Question CCR for recreational depths

[ND5342]

I will say there is one more bad thing about shallow CCR. Buoyancy is harder to maintain in the shallow side of diving. Open circuit you can make minor adjustments with lung volume. CCR that doesn't happen. In deeper water, a minor change in depth doesn't really affect the overall volume of air you are breathing (lungs+counterlungs). While in shallow waters a small change in depth has a larger change in gas volume.

It is common to do 20' deco stops on CCR where OC will do 10'. OC on 10' will conserve more gas, you can stretch out your deco bottle. 20' on CCR is easier to maintain the depth. Being CCR and on (nearly) 100% O2 you still get (nearly) the same deco at 20' as you would at 10'. The difference is 20' is a lot easier to hold than 10' on CCR.

Hence the RD1 designer's philosophy on getting new rebreather divers on an O2 rebreather first to sort out the buoyancy issue before moving on to PO2 management.

 

[Angelo Farina]

I worked at a NATO oceanographic lab in La Spezia 1968-1972. There was not a lot of diving at the lab, but what there was was done with steel tanks and compressed air. I never saw a CCR, pure-O2 or otherwise. The diving was done by Italian Navy folks who also were lab employees. The only other diving I saw in Italy during those times was on Christmas Eve in 1969 in Tellaro, when a new tradition was begun with scuba divers arriving on shore at midnight carrying a Christ child doll. Those divers also wore tanks, not rebreathers, I'm pretty certain.

Nice. The Nato Saclant center. Just beside Varignano, the base of Italian Comsubin.
At the end of the sixties the Aro was already being replaced by steel twin tanks. It was only used as a didactical tool.
The great diffusion of the ARO started with the first diving courses in 1950, and continued in the sixties, when progressive shift to ARA (air tanks) occured.
Memory has almost been lost of those glorious years.

 

[huwporter]

I guess that "different discussion" is what I was asking for. Please?

Without getting in to a lesson about how SCRs work - the key differences are that 1. SCRs do NOT maintain a constant PO2 at any depth; PO2 varies with depth and will be lower for a given drive gas when shallow compared to deep; and 2. the PO2 in the loop is always lower than the PO2 you would have if breathing the drive gas directly (i.e. on OC), and this PO2 drop is greater when shallow than deep. So on SCR you always start off with fewer moles of oxygen in the system when shallow than when deep. (Yes, I'm leaving out cases of doing a shallow drive gas switch to oxygen.)

 

[RainPilot]

I don't think those words mean what you think they mean. You're a single first stage failure away from unrecoverable issues.

Using dilout, if the O2 first stage fails you bail out to the dil and ascend. If the dilout 1st stage fails, then you ascend as normal since you shouldn't need to add gas anywhere (assuming you weren't horribly negative at the time of failure)

Note, I am NOT saying that only having a single OC usable tank in the water is a great idea at all but it is do-able. Personally, even for shallow (20-30ft reef work) I will always sling an OC stage for bailout or, more often, having something for the OC buddies to breathe if they need it. Also, my 3L dil tank doesn't go terribly far in terms of a slow ascent from any depth, so a slung AL40 gives me the warm and fuzzies.

Training is a bigger cost. You need a full week's training for MOD1 and must spend time practising. Lots of time practising.

Depends on the unit. Air dil, NDL training is usually about 120-240 min confined water and then around 240 min open water. Generally I will do 4 OW dives of about 90 minutes or so, after a full pool day, can be done on a long weekend IF the student is squared away, reference to your point below:

Rebreather's are not for everyone. This is the attitude thing. You must take care of the rebreather; you must do the post dive maintenance; you must do thorough cleaning; you must store it correctly; you must use checklists for assembly; you must use checklists when you kit up and turn the unit on; you must be pedantic and that final check before you jump in. Preparation will be hours in the early days. Rebreathers are most definitely not for people who throw their kit into a heap and leave it.

This. This is the biggest limiting factor for who should dive CCR. If someone has the right attitude toward consistent procedures and a borderline OCD aversion to skipping steps "just because" then there should be no reason they cant dive a CCR in any depth for any diving they already do.

 

[L13]

Without getting in to a lesson about how SCRs work - the key differences are that 1. SCRs do NOT maintain a constant PO2 at any depth; PO2 varies with depth and will be lower for a given drive gas when shallow compared to deep; and 2. the PO2 in the loop is always lower than the PO2 you would have if breathing the drive gas directly (i.e. on OC), and this PO2 drop is greater when shallow than deep. So on SCR you always start off with fewer moles of oxygen in the system when shallow than when deep. (Yes, I'm leaving out cases of doing a shallow drive gas switch to oxygen.)

Arrgghh, I didn't catch the the switch from CCR to SCR. I understand the difference(I think). I thought you were referring to additional details on CCR. But thanks for responding to my request.

 

[davehicks]

??? No it doesn't - there is no difference between a failed solenoid (on CCR) and a failed/blocked orifice (on MCCR) in this scenario.

Except that a blocked orifice is very unlikely. On an eCCR with electronics, wiring, batteries, etc there are far more likely failure points.

 

[Wibble]

Except that a blocked orifice is very unlikely. On an eCCR with electronics, wiring, batteries, etc there are far more likely failure points.

Also a manual CCR pilot would be far more switched on about monitoring their unit during a dive as there’s no benevolent computer watching over them. A full electronic CCR enables the pilot to be more relaxed about their PPO2 monitoring.

 

[happy-diver]

Fairdinkum blah blah blah blah blah life will be over soon anyway

Ha ha ha ha ha ha ha see what happens divers go out diving and suddenly they come back pilots
and on landing, somehow overshoot the runway and end up with their head stuck in a cows bum

 

[huwporter]

Except that a blocked orifice is very unlikely. On an eCCR with electronics, wiring, batteries, etc there are far more likely failure points.

Also a manual CCR pilot would be far more switched on about monitoring their unit during a dive as there’s no benevolent computer watching over them. A full electronic CCR enables the pilot to be more relaxed about their PPO2 monitoring.

All that may be true, but the discussion is (or at least was) about whether that MCCR is more dangerous when shallow than that same MCCR is when deep, not whether MCCR or ECCR is more dangerous overall.

 

[billgraham]

It may only be slightly less than common, but 10/50 is used by quite a few divers in my experience due to it being, heliar, and rather inconsequential with regard to helium usage to begin with. I don't know the protocol for diving with that mix near the surface or at the surface aside from always have a breathable gas in your loop.

But the tradeoff to a CCR is that wreck sites change frequently on a charter where you may expect 150+' dives weather could change and be on a 80' wreck or less. I bet some bring back up Dil but not all as the philosophy of "the gas maker is on your back" is pretty prevalent.

It's easy to mix but I began using dil as close to normoxic as I could achieve for the dive after a number of accidents occurred at or near the surface. I was doing the tie in a lot of the time which involved a surface swim and I didn't like that idea with the 10/50.