mCCR 1 Class - Carlsbad, CA March 2010

[TSandM]

Nick, I was doing a bit of reading in Mel Clark's book the other night, and she made the comment that back-mounted counterlungs (which it sounds like are what the MC90 has?) have very different work of breathing in different positions. Did you guys play with that at all?

 

[lamont]

OC bailout is calculated on having enough OC between two divers for one of them to bail based on the premise that it is highly unlikely that both breathers will have total failures. (I know a little alpinist for UTD )

a lot of the non-DIR rebreather community feels that team bailout is also unsafe.

this seems very much like violating the rule of thirds in a cave, which can you do many times safely, and build complacency, before you find out why it's a bad idea.

 

[Jeff_S]

Hi Lynne,

I played with that quite a bit (on purpose - really). The most surprising part was rolling on my back, horizontal looking up. With OC the regs are terrible in that position, but the rebreather was easy to breathe, no real difference. Same at 100', I couldn't tell the difference from breathing in the shallows.

Jeff

 

[limeyx]

Nick, I was doing a bit of reading in Mel Clark's book the other night, and she made the comment that back-mounted counterlungs (which it sounds like are what the MC90 has?) have very different work of breathing in different positions. Did you guys play with that at all?

I have only the one RB experience so with that said

#1: I dont think they are technically "back mount" -- they are around the head like a nice airplane pillow

#2 the WOB is just simply not an issue here, way better than the Apeks bailout reg. No issues at all.

 

[limeyx]

Hi Lynne,

I played with that quite a bit (on purpose - really). The most surprising part was rolling on my back, horizontal looking up. With OC the regs are terrible in that position, but the rebreather was easy to breathe, no real difference. Same at 100', I couldn't tell the difference from breathing in the shallows.

Jeff

Exactly the same here. Breathed awesome in every position I tried it

 

[Blackwood]

We each carry enough gas to get 2 divers to the next breathable source of gas in the event of a single "major failure"

How do you plan maximum time at depth as it relates to available gas?

Do you simply never incur more open circuit deco (PO₂ decreasing as you ascend) than you could do if you lost the O2?

 

[Jeff_S]

How do you plan maximum time at depth as it relates to available gas?

Do you simply never incur more open circuit deco (PO₂ decreasing as you ascend) than you could do if you lost the O2?

Hi Marc,

Think of it more about taking the gas you need for the dive. If all you carry is O2, then you need enough back gas to get to that bottle if the rebreather fails. The MC90 is so named because it has two 45 cft tanks - 90 cft total. So just like rock bottom calculations on open circuit, if you don't have enough backgas to get to 20'/6m, then you can add a 70'/21m bottle or convert the unit to an MC160 by going to 80's instead of the 45's. In The Pit they dove it with 120's.

If all goes well, you won't use any of it. But it has to be there in case you bail out. You don't want to be in a position of not having enough gas to ascend on OC, and everyone carries their own bailout - no sharing bailout among the team.

And just like in OC, we plan for one major failure, so that would be either the CC or the OC. If nothing fails, then you're carrying two completely independent breathing systems. If everything fails (like you break an isolator and loose all you gas, your buddy has OC bailout gas for you.

Does that answer your question?

Jeff

 

[Blackwood]

To be fair, this is more of a general closed-circuit question than a MC90 one.

Does that answer your question?
Jeff

Not really, unless you are saying you bring an O2 bottle in addition to the one plugged into the unit.

One of the strengths/pros I often hear about CCR is that you have more-or-less infinite gas. Something goes wrong at depth? No worries, you can fix it because you use an insignificant amount of dil and consume very little oxygen.

To me, though, it seems that your bottom time is still functionally limited by how much deco you can do if the breather goes belly up.

Say you take a CCR to 100 feet. Do you have enough 'bottom gas' to stay for 3 hours? Yah, sure probably longer (depending on the volume of O2 you bring). What happens if after that 3 hours you have a catastrophic failure (burst disc, however unlikely) which drains the oxygen bottle at depth. Now you're bailing out to OC, and don't have nearly enough backgas for that decompression obligation.

So I guess this:

Think of it more about taking the gas you need for the dive.

probably answers my question, it just makes me wonder about that "infinite gas" strength (which may just be conventional 'wisdom' and not reflect the views of actual CCR divers).

I could very easily be missing something, since I know next to nothing about CCR. Appreciate any insight.

 

[rjack321]

Say you take a CCR to 100 feet. Do you have enough 'bottom gas' to stay for 3 hours? Yah, sure probably longer (depending on the volume of O2 you bring). What happens if after that 3 hours you have a catastrophic failure (burst disc, however unlikely) which drains the oxygen bottle at depth. Now you're bailing out to OC, and don't have nearly enough backgas for that decompression obligation.

If you were doing a 3hr 100ft dive (on OC) you would have enough oxygen to deco out. Or enough backgas to do the deco (not likely). So you'd be adding another deco gas, a support diver, or safety gas. Same thing for the MC90. In fact 90cf of dil/backgas ain't gonna cut it either. You need a backup to the O2 bottle and additional OC gas to even get up to 20ft in this case. It can take various forms obviously, an AL80 of EAN50 would be a good start.

 

[Jeff_S]

probably answers my question, it just makes me wonder about that "infinite gas" strength (which may just be conventional 'wisdom' and not reflect the views of actual CCR divers).

Hey Marc,

You still have to deco, so it's the same as loading up with 130's and a stage, but only taking a 70' bottle. With the MC90 (or 160, or whatever), your dive is still limited by the amount of open circuit deco gas you can carry. And, of course, some of that deco gas can be back gas, depending on how you plan your dive.

If all goes well, you won't use the OC gas, but it's there if you have a problem with the rebreather.

js