Two recent articles on Nitrox

[limeyx]

That deco planner doesn't like He. Its just a piece of software.

Yes, that was my conclusion also
The question is is that a deco planner issue, or reality?
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.

However, this seems to be far from a universal attitude toward He, as we can see from people who teach/take deco classes without using any He, which is something I would not personally do.

The O2 window has always baffled me though honestly, and each time I hear a (different ) explanation, I think I have it, and then I either think some more or hear another view and out it goes.

 

[TheRedHead]

And it's not really the %N2 we care about in this case, it's the PP, right?

Inside the cells, the % concentration matters. The way I understand it, the %concentration is proportional to the PP until the compartment is "saturated." Different "tissues" have different rates of saturation. But the more gas presented to the tissue, the faster it will saturate. And the converse is true for desaturation.

If you dive with nonstandard mixes and deco with 80/20, the GUE system doesn't work so well. That's when you need the software or Ph.D

 

[limeyx]

Inside the cells, the % concentration matters. The way I understand it, the %concentration is proportional to the PP until the compartment is "saturated." Different "tissues" have different rates of saturation. But the more gas presented to the tissue, the faster it will saturate. And the converse is true for desaturation.

If you dive with nonstandard mixes and deco with 80/20, the GUE system doesn't work so well. That's when you need the software or Ph.D

or dont dive those mixes

But honestly, most of the GUE mixes are not "magic" (maybe not withstanding the 80%/100% O2 argument).

The bottom mixes are just convenient amounts of He to reduce END to < 100 feet (assuming O2 is narcotic) and that can be topped with 32% to get a fill.

The deco mixes are designed (as far as I understand it) to still give END < 100, but to try to get PPO2 close to 1.6 at each gas switch.

If you used "nonstandard" gases, you'd just need another system (as long as the system was well thought out)

 

[TheRedHead]

A lot of people dive with an END > 100 feet, but best mixes are a different argument One I don't want to have.

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.

 

[limeyx]

A lot of people dive with an END > 100 feet

Still doesn't make it a good idea
And I think it's one thing (still not a good one) on a dive that goes well, but when things go tits up? I'd rather have my wits about me.

I think H2Andy had an interesting END > 100 experience recently .. (not that I am trying to pick on him, but it was interesting to read about)

 

[TheRedHead]

I think H2Andy had an interesting END > 100 experience recently .. (not that I am trying to pick on him, but it was interesting to read about)

Do you have a link?

I'm not crazy about being narced either...

 

[do it easy]

The O2 window has always baffled me though honestly, and each time I hear a (different ) explanation, I think I have it, and then I either think some more or hear another view and out it goes

I hear you about the oxygen window. My interpretation is a bit backwards- deco at 1.6 ppO2 will result in efficient deco (or "a wide open O2 window"), but is not the physiological mechanism that causes the efficient deco. 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 CO2 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...

 

[limeyx]

Do you have a link?

I'm not crazy about being narced either...

http://www.scubaboard.com/showthread.php?t=144892

read his first post. Of course, it could be plenty of other contributing factors (stress from the class etc) but I think that emphasizes my point -- you only really need the He when you really need it...

Of course, if you believe O2 is not narcotic (iI honselty dont know what the truth is there, so assume that it is) then you probably only have a little bit of heliium in the mix anyway (I have heard things like 26/17)

 

[TheRedHead]

http://www.scubaboard.com/showthread.php?t=144892

read his first post. Of course, it could be plenty of other contributing factors (stress from the class etc) but I think that emphasizes my point -- you only really need the He when you really need it...

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.

 

[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.

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.

well, I dont think the He has as much effect on the deco as some people think.
GUE requires an END < 100 feet, including the assumption that O2 is narcotic.
This means adding more He.

For a typical "Tech1" dive (say 150-160 feet for 20-30 mins) you are looking at 30 mins max on 50% deco bottle.

For a tech2 dive (say 240 for i think 20-30 mins) I think it's something like 60 mins deco on 50% and 100% (I could easily be wrong here as I have never taken tech2 so these are approx) GUE guidelines say for ocean diving, try to limit to 90 min exposure.

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