Checklists in Rebreather Diving

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There is no difference between valves up and back mount. Other than you have a can between them that usually takes the beating not the valves. Valves up is a nonissue and just because it’s been done one way by everybody doesn’t mean valves up is bad. It makes everything stay the same as in bm so you have inherent muscle memory in case you need to do a valve drill. Again, if valves up was a danger I don’t think they would have chose that orientation to do a world record cave dive. It is a nonissue. Give it a try and see for yourself.
There is a difference between Open Circuit, with all the compromises that system imposes, and Closed Circuit which has a whole host of other compromises.

As mentioned, OC has valves up because of the weight of the cylinders. Commie divers invert them for ease of access to the valves, but have "dive butlers" to dress/undress them. Some people dive inverted doubles/twinset but this requires a protective stand around the valves to protect them when sitting on the bench.

A backmount CCR is completely different. For a start there's the low amount of oxygen and diluent which means 2 or 3 litre cylinders. The "box" includes a convenient scrubber unit to attach the cylinders (plus suit inflate, batteries, etc.) and normally has a stand as you don't want your fragile $£€10k+++ unit falling over. Most people can reach valves near their hips; not all people can contort themselves to twiddle knobs behind their head -- hence the endless valve drilling on twinsets/doubles.

As for the suggestion that I deliberately put the valves in the most vulnerable position -- behind my head -- is a crazy non-starter. The valves live near my backside just like all normal units. They're nicely tucked away from any danger; along with the hoses.


When diving on wrecks, you need to ensure that the top of the unit's as streamlined and tough as possible as you will -- not might, you will -- bash the wreck and any other fishing apparatus strewn over the wreck. Cave restrictions will be the same; bashing the top of the unit is not uncommon. Why are "cave shields" sold for JJs and other rebreathers?


Appreciate that we dive with what we have. Just making the point that some of the design decisions are common sense as CCR isn't OC.
 
They want to show that a partially failing scrubber does not cause direct problems, but later maybe yes.

With more than 1000 hours on ccr, yesterday I had my first strange feeling what must be a CO2 problem on a really shallow dive with my ccr. I was diving my ccr with new cells, the scubber has a CE limit of 3 hours and only 45-60 minutes were done on this scrubber a week before. Why do I write 45-60 minutes? That has to do with a teaching dive, we practised some skills like bo, descends, etc, so went up for a couple of times, down again, flushes, etc. So I did not feel anything strange during that dive, where the in wate time was 65 minutes, but the time under water some less. So if there was something wrong with this scrubber, I would probably not feel it because of breathing normal air between excersizes. Max depth was 15m.
So yesterday I took my ccr again with the idea of doing 2 shallow dives, or 1 longer one on ccr and the second on oc. This was a fun dive, and after half an hour we split up because I wanted to take some pictures of small snails. I also checked the celles as they were new, if they lookedTo take these pictures, I was diving in strange positions, but really shallow, between 5 and 8m depth. I felt some headache starting, but not that serious. I also had to pee. After about 75minutes I had to decided that I had to go out due to my peeing need. I swam back to the entrance point, a little bit faster than normal because I was not looking anymore for small critters. But it was not a hurry swim. Only at a depth of 4m. I only felt that I was a little bit out of breath, something that is not normal, even not with this a little bit faster swimming. But I still could hold my breath. I tested this because I was thinking that this could be CO2. Yes, I could swim back at surface, yes, I could do a bo, but then was already back to the entrance before I had to take action. When taking off my fins, I was still a little bit out of breath. For sure not normal. Back at the car I had only a slight headache left. Drank some water, and for the second dive I took my oc twinset.
All signs are there for a small CO2 breakthrough, nothing that you will see with a prebreathe, nothing that I felt within the first hour of the dive, it started around 75 minutes, when I was getting a little bit cold also. I checked all at home again, no orings broken, all orings in place, no leaks in the mouthpiece (I also had checked this before assembling), so I know my unit must be ok.
The tempstick still did show 5 out of 6 stripes.
If I was deeper or had been further away from the entance point I had to do a bo or swim back at surface. I don't have a CO2 sensor, but have tempstick, but haven ever seen a breakthrough on the tempstick (have seen this with another diver). On the dive with the twinset, I did not feel anything strange and also no headache appeared again.

So everything is checked, even a prebreathe, but nothing was wrong then. But if you want to test if people feel a partially failing scrubber, the test mentioned above is a good one. You see that most people won't feel it then. If a scrubber only filters out 99% of the CO2, it will take a while before you feel it as a diver. This is what this test shows. If the prebreathe would have taken 30 minutes, more would have gone from the loop with a partially failing scrubber.

So this week I will dive my unit again before going to trimix depths next week.

But I won't say a prebreathe is useless and a waiste of time. There are failures that can be found then. But not all.
That was a long story about NOT having a CO2 incident. I have had one before and what you describe is probably not it. Rapid breathing, anxiety, distraction, irritation were my experience. Headaches i have only had after an OC dive as a newby diver skip breathing.

I have also nearly exhausted my scrubber without breakthroughs due to unexpected high workload and that you can tell as the gas gets sort of a rough raspy feeling as you breath. Slowing down and flushing generally take care of this, followed by ending the dive.
 
And as a seasond cave diving expert, wibble would know.
What do you know from your quarry diving?

Again, address the content not attack the man.
 
There is a difference between Open Circuit, with all the compromises that system imposes, and Closed Circuit which has a whole host of other compromises.

As mentioned, OC has valves up because of the weight of the cylinders. Commie divers invert them for ease of access to the valves, but have "dive butlers" to dress/undress them. Some people dive inverted doubles/twinset but this requires a protective stand around the valves to protect them when sitting on the bench.

A backmount CCR is completely different. For a start there's the low amount of oxygen and diluent which means 2 or 3 litre cylinders. The "box" includes a convenient scrubber unit to attach the cylinders (plus suit inflate, batteries, etc.) and normally has a stand as you don't want your fragile $£€10k+++ unit falling over. Most people can reach valves near their hips; not all people can contort themselves to twiddle knobs behind their head -- hence the endless valve drilling on twinsets/doubles.

As for the suggestion that I deliberately put the valves in the most vulnerable position -- behind my head -- is a crazy non-starter. The valves live near my backside just like all normal units. They're nicely tucked away from any danger; along with the hoses.


When diving on wrecks, you need to ensure that the top of the unit's as streamlined and tough as possible as you will -- not might, you will -- bash the wreck and any other fishing apparatus strewn over the wreck. Cave restrictions will be the same; bashing the top of the unit is not uncommon. Why are "cave shields" sold for JJs and other rebreathers?


Appreciate that we dive with what we have. Just making the point that some of the design decisions are common sense as CCR isn't OC.
You don’t even cave dive.
 
You don’t even cave dive.
Is the statement wrong? Compare wrecks with caves? Maybe not as wrecks degrade over time, like mines. If so, please correct it.

But let's think about it. Do caves have something above you? Do caves have restrictions that squeeze you? Do you try to go through small 'openings'? Do you have rocks descending from the ceiling which you need to squeeze under?

How about addressing the content.
 
Is anyone running 7 litre / "LP50" onboard dilout cylinders with valves down? How does it dive & trim?

It's not surprising the DIR / GUE manifolded twinset CCR has the valves up. It might even trim the rig well. I'm not sure I would want two other additional valves right next to my oxygen valve, mixups could happen. But those upright valves are definitely asking for a beating. Maybe that's why they need all the extra valves :D

It probably makes sense for packing boat divers onto a high end boat (with a lift) since there's one less extra bailout cylinder to wrangle.

How much longer is the GUE CCR checklist? :facepalm:

CCR_IMG_3084JPG.jpg
 
https://www.shearwater.com/products/perdix-ai/

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