Question Running a rebreather on only dilluent

[Wibble]

How do divers manage Boyancy with these units?

With constant gas injection the loop would become more and more boyant until you dump gas and suddenly become negative?

And that is the right question. CCR and SCR are very different to open circuit and take quite a lot of time to master.

Can only give you my experience which I reckon on it took me over 200 hours to feel comfortable with a CCR on deco ascents in the sea especially at 6m/20ft without holding on to anything. You need to know the unit and predict what will be happening in the near future. On SCR this means controlling the loop as it dumps out of an over pressure valve.

 

[nadwidny]

And possibly like 5+% enriched argon and whatever else gets stuck on the "oxygen" side of the membrane on a ~5:1 enrichment ratio (CO2, CO, VOC, ...)

Possible vs reality. I've done analysis on membrane generated O2. None those values have ever broken standards for Medical O2 here in Canada. I don't even remember them showing up in any appreciable numbers in the samples I've done.

 

[justinthedeeps]

Possible vs reality. I've done analysis on membrane generated O2. None those values have ever broken standards for Medical O2 here in Canada. I don't even remember them showing up in any appreciable numbers in the samples I've done.

Haven't measured it myself, have heard accounts of higher argon in some membrane or pressure swing systems though. Unless it was actually welding blends misused for diving

Does argon escape through the membrane just as well as nitrogen does? Or does it depend on membrane type? Any multi-membrane systems?

How about CO or CO2? There is more than just oxygen and nitrogen in the atmospheric input.

 

[rjack321]

Lots of good discussion here. I tend to agree teaching SCR mode in an entry level/no deco/no overhead situation unnecessarily complicates things because bailing out to Open Circuit is the best, quickest and safest option.

Being able to use SCR mode in a cave environment is critical and must be taught and the student must master this skill.

I dive double LP85s in the cave. Tanks are generally filled to around 3500 psi which is about 220 cubic feet of gas. (35*6.4) If I switch to SCR mode and use a four-breath cycle while diving SCR mode I turn those 220 cubic feet into 880 cubic feet, essentially a "Gas Extender".

Except it doesn't help you at all if the unit if flooded or you have a co2 hit. So it's not like SCR let's you somehow bring less BO.

Once again, there are virtually zero recorded "near misses" that SCR mode somehow kept from being a fatality. In a cave or otherwise. If someone knows a report of this, let's see it.

 

[Wibble]

However, SCR mode is another tool in the toolbox for sorting issues whilst you’re a long way to fresh air. Even if it’s a temporary state whilst sorting out other more pressing issues (stuck in a restriction with no oxygen, use the ADV/MAV until you can sort yourself out…)

CCR isn’t binary, it’s subtle with loads of options.

 

[Capt Jim Wyatt]

Except it doesn't help you at all if the unit if flooded or you have a co2 hit. So it's not like SCR let's you somehow bring less BO.

Very true. If the scrubber is flooded SCR mode obviously will not work. In the event of a controller failure then SCR mode is a great choice. whether relying on ideal or real gas laws.

Because you cannot predict the failure you cannot bring less BO under the assumption you can use SCR mode to make 220 FT3 into 880 FT3.

 

[kensuf]

Well that's f'ing dumb. I wonder what the rationale is. One of the biggest benefits of ccrs is time and techniques to get your butt out of a pickle.

I support removing SCR mode as a mandatory skill in MOD1.

My rationale is this: MOD1 is an OW course with light deco, the best option is a bailout to head up. SCR mode, especially when used as a gas extender in the event of a loss of monitoring, adds an increased risk of hypoxia. I would rather have my MOD1 students get really proficient at bailing out and proper planning than learning a "party trick" that we only do once or twice in class to check a box.

For CCR Cave, absolutely SCR mode is useful. MOD3, can be very useful as well, but the diver should still have enough OC gas as a bailout option. MOD1, no way Jose.

 

[doctormike]

I'm not a cave diver or MOD3 yet.

But am I correct in assuming that the pro-SCR contingent feels that its main use case would be no O2 or electronics as an alternative to bailout, while the anti-SCR contingent feels that it's use case would only be a double failure of O2/electronics AND bailout?

 

[kensuf]

I'm not a cave diver or MOD3 yet.

But am I correct in assuming that the pro-SCR contingent feels that its main use case would be no O2 or electronics as an alternative to bailout, while the anti-SCR contingent feels that it's use case would only be a double failure of O2/electronics AND bailout?

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).

 

[Underwater Tourist]

Once again, there are virtually zero recorded "near misses" that SCR mode somehow kept from being a fatality.

On top of that

There are cases when SCR mode contributed to a "near miss"

I know of a recent one in MX where a very experienced ccr diver had a near miss (hypoxia) exiting the dive in SCR instead of simply bailing out