Crazy question from the family table

[JohnnyC]

You can manually run a rebreather like an O2 rebreather, but nobody I know is going to deco out at 1.6 for that long.

On dives of that length you’re really taxing your O2 exposure. Hence why most people doing loooooong rebreather dives are running lower PO2’s.

Remember, 1.6 only allows for 45 minutes single exposure. And it’s not like your time spent at lower PO2 gets ignored. So when you look st 5 hours of deco, running your rebreather at 1.6 isn’t the greatest idea.

 

[taimen]

Yes, I knew the oxygen exposure thing. But in this (hypothetical) case you would blow oxygen exposure limits with that OC deco at 1.6 too. Wouldn't you?

 

[broncobowsher]

I would consider that anyone doing that epic of a dive OC would be taking air breaks during deco. Yea, you are pretty much hitting the pause button on deco, but you are trying to avoid getting burned out on such high O2. A little beyond the quick question while sitting around the table with some family while having a few drinks and only dealing with the basics.

 

[Superlyte27]

Honestly, the 1.6 at 20' on CCR depends on how cold or hungry or dehydrated you might become.

A long time ago, my 6 hour planned dive became 10 hours when I got lost in a cave for more than 30 minutes at 300'+

I hadn't planned to be in the water that long. I had undergarments and battery heat for a 6 hour dive, not a 10 hour dive.

I accelerated my deco I believe the risk from cold, dehydration and exhaustion was greater than the risk of oxtox. But, I have a lot of high CNS dives both commercially and recreationally and am pretty confident in my personal physiology.

 

[PfcAJ]

You can manually run a rebreather like an O2 rebreather, but nobody I know is going to deco out at 1.6 for that long.

On dives of that length you’re really taxing your O2 exposure. Hence why most people doing loooooong rebreather dives are running lower PO2’s.

Remember, 1.6 only allows for 45 minutes single exposure. And it’s not like your time spent at lower PO2 gets ignored. So when you look st 5 hours of deco, running your rebreather at 1.6 isn’t the greatest idea.

The CNS clock is a very poor method for tracking oxygen exposure.

Theres no shortage of people blowing past CNS% without showing any oxygen toxicity symptoms.

But there's also cases of people toxing far below "100%".

Its almost a worthless measure.

 

[JohnnyC]

The CNS clock is a very poor method for tracking oxygen exposure.

Theres no shortage of people blowing past CNS% without showing any oxygen toxicity symptoms.

But there's also cases of people toxing far below "100%".

Its almost a worthless measure.

Sure, but your lungs are gonna feel it doing 5 hours of deco at constant 1.6, even with air breaks. It's not so much the overall exposure as it is the extended exposure to high PO2.

Point is, running a consistently high PO2 on a rebreather for hours at a time isn't a great idea. PO2 drop on OC keeps you off high PO2's for most of your deco time until you get shallow and you're doing lots of time on 50% and O2. The limitation doesn't exist on CCR, and you can spend as much time torching your lungs as you like. I don't know anyone who actually decos that way without lots of experience with their own personal physiology, but in response to running a rebreather as an O2 rebreather, you can't ignore that realistically the same person can run high PO2's for the entire ascent.

 

[rjack321]

Nobody would do that do that dive that way.

There's a few things you'd have to assume. First, it depends on where you stand on helium penalty. Second, you'd have to stay on your Dil, which no one that I know does. We are plugging in air as soon as we're 150'ish feet. And last, remember that a perfect PPO2 for the entire dive is better than a perfect PPO2 just for the max depth of the dive, at least for the purposes of deco.

I have 100's of dives at 300+ on CCR with buddy on OC, my deco is significantly less than theirs.

I didn't say anyone would actually do that silly, just that it would (in theory) be shorter on OC primarily because of the inert change.
And not everyone switches dil to air, especially at 150ft which is pretty dense at the depth. The deco dil is a huge determinant of deco time on CCR.

The big difference seems to be the 6m stop. In theory I can see how doing a 5 hour stop at 1.2 would be shortened by doing it on OC 100% but the gas amounts get pretty big pretty fast. Using an O2 breather for that deco stop would probably be the optimal ito deco duration.

Actually if you play around in multideco the time differences are not on the 20ft stops. The big shift in time is (or isn't) a result of the inerts in the 100-20ft stop range. On OC you drop the helium out of your inerts completely. The merits of a dil switch to functionally do the same thing on CCR are not as universally accepted.

 

[rjack321]

Can't you manually use your CCR like an O2 rebreather for the last stop and deco at 1.6?

yes but you're going to fry your lungs. ppO2 of 1.6 has a "max" daily dose of 45mins and this whole plan is way way over on OTUs so you end up needing lots of low ppO2 gas breaks. And a habitat.

 

[Superlyte27]

yes but you're going to fry your lungs. ppO2 of 1.6 has a "max" daily dose of 45mins and this whole plan is way way over on OTUs so you end up needing lots of low ppO2 gas breaks. And a habitat.

Rjack, we appreciate the 45 minutes, but honestly man, people have been doing as much as 3+ hours at 20' on pure oxygen OC for the last 10 years. I strictly dive CCR now for fun dives, but I still have OC friends doing 2-3 hours at 1.6

 

[PfcAJ]

Sure, but your lungs are gonna feel it doing 5 hours of deco at constant 1.6, even with air breaks. It's not so much the overall exposure as it is the extended exposure to high PO2.

Point is, running a consistently high PO2 on a rebreather for hours at a time isn't a great idea. PO2 drop on OC keeps you off high PO2's for most of your deco time until you get shallow and you're doing lots of time on 50% and O2. The limitation doesn't exist on CCR, and you can spend as much time torching your lungs as you like. I don't know anyone who actually decos that way without lots of experience with their own personal physiology, but in response to running a rebreather as an O2 rebreather, you can't ignore that realistically the same person can run high PO2's for the entire ascent.

Lung stuff is OTU related, totally separate idea from CNS%.