Question Running a rebreather on only dilluent

[Gone for diving]

Can anyone tell me what is the difference that something gets labeled a gas extender vs SCR.

As far as I can understand they are identical...... maybe one doesn't have po2 sensors????
Or one dumps more gas???

 

[Wibble]

They're the same thing.

SCRs come in manual and electronic versions.

Manual ones leak at a constant rate and inject based upon pressure. Electronic ones monitor PPO2 and should be more frugal with gas consumption.

 

[Capt Jim Wyatt]

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".

 

[nadwidny]

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.

I've been to the arse end of the universe. Membrane compressors were putting out ~90% give or take a percent depending on the day.

 

[grantctobin]

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)

Just for teaching sake of other readers, much much closer to 205 than to 220.

 

[Capt Jim Wyatt]

Just for teaching sake of other readers, much much closer to 205 than to 220.

Hijacking the thread, but:

LP 85 rated pressure is 2640 psi.

(85/2640)*2*100=6.439 cubic feet per 100 psi for doubles.

35*6.439=225 cubic feet

 

[grantctobin]

Hijacking the thread, but:

LP 85 rated pressure is 2640 psi.

(85/2640)*2*100=6.439 cubic feet per 100 psi for doubles.

35*6.439=225 cubic feet

=\

A LP85 at 2640psi isn’t 85cf regardless of which gas you’re using. If said gas was linearly compressible and ideal, it would be 82.5cf. As pressure increases, that gas is even less compressible. 82.5/zFactor 32% at 3500psi is 1.045 = 105cf per bottle. Lower if there’s helium in the mix.

 

[mr_v]

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"

As with many things scuba, the answer is "it depends." I am sure nuances get lost in SB discussions. A competent instructor should cover multiple SCR scenarios including using the best gas for SCR and finding O2 options for when you're getting shallow. I learned SCR early and I am glad I did.

I am with Jim on SCR in caves - it is nice to almost 4x your gas.

 

[Tom_Ivan]

Manual ones leak at a constant rate and inject based upon pressure. Electronic ones monitor PPO2 and should be more frugal with gas consumption.

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?

 

[justinthedeeps]

I've been to the arse end of the universe. Membrane compressors were putting out ~90% give or take a percent depending on the day.

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, ...)