Two recent articles on Nitrox

[limeyx]

My class didn't require me to dive to 150 feet on air even though the cert is to 150 feet. You can do accelerated deco without diving to the max. The next class introduces Trimix. TDI is moving away from deep air. I dived to 142 feet on air and I was happily narced. Too happy. This was before I took the class.

But the really deep dives often have greater ENDs just because of the depth. Increasing He too much creates an enormous deco obligation. I don't do those dives and probably never will.

Was also interesting on what you thought of a class that certs you to do something that you might find "unreasonable" (like 150 on air) or do you not consider 150 on air unreasonable?

 

[TheRedHead]

Decoplanner says 240 at 25 mins is 80 mins deco with 50% and 100% deco gases. So doesn't seem (comparatively) enormous ...

I was thinking about Gary Gentile's description of the Lusitania expedition and how they chose their mix. Over 300 feet and they wanted to keep the number of deco cylinders at a min because they couldn't stage their bottles. I think they settled on an END of 130 feet with 1 deco bottle and surface supplied O2. I don't remember the exact mix, but they went through a vetting process. I'll have to look it up.

 

[limeyx]

I was thinking about Gary Gentile's description of the Lusitania expedition and how they chose their mix. Over 300 feet and they wanted to keep the number of deco cylinders at a min because they couldn't stage their bottles. I think they settled on an END of 130 feet with 1 deco bottle and surface supplied O2. I don't remember the exact mix, but they went through a vetting process. I'll have to look it up.

Would be interesting to see what they did, but I think a 300 foot dive with one deco gas is pretty hairy unless you have plenty of support divers.

I'd definitely leave that for someone else

 

[TheRedHead]

Was also interesting on what you thought of a class that certs you to do something that you might find "unreasonable" (like 150 on air) or do you not consider 150 on air unreasonable?

I think it is very hepful to those people who would do it anyway. TDI leaves much to the instructor as to what you will get out of it. Our deepest checkout dive was 122 feet on a wreck with bad viz so it could have seemed deeper than Andy's 150 foot dive in clear water with no current.

I also think you work up a tolerance to narcosis over a period of time, just like the drinker becomes more tolerant of alcohol. Building up to deep dives would also make sense. I see that Andy didn't do progressively deeper dives in his course either.

 

[TheRedHead]

Would be interesting to see what they did, but I think a 300 foot dive with one deco gas is pretty hairy unless you have plenty of support divers.

I'd definitely leave that for someone else

I looked it up and they did 2 deco gasses for that one. When he dived the USS Monitor at 235 feet, he selected 19/30 with an END of 137 feet. He also considered 17/17 (END 187) and 17/50 (required 3 deco gasses). That was the one I was thinking about.

With 19/30, he carried 1 deco gas and used surface supplied oxygen.

I'm not going to be doing any of those dives soon.

 

[vondo]

a subtle point is that a reasonable diver doesn't do a square profile dive right to the table limits, and then immediately ascend. He will take his time going up, doing a few short stops on the way, and then hang shallow for a while. The reduced N2, even in lean mixtures like EAN32, will somewhat accelerate the offgassing.

Do the basic nitrox dive computers take this into account? I have an Oceanic Veo 200, so I'm most curious about that one? Obviously, the tables don't since one is given no credit for a slow ascent or safety stop, but do the entry level Nitrox computers change their "decompression" algorithms based on the gas being breathed?

 

[Charlie99]

Do the basic nitrox dive computers take this into account? I have an Oceanic Veo 200, so I'm most curious about that one? Obviously, the tables don't since one is given no credit for a slow ascent or safety stop, but do the entry level Nitrox computers change their "decompression" algorithms based on the gas being breathed?

Sure. Dive computers, unlike tables, give you credit of the offgassing that takes place when you are shallow. This offgassing is a bit faster on nitrox than air, and yes, the nitrox computers track that, just like they track the reduced ongassing of N2 when diving nitrox.

It's there, but isn't a huge effect on a case where you just ascend, hang for 3 minutes, then surface. Doing a good, gradual ascent to minimize bubbling is a much more significant effect in real life ---- bubbles offgas very slowly. N2 still dissolved in tissues will offgas nicely during the SI.

 

[Charlie99]

I've heard it described as partial pressure vacancies, which makes sense to me. In my mind, the "oxygen window" exploits the fact that you body handles O2 differently than it does other inert gasses, so this makes room for the other gasses to be dissolved/offgassed. Of course, I could be completely wrong about this...

That's a pretty good way of describing it.

Saying the same as you did above, but fleshing it out a bit: In the gas you are breathing, O2 is part of the pressure, leaving lower ppN2 (and/or ppHe) for a given depth. Ongas/offgas is set by the ppN2 (and/or ppHe) without any reference to how much O2 is there, since O2 is metabolized. But OTOH, whether or not a bubble will grow, is dependent upon the total ambient pressure. So a high fraction O2 mix lets you stay deep (for total ambient pressure) to reduce bubble formation, but at the same time have a reduced ppN2 in the breathing gas, thereby promoting offgassing.

 

[Charlie99]

From what I understand, it's a deco planner issue where it unfairly penalizes you for usng He, which is why I plan all dives on it (if I am going to use it at all) with no He in the mixes.

Search for comments here on SB by Bruce Weinke regarding the "mass transport limit".

For deep, long dives, the lower solubility of helium means that there is less total dissolved gas. This gives signficant advantages in decompression.

For a short dive, though, the faster ongass of helium means that deco can be a bit longer on helium (the model math that forces this is that helium more quickly drives a slightly slower compartment to its limits. Slower compartments have lower M-values). Or to put it another way, NDLs will be lower for a breathing gas with helium.

The transition between a short dive where helium increases deco slightly, and deep,long dives where helium will decrease deco is roughly at the depth/time combo that would be air NDL, or a tad beyond.

Plug an agressive dive into decoplanner -- you will probably find that increased helium reduces deco. In the typical "rec triox" sort of dive, BRW's studies and calculations agree with Decoplanner in that deco is slightly increased / NDL is slightly reduced.

Just like life in general, diving is all about tradeoffs. In a lot of cases, reduced narcosis is worth a bit less NDL or slightly increased deco.

 

[Charlie99]

One of my instructors made a good case for 80% vs. 100% since you get almost as much benefit with decreased OTUs and use it earlier.

Most people will agree that there are dives with a single deco gas where EAN50% is better than O2, and some that are vice versa. Even without running lots of trial dives in a deco program it isn't all that hard to believe that there would be some dives where an intermediate gas (such as 80%) is optimum.

I'd have a lot harder time believing in EAN80 as the optimum deco gas for dives where you have 2 gases.