Question Running a rebreather on only dilluent

[doctormike]

I'm in the anti-SCR for MOD1 divers contingent.

In the scenario where you lose O2, but retain monitoring, SCR mode may be acceptable in certain environments (ex. cave) because you still have the ability to monitor the PO2 and can flush when it drops too low.

In the scenario where you lose monitoring, but retain O2, you better shut your O2 down and just go to a flush/breathing cycle. Trying to add O2 that you've metabolized based on the volume of gas in your loop may seem like a great idea when written down in a textbook, but it's not super realistic in the real world.

In the scenario where you lose monitoring AND O2, a flush/breathing cycle may be acceptable, but again, only in environments where it makes sense (ex. cave).

I am too. But just trying to learn more from the experienced divers here.

The only reason you would HAVE to do it would be bailout failure, right? "Failure" meaning not only complete loss of bailout, but also being in a situation where you don't have enough contingency bailout.

I guess what I would like to learn from the pro-SCR crowd is about the situations where it would be preferable to just bailing out, apart from unlikely scenarios with multiple failures. I would put losing monitoring AND O2 in that category of multiple failures, since I can't think of one thing that would cause you to lose both but still have a functional loop and scrubber.

If there was no risk to SCR, I guess it always helps to have more time. But it sounds like (1) there is some risk of hypoxia, and (2) the option may have encouraged some divers to skimp on bailout reserves.

Thanks for the discussion!

 

[Capt Jim Wyatt]

For BO and diluent when I dive to depths of no more than 120 feet, I carry EAN30 which is banked at CCDS. There is less risk of hypoxia using EAN30 than air while in SCR mode.

Divers who plan SCR as a contingency need to practice SCR with a functioning controller so as to have a good idea of how quickly the loop goes hypoxic. This should be practiced at various depths. I have found 10 breath cycles to not go hypoxic at 100 ffw with EAN30 diluent.

Has anyone else here practiced this to determine when the loop goes hypoxic? Whether using air, EAN or Tmx?

I say again that I too am a proponent of changing the standard to not teaching SCR mode in an entry level rebreather course which is not done in an overhead environment, has limited or no decompression, and where the student carries adequate bailout.

 

[inquis]

In the scenario where you lose monitoring, but retain O2, you better shut your O2 down and just go to a flush/breathing cycle.

Would you have the same thought for an mCCR (still in a suitable environment, like a cave)?

 

[Capt Jim Wyatt]

Would you have the same thought for an mCCR (still in a suitable environment, like a cave)?

Yes. Shutdown O2 and SCR on diluent/BO gas.

 

[inquis]

Yes. Shutdown O2 and SCR on diluent/BO gas.

On mCCR, the fixed O2 addition rate (though still less than metabolism) seems like it adds margin and reduces buoyancy loss (still using the same flush cadence, of course). Is the desire for O2 shutoff due to the possibility of IP increase that couldn't be detected since both monitors are dead? Or something else?

 

[rjack321]

For BO and diluent when I dive to depths of no more than 120 feet, I carry EAN30 which is banked at CCDS. There is less risk of hypoxia using EAN30 than air while in SCR mode.

Divers who plan SCR as a contingency need to practice SCR with a functioning controller so as to have a good idea of how quickly the loop goes hypoxic. This should be practiced at various depths. I have found 10 breath cycles to not go hypoxic at 100 ffw with EAN30 diluent.

Has anyone else here practiced this to determine when the loop goes hypoxic? Whether using air, EAN or Tmx?

I say again that I too am a proponent of changing the standard to not teaching SCR mode in an entry level rebreather course which is not done in an overhead environment, has limited or no decompression, and where the student carries adequate bailout.

I practiced SCR in JB for a bit during a class with a functioning controller. Also in Ginnie, although I had my eCCR set to the lowest 0.5 ppO2 setting kinda as a parachute. In part because using the 0.21 surface setting is not a manufacturer approved thing underwater. I wouldn't describe it as inherently dangerous with a working monitor and just no O2.

My personal diving is nothing like those caves and sawtooth all over - SCR is totally useless for me to exit those caves.

It's much easier to just not run out of O2 in the first place by actually having a legit gauge on your O2 instead of not having a gauge or using button gauges which you can't double check during a dive.

 

[rjack321]

On mCCR, the fixed O2 addition rate (though still less than metabolism) seems like it adds margin and reduces buoyancy loss (still using the same flush cadence, of course). Is the desire for O2 shutoff due to the possibility of IP increase that couldn't be detected since both monitors are dead? Or something else?

It's because you can guestimate your ppO2 drop with 3 variables, depth, SCR gas, and breaths.

You can't estimate your ppO2 drop worth a damn with 4 variables, depth, SCR gas, breaths, and O2 flow.

 

[kensuf]

Would you have the same thought for an mCCR (still in a suitable environment, like a cave)?

Absolutely. You have no way to know if your metabolic use of oxygen is exceeding the flow of oxygen without monitoring.

Rule #1 of rebreathers: ALWAYS KNOW YOUR PO2!

 

[inquis]

It's because you can guestimate your ppO2 drop with 3 variables, depth, SCR gas, and breaths.

You can't estimate your ppO2 drop worth a damn with 4 variables, depth, SCR gas, breaths, and O2 flow.

I was wondering about still using the 3-variable guess & flush cadence, but keeping the (Edit: below-metabolism) O2 addition to reduce the buoyancy impact of being on SCR.

EDIT: I now see the potential to exceed metabolism makes it a non-starter.

 

[Capt Jim Wyatt]

Rule #1 of rebreathers: ALWAYS KNOW YOUR PO2!

Absolutely!!