol? I wouldn’t dive a divator for example without a BOV.

 

[tbone1004]

@Dr Herring I'm not using a FFM with my rebreather because I don't need one. If I did, I would have a BOV on it though. Switch over to the BOV when you need to bailout, then if you need to get out of the mask you can later.

 

[DA Aquamaster]

@Dr Herring you should be using a BOV, when you need to bailout you don't want to have to remove the whole mask if you don't have to.
Why are you wanting the FFM in the first place?

I used the KM-48 for cold water technical diving. With the nose and face portion of the mask separate from the oral cavity and its removeable pod, you have a different set of options than with a traditional full face mask.

With a traditional full face mask and a BOV, you'd switch the BOV lever to bail out and that would work for most bailout scenarios although a loop that is flooded and caustic can put enough caustic into the BOV when you switch to make that less than ideal.

With the KM-48 and a DSV in the rebreather pod, you'd have to close the loop, then remove the pod with the DSV in it to switch to your bail out second stage. That bailout second stage could have a pod or not.

Alternatively, you could have the rebreather pod with a BOV installed and just switch like you would with any other BOV. The added advantage is that if you get caustic in the BOV side, you can remove the pod, flush it all out and then re-install the pod on the mask. It potentially gives you the best of both worlds.

And as noted above, on the KM-48 you can exhale though your nose and vent gas out of the loop hands free, or close the BOV and vent through the BOV, or tilt the de-watering valve in the mask and exhale to remove gas from the loop that way was well.

It's very flexible.

 

[rjack321]

The JJ doesn't really have an OPV that you can readily vent from. But the Meg and other units (Hammerhead, AP, XCCR) with OTS or TOS counterlungs all have functional OPVs that are easy to vent the loop from.
Are you heart set on the JJ (they are made nearby...) or possibly looking at other units?

 

[tbone1004]

@rjack321 managing loop volume from them though is a bit of a PITA with some contortions required. Much easier to use a BOV and manage it that way. Just switch over to OC mode and you can get the volume down there.

@DA Aquamaster BOV's don't solve everything, but they certainly expedite bailing out from the 3 H's and if I was going to use my CCR with my FFM, a BOV would be a 100% requirement for me regardless of a KM-48 or similar

 

[rjack321]

@rjack321 managing loop volume from them though is a bit of a PITA with some contortions required. Much easier to use a BOV and manage it that way. Just switch over to OC mode and you can get the volume down there.

When you close the loop because you have caustic crap in the loop you cant really vent it that way. Its not insurmountable but definitely a weakness for the JJ, Revo, some Kiss units, SF2, and others which either don't really have an OPV, its cranked down, or hard to access. That's why I asked if the OP was set on a JJ, as units with OTS and TOS counterlungs generally have accessible OPVs, open it up, roll the shoulder and dump like its a drysuit.