Nitrox mixes above 40%?

[iamjblevine]

Not only that, but they discovered that for reasons that are unknown, the PPO2 at which you tox in the chamber is much higher than when under water.

I thought the reason was no stress/exertion in the chamber compared to the stress and exertion present during a dive. Not certain though.

 

[HIGHwing]

It seems most if not all agencies would set 40% as max for recreational diving. But some of the recreational rebreathers(CCR or SCR) will allow much higher % use eg. Dragger Dolphin SCR(60%). Does it mean you have to be tec certified before you can venture into rebreather?

In short, no.

 

[Centrals]

In short, no.

If you need to be nitrox certified(OC) before you can dive with it(up to 40%). Why there is exception for CCR and SCR? Are you suggesting that the respective courses will deal with the issue, ie. higher than 40% and all the way to 100%(CCR)?

 

[Rhone Man]

I have often thought it make make sense for liveaboards to make 75% available on the hang bar for more efficient safety stops, but I have never seen it and I am guessing it will never happen for lots of different reasons.

 

[HIGHwing]

If you need to be nitrox certified(OC) before you can dive with it(up to 40%). Why there is exception for CCR and SCR? Are you suggesting that the respective courses will deal with the issue, ie. higher than 40% and all the way to 100%(CCR)?

You do need a basic Enriched Air Certification, but you don't need Tec 40 or TDI Advanced Nitrox. Fully closed rebreathers are a very fancy adaptation of a breathing in and out of a zip-lock bag. Clearly CO2 is going to be a bit of an issue in my scenario, so we have to toss in a little soda lime in our bag.

Picture sitting on a boat breathing in and out of the bag getting ready for the dive. We quickly discover this is workable for a short period of time, but breathing from a plastic bag has a few problems long term. As we leave the surface and descend to any depth the air inside the bag almost instantly crushes leaving us nothing to breathe. The rebreather diver solves this problem by adding gas with a fancy name, diluient, a.k.a. "dil". Dil provides additional gas in the plastic bag which reinflates it making it possible to continue breathing. Dil doesn't have to be a special gas, in fact, it can simply be air, an enriched air mix, or something more exotic like Trimix/Heliox. By now you're probably wondering how we didn't pass out from a lack of oxygen?? Let's first reexamine the sources of oxygen available to the rebreather diver. For practical purposes there are three sources of oxygen, 1) oxygen expired by the diver 2) oxygen fraction in the diluient 3) oxygen cylinder.

When diving closed circuit you're operating a nitrox (trimix) mixing station on your back in real time. Instead of selecting a fraction of oxygen, i.e. I'm diving 36% and I can't go below 100ft, you're selecting a partial pressure of oxygen (a.k.a. PO2, or PPO2) for the entire duration (or majority) of a dive. Instead of planning for a peak PO2 at the very deepest (and often shortest) portion of the dive what a rebreather does is give you the ability to mix the optimized gas mixutre for every depth. If you want to raise your PO2 you can put a little shot of oxygen into the zip lock bag, if you descend and discover your PO2 climbs (from increased pressure) you can add a little shot of dil. Adding or removing oxygen and dil is essentially the same as partial pressure blending into a scuba tank which will be breathed by an Open Circuit diver. By blending oxygen and dil together we do in fact change the fraction of oxygen, i.e. we might be breathing 32% Enriched Air, but we don't care because we track oxygen against our exposure (time) and dosage (PO2). A rebreather diver simply ignores fractions placing PO2 above all else.

Consider: If you plan a single tank open circuit dive and you get a blend of Nitrox which has a 1.4 PO2 at the Max Operating Depth (MOD), you're probably only getting the maximum advantage of that gas for exactly as long as you're at the MOD for the gas. With a rebreather you're going to manage to a target PO2, a.k.a. "the set point" or "SP" and the rebreather (or rebreather diver) can mix gas constantly to maintain the optimal gas for depth. The real difference is the rebreather diver stays at the MOD for the gas at all times by using a source of 100% oxygen and dil to keep the PO2 constant.

That's basically how it works without really explaining all the nuances. As you can see, you're more of a practitioner of physics & physiology. At some point diving a rebreather you start looking at oxygen as a poison rather than a helpful drug. In effect, dosage, duration and exposure become much more interesting when you can bathe your body in it for three, four, and five hour exposures. Just sayin'

Now that we have established how a rebreather is a $10,000 zip-lock bag. Tec 40 and/or Advanced Nitrox/Deco Procedures address making short decompression dives on open circuit with a focus on planning, equipment, and execution. As you can see the basic rebreather course has topics which are far afield and may or may not even include decompression in the courses (depending on agency, rebreather type, and diver experience). You'll certainly cover nitrox adequately in any CCR class, including an in-depth review of the physics and physiology associated.

 

[Doppler]

If you need to be nitrox certified(OC) before you can dive with it(up to 40%). Why there is exception for CCR and SCR? Are you suggesting that the respective courses will deal with the issue, ie. higher than 40% and all the way to 100%(CCR)?

the basic, air diluent user classes for rebreathers (the ones with which I am familiar at any rate) cover the issues and material traditionally used in a stand-alone advanced nitrox class.

 

[billgraham]

FWIW, I am aware of at least one incident where a diver toxed at a PPO2 of 1.4. Different people have differing sensitivities to high PPO2's.

 

[HIGHwing]

FWIW, I am aware of at least one incident where a diver toxed at a PPO2 of 1.4. Different people have differing sensitivities to high PPO2's.

Agreed. At a 1.4 PO2 [dosage] given enough time [exposure] the tables show it is easily possible to tox within otherwise sport diving limits on a typical diving day (especially live-aboard situations). Of note is many rebreather divers are favoring a setpoint of 1.3 or even 1.2 (I choose the latter) to reduce the update intensity, which considers being at that SP for longer durations that OC. If we're getting into single-day exposures extending beyond four hours, I'll back it down to 1.0 or 0.7 for the reason you bring forward.

 

[billgraham]

Yeah, even though you've theoretically got three hours at 1.3, factor in some cold, work, sensor inaccuracy etc. -- probably makes sense to dial it back one notch. Steve, am I right in recalling that you wrote an article on the wisdom of the "half life" concept ? Being a one dive a day person, I don't consider it much.

 

[CT-Rich]

I have often thought it make make sense for liveaboards to make 75% available on the hang bar for more efficient safety stops, but I have never seen it and I am guessing it will never happen for lots of different reasons.

Just finished "The Last Dive." They mention several times about hanging reg with pure O2 over the side to aid in decompression. I am not a deep tech diver, but I imagine that is still done on some boats. Although, the techology has probably evolved extensively since the early '90s. I imagine a lot of the equivelent divers today would be using CCR rather than carrying elaborate selections of tanks with different mixes on the extreme dives described in the book. I you want to read about the penalties of ox-tox it describes several instances and gives some very dramatic descriptions of what happens when you screw the pooch in really deep water.