Question Running a rebreather on only dilluent

[DiveTucson]

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.

Because SCR on a vertical ascent like you would have in a an “air Dil no deco course” is dumb and not practical and honestly borderline dangerous. SCR is not designed for ascents.

It’s easy enough to carry enough bailout to make a direct ascent to the surface.

It very much has an important role in cave and other advanced CCR courses but in my opinion as zero benefit in initial level CCR courses.

 

[davehicks]

Scr mode is difficult? Unless you have a nearly hypoxic mix it’s really not hard at all. And I only dive mccr. I mean you’re not going to enjoy seeing pretty fishies or enjoy your way out of a wreck but I’ve run scr mode for extended periods on cave dives just for practice and it’s a breeze.

I think we are saying the same things. No, it is not difficult but it is not something that can fade into the background with all the loop flushing. It is a practice/recovery skill. Not a standard diving mode, which the OP seems to be enquiring about.

 

[justinthedeeps]

@Tom_Ivan I guess you meant "run a -closed circuit- rebreather." Since you describe a semi-closed rebreather.

You'd best have an automatic passive or variable flow input of the diluent in order to make this routinely safe. That's what the SCRs do.
The Mares Horizon I think even uses ppO2 sensors to drive a solenoid that adds fresh "diluent" at an appropriate flow rate (?)

Otherwise, sounds like another way to drown if anything else is going on (task loading, actual emergency etc)

Thanks to instructor @DiveTucson for pointing out the extreme danger of trying to do this especially as an [emergency] ascent, versus open circuit.

When prompted to do the SCR drill on an open water course (e.g. "loss of O2"), we had to pretend that it was a good idea, even though we all knew we should just bail out and stop d**ing around.

 

[Tracy]

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.

At the intro level, you have enough bailout to just bailout and end the dive. No reason to complicate what is already stressful to a new rebreather diver.

 

[mr_v]

I can see how SCR may complicate entry level courses. However, the procedure isn't dangerous if taught and practice correctly - at 1.6 atm you should be flushing and breathing O2. Advanced courses will also be much easier if you have SCR basics dialed in.

 

[rjack321]

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.

Why teach something that would never be used in a no-deco non-overhead MOD1 course? SCR is a terrible and dangerous choice for a MOD1 CCR diver in peril. Just bail and go home. Don't dick around with so many air loop flushes that you might as well be on OC anyway.

 

[rjack321]

I can see how SCR may complicate entry level courses. However, the procedure isn't dangerous if taught and practice correctly - at 1.6 atm you should be flushing and breathing O2. Advanced courses will also be much easier if you have SCR basics dialed in.

There's no O2 in SCR mode.
Also dumb for an MOD1 diver's ascent.

SCR has been a kludgey excuse for far too many divers to carry insufficient BO for decades. And rarely if ever been credited with a "save"

 

[rjack321]

Any paticular reason? I realise it wouldn't be ideal as far as PO2 etc is concirned

Because SCR with air is a ton of work on ascent to not go hypoxic and die. Unless you are using something like 50% in which case you can't take it very deep or you'll be below the MOD to bail to that gas.

SCR's have not been commercially viable for a couple decades. Both relatively recent SCR models were commercial flops. Too many compromises and limitations.

 

[Wibble]

One other non-electronic mechanical SCR is the RB80 “range extender” used with large gas sources such as twinsets so you can use all your experience with OC valves. It has a large scrubber and still needs bailout options should it flood, caustic or CO2 hit. It is a very specialist unit designed with long scrubber duration for long cave penetrations without restrictions.

Now superseded by the modified JJ with manifolded diluent or any large scrubber CCR — the dual Megladon used for the Pierce Resurgence in NZ

 

[Wibble]

SCR mode on a CCR gives options for a narrow use case; loosing your oxygen. In theory you should have sufficient bailouts for all dives, especially on sea dives.

Most CCR divers don’t like using their deep bailouts due to the cost of refilling. SCR mode could be an option to save an expensive rich helium mix by plumbing in (offboarding) your rich decompression gas, such as your 50% or richer if carried. Caveat here is the loop helium fraction will be diluted, but you’d be heading up anyway.

There is, I believe, a SCR mouthpiece available (Kiss??) which might be a viable solution to converting a CCR into a SCR if pure oxygen is not available. For example if diving at the arse end of the universe where only membrane compressors are available.

In theory you could use, say, 40% (max O2 from a membrane compressor) in a oxygen cylinder on any CCR where the oxygen solenoid would frequently fire and you would be constantly venting the loop to dump the excess loop gas. Effectively this is like the Mares Horizon or Hollis Explorer SCR which are viable in places where oxygen supplies are limited. These units aren’t built with large scrubbers and are intended as “recreational rebreathers”, giving partial CCR benefits.