Question Running a rebreather on only dilluent

[Tom_Ivan]

Continuing my barrage of rebreather questions.

Any reason you couldn't theoreticlly run a rebreather on dill only? I guess you'd have to accept your PO2 would flucuate and it would have to be a manual CCR as an automatic would keep trying to inject O2 to get to a setpoint that is potentailly impossible with your dill mix?

 

[scubayinzer412]

If iam Not wrong, that would be SCR Mode

 

[davehicks]

If iam Not wrong, that would be SCR Mode

Correct. You should do this in training. But not something you should plan to do on purpose.

 

[Tom_Ivan]

...But not something you should plan to do on purpose.

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

 

[Alekseolsen]

If you want an SCR purchase an SCR In SCR mode you breathe the loop content a few times (higher PO2 on DIL means you can breathe it more times), and then vent, refill DIK and repeat. VERY useful for getting your sorry ass out of a cave (uses a lot less bailout gas), but comes with the price of longer deco (given lower PO2), and would be very annoying as "default dive mode"

 

[davehicks]

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

It is a pain in the ass to do, not automomic, and not something you can do while enjoying other standard pleasures of diving. It's a fallback when other sh@t goes wrong, more dire than bailing to oc, which would be the first bailout option.

 

[rddvet]

It is a pain in the ass to do, not automomic, and not something you can do while enjoying other standard pleasures of diving. It's a fallback when other sh@t goes wrong, more dire than bailing to oc, which would be the first bailout option.

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.

 

[Tracy]

Correct. You should do this in training. But not something you should plan to do on purpose.

It has been removed from intro courses. It is now just taught in advanced courses like cave and trimix.

 

[doctormike]

Continuing my barrage of rebreather questions.

Any reason you couldn't theoreticlly run a rebreather on dill only? I guess you'd have to accept your PO2 would flucuate and it would have to be a manual CCR as an automatic would keep trying to inject O2 to get to a setpoint that is potentailly impossible with your dill mix?

Since you might not be familiar with the system you are asking about, as implied upthread, an SCR (semi-closed circut rebreather) is a real thing that you can buy. There are a few that I can think of that have been brought to market - the Mares Horizon and the now discontinued Hollis Explorer.

Fun Fact - Steven Tyler of Aerosmith trained on an Explorer!

The idea is that you have a single tank of nitrox which is vented into the loop. So unlike a CCR, which only vents the loop on ascent during normal operation, the SCR does continually bubble as the nitrox replenishes the gas in the loop as the oxygen is metabolized (not as much as open circuit).

And the SCR mode described above is an emergency CCR procedure - you use your CCR as an SCR. That would be done if you had no O2 or electronics - you fill the loop with your diluent (which you know has enough oxygen to support life), take a few breathes and then replace it, presumably before the PO2 drops too much. It gets you further than just breathing your diluent as an open circuit gas supply, since you aren't throwing away usable oxygen with every breath. Without sensors, you don't know the PO2 of what you are breathing, but by limiting yourself to a small number of breaths between gas exchange, you extend the supply while making it unlikely that you will become hypoxic.

 

[rddvet]

It has been removed from intro courses. It is now just taught in advanced courses like cave and trimix.

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.