Question SCR Semi Closed Rebreather

[rjack321]

I guess I am a little shocked people (including the OP's dive shop) are recommending SCR or CCR at all - to extend the OP's dives when they're currently diving small tanks (63s, 80s, and 72s) on air.

Just doubling up a set of 72s (or a single 130) and using nitrox would be a huge potential increase in bottom times and plenty for the OP to work with for the next several years of busy parenting time. Come back to us in 2026+ if a larger cylinders and nitrox are now a newfound limitation.

 

[PfcAJ]

As I said in another post, I can't dive doubles due to limited range of motion in my right shoulder. I can't reach isolator valve with my right hand.

Can you reach it with your left hand?

Do you even need the function of an isolator? A straight crossbar would give you double the gas and two regs. If something breaks you’re no worse off than if something breaks and you’re diving a single tank.

 

[Wibble]

Can you reach it with your left hand?

Do you even need the function of an isolator? A straight crossbar would give you double the gas and two regs. If something breaks you’re no worse off than if something breaks and you’re diving a single tank.

Could also dive "independent doubles" which is two separate cylinders backmounted. If there's a manifold it will be turned off before diving. You then have two independent tanks which you breathe a bit from one, then switch over to the other and breathe that down and switch back.

As I said in another post, I can't dive doubles due to limited range of motion in my right shoulder. I can't reach isolator valve with my right hand.

With your limited RH shoulder motion your best route is to look at sidemount. This has two independent tanks either side of you where the valves, regulators and SPGs are in front of you and very easy to reach. In the water sidemount is unrivalled for stability, streamlining, safety and comfort. They're very easy to learn with few new skills to learn.

A pair of ali80s could be easily rented; if you're doing short dives off a boat you could take both of your tanks with you and come out with enough gas for another dive. If you've longer dives, you've double the gas -- but be aware of No-Decompression Limits.

 

[sprockjohnson]

As others have said, you would probably be best off sticking with sidemount and getting Nitrox certified. SCR has its place but not so much as a bottom time extender since you can't easily vary your mix on the fly to adjust your NDL.

I have a Draeger Dolphin which I picked up for $300 a few years ago and after a little bit of maintenance and component replacement it has proved to be an extremely easy and fun SCR for recreational profile dives. I also have two mCCR's and the SCR is truly an easy piece of equipment to dive with in comparison since it doesn't require o2 cells and you can simply set the flow, do the algebra to figure out the estimated inspired Fo2, and then plug that number into your nitrox computer and away you go. I converted mine to use a single sidemounted AL80 of whatever flavor of nitrox I blend up and with Nx40 and a flow rate of around 8lpm through a needle valve that one tank will give me about 2.5-3 hours of diving, which nicely coincides with the scrubber duration.

However- as easy and fun as the SCR is to dive it really is not any more beneficial than OC for your dive profile and it adds the extra cleaning/ sanitizing and the requirement for scrubber material. If you want a tool that you can grow with then get a CCR. If you are curious and want to get an SCR then go for it and it will be fun but just realize its limitations.

 

[GiraffeMarineSalvage]

Your FIO2 is always a known, if it looks off you flush by breathing out your nose and get instant verification . This simple fact makes SCR quit a bit safer than CCR. I fact, there really isn't much stopping you from using it with a failed O2 sensor.

Wouldn't a CCR Dil-flush (with PPO2 of Dil at that depth provided by petrel or other computer) basically get you the same verification, and even though it requires conscious thought a CCR can be operated in "SCR mode?"
(Asking purely to learn more on relative pros and cons of different systems, all I personally know is my Triton CCR...)

 

[Cio]

Wouldn't a CCR Dil-flush (with PPO2 of Dil at that depth provided by petrel or other computer) basically get you the same verification, and even though it requires conscious thought a CCR can be operated in "SCR mode?"
(Asking purely to learn more on relative pros and cons of different systems, all I personally know is my Triton CCR...)

You can dil flush a CCR but there is an oxygen circuit in there, so additional unknowns that make the process not as straight forward, like if there was excess o2 being added due to an oring or valve issue.

If you positively shut down/disconnect the O2 you are essentially a manual SCR , so the SCR rules apply. The safety side of here really applies to the fact that a CCR can add more O2 than you need and even with dil flushes, you need to put more thought into what you are doing. I’m no expert here, only pointing out a basic principal that most SCRs are less complex (no O2 system) and eliminate the chance of hyperoxia, (no O2 system ) which does give them an edge safety wise.

Any CCR requires cells to verify operation, SCR is operating in a realitivly known, FIO2, the cell is really just helping he computer calculate deco. Shallow depth is sketchy so RB80 divers use OC and GEM diver breath out their nose.

 

[Tracy]

I believe the 23psi/min would be on a LP72. I don't really dive the HP100 unless I doing a deep dive (below 90ft), and then I'll sling my LP50 for out of air emergencies. During 2022 I think I use the HP100 maybe on 2 dives, most of my 2022 diving was working as a DM with OW students I did a few deep dives with an AOW class.

Im thinking ill just stick with sidemount (forget SCR and CCR for now) and look to get Nitrox this year and take advantage of the Nitrox. Im in no rush to get into more complex setups I just want to maximize the time underwater when I am able to get away from my family and dive for fun.

Steel 72 and HP100 are the same tank volume.
His 23 psi/min would stand the same between those two.

 

[GiraffeMarineSalvage]

Cio, Thanks for taking the time to explain your thinking, I can see how eliminating pure O2 could potentially eliminate the risk of hyperoxia from a mechanical issue (such as an eCCR solenoid unlikely but possibly getting stuck open, which was part of what personally drew me to mCCR) but if SCR like the horizon is advocating accelerated deco hyperoxia from wrong mix is the same risk as OC accelerated deco? (Yes, that would be pure diver error vs mechanical error but just as fatal... Mechanical issue of loss of gas addition in SCR I'm assuming is pretty easily noticed by both lack of interment bubbles and that at a constant depth there would be a noticeable buoyancy change similar to when CCR diver has metabolised O2 from loop faster than leaky valve and needs to manually add more?).

I think we're in agreement shallow depth seems like the sketchiest place for all rebreathers, regardless of SCR/CCR. Perhaps the close second sketchiest would be too fast of a drop to depth (which takes more PPO2 awareness from CCR diver knowing required PPO2 to begin rapid decent, as opposed to SCR diver simply being able to be confident a rapid descent to a planned depth won't result in hyperopia due to mix and flow rate which have already chosen before even entering the water?).

 

[PfcAJ]

You can dil flush a CCR but there is an oxygen circuit in there, so additional unknowns that make the process not as straight forward, like if there was excess o2 being added due to an oring or valve issue.

If you positively shut down/disconnect the O2 you are essentially a manual SCR , so the SCR rules apply. The safety side of here really applies to the fact that a CCR can add more O2 than you need and even with dil flushes, you need to put more thought into what you are doing. I’m no expert here, only pointing out a basic principal that most SCRs are less complex (no O2 system) and eliminate the chance of hyperoxia, (no O2 system ) which does give them an edge safety wise.

Any CCR requires cells to verify operation, SCR is operating in a realitivly known, FIO2, the cell is really just helping he computer calculate deco. Shallow depth is sketchy so RB80 divers use OC and GEM diver breath out their nose.

The o2 drop is a real thing on the rb80. 32% is a no-go above 20ft. The minimum operating depth of hypoxic gases is surprisingly deep.

 

[Cio]

The o2 drop is a real thing on the rb80. 32% is a no-go above 20ft. The minimum operating depth of hypoxic gases is surprisingly deep.

That’s why in mentioned it at the bottom of my post, SCR is not without risk, and, shallow , on scr is arguably more dangerous. if you don’t have a rich gas and just breathe, you willl pass out, a CCR should be keeping you alive, even shallow.

Cio, Thanks for taking the time to explain your thinking, I can see how eliminating pure O2 could potentially eliminate the risk of hyperoxia from a mechanical issue (such as an eCCR solenoid unlikely but possibly getting stuck open, which was part of what personally drew me to mCCR) but if SCR like the horizon is advocating accelerated deco hyperoxia from wrong mix is the same risk as OC accelerated deco? (Yes, that would be pure diver error vs mechanical error but just as fatal... Mechanical issue of loss of gas addition in SCR I'm assuming is pretty easily noticed by both lack of interment bubbles and that at a constant depth there would be a noticeable buoyancy change similar to when CCR diver has metabolised O2 from loop faster than leaky valve and needs to manually add more?).

I think we're in agreement shallow depth seems like the sketchiest place for all rebreathers, regardless of SCR/CCR. Perhaps the close second sketchiest would be too fast of a drop to depth (which takes more PPO2 awareness from CCR diver knowing required PPO2 to begin rapid decent, as opposed to SCR diver simply being able to be confident a rapid descent to a planned depth won't result in hyperopia due to mix and flow rate which have already chosen before even entering the water?).

Hyperoxia, buy using the wrong gas, isn’t likely on the horizon. The horizon should able to maintain proper ppo2 even with a rich gas. I’m sure the electronics will go wild on you with warnings but that’s one thing an electronic scr has going for it. Within reason I’d imagine the horizon can up the ratio enough to compensate. Other SCR will off you quick with a rich gas.

No risk with rapid decent on SCR, I actually pitch head down, which on a GEM , will stop the purge, on ascent I pitch head up which increases the purge and avoids low ppo2. What’s nice is that, if I’m busy for a moment and shallow, I breathe every other breath out my nose, this keeps the loop fresh enough. I went to a Nerd since anything below 32 is not easy to keep up with above 20 feet.