How do dive computers calculate nitrogen absorbtion and clearance when using nitrox?

[boulderjohn]

Not uncommon to surface from a deco dive and have the fast tissues on gassing Nitrogen. One of the cool features of a Shearwater.
First saw it in my rebreather class where we spent a couple hours in a pool on pretty much pure O2.
If you have a Shearwater (or can borrow one) and have a pool with a deep end, set it to 99% O2 and put it at the bottom of the pool for a couple hours. fish it out and look at the tissue loading. It is the inverse of a dive as far as Nitrogen loading goes.

Yes, this is true, but I wish Shearwater would take it a step further.

Once you are out of the water, it assumes you are breathing air. You don't have to be breathing air, though. You can be breathing nitrox or oxygen instead with the goal of increasing the offgassing gradient in those middle tissues. I wish we had the option of telling the computer what we are breathing during the SI.

 

[GJC]

That's very interesting. What if you breathed O2 pre dive and washed out all your nitrogen?

How much benefit would that be and how much longer would your NDL be?

 

[broncobowsher]

I've heard of people doing an O2 prebreath before a big deco dive. What will it get you? How long do you prebreath? A 5-minute O2 prebreath won't do anything. The fast tissues might get a slight headstart, but as soon as you start diving it goes away. The medium to medium fast tissues are typically the limiting factor. So you would need to prebreath long enough that you pull down the nitrogen loading in those tissues. That much O2 for that long, before the dive and for deco, you would likely be pushing the O2 limits.

 

[Angelo Farina]

This suggests another question.
Consider a diver who lives on an high mountain, 2000 m or more.
His body contains much less Nitrogen.
He comes down to the sea and immediately dives.
Does the computer take into account his initial lower Nitrogen content?
And, after surfacing, does the computer warn him if he climbs up the mountain back home too early?

 

[tursiops]

That's very interesting. What if you breathed O2 pre dive and washed out all your nitrogen?

How much benefit would that be and how much longer would your NDL be?

Astronauts used to prebreath pure O2 for 24h (I think) prior to a launch, to prevent DCS during the rapid ascent from sea level to the ambient pressure in the capsule (0.34 ATM), and they prebreath for 2h20m prior to a spacewalk from the ISS; the space suits operate at 0.29 ATM of O2, while the ISS is maintained at 1 ATM of an air mix.

 

[broncobowsher]

Coming out of a mountain to go dive, non-issue. Slight margin of safety.
Return to altitude, Ever hear of "do not fly", same thing applies.

 

[tmassey]

Serious, but admittedly ignorant question:

Why isn’t this covered in nitrox class?

In a related discussion of an advanced nitrox class, it seems that my 13-year-old nitrox class is different than modern nitrox classes. All the math seems to have been eliminated, and it is now computer focused. All of that math burden has now been shifted to the advanced class.

I actually have no problem with any of that: you don’t need to know the math behind an internal combustion engine in order to get a drivers license. But, if they don’t teach you how a computer handles your decompression limit in a modern computer-based nitrox class, then exactly what do they teach?

Is it literally 15 minutes of: don’t exceed MOD, and here’s how you change the numbers on your computer screen?

I’m really trying not to be grumpy old guy. I don’t care if no one has to do another math formula ever again. But if they aren’t explaining what your computer is doing for you in a computer-based class, what are you paying for?

And just to make this doubly clear: this is in no way any type of shot at the original poster. I think they asked what is a brilliant question. Just one that I would’ve been expected to cover in the first five minutes of the class! Because the answer really is: exactly the same way the computer kept track of air, just using a slightly smaller number for the percentage of nitrogen.... Which is the whole reason for nitrox!

 

[tmassey]

That's very interesting. What if you breathed O2 pre dive and washed out all your nitrogen?

How much benefit would that be and how much longer would your NDL be?

Speaking of good questions… This is another one.

Understanding exactly what that would do to your NDL and decompression situation is complex. It has to do with the fact that your body adds and removes nitrogen at different speeds in different areas. Or at least, that’s the simplification we use for what the body is actually doing, seeing as we don’t actually know!

How changes like pre-breathing oxygen, post-breathing oxygen, or breathing oxygen on a Deco stop actually affect your body will change based on which types of tissue were talking about, how long the dive was, how deep the dive was, etc. The answer is not straightforward.

Getting the answer requires having a much deeper understanding of how decompression is modeled within your body. Deco for Divers and Deep into Deco are two books that are often recommended as a detailed overview of decompression.

But be warned: it’s a slippery slope. Today it’s a simple question, tomorrow it’s a 300 foot dive.

But to give you a simple answer that pretty much echoes the answers you’ve gotten above: assuming you’re not going to pre-breathe oxygen for hours in advance, it will only make a difference in lowering nitrogen in tissues that fill back up very quickly anyway. So even if you completely drained them, they will fill up no matter what you did. And once at maximum, they won’t add more, so the pre-breathe does nothing. They get full either way. The other tissues that might benefit from a pre-breathe respond too slowly to improve from a *brief* pre-breathe.

Now if you did pre-breathe for hours in advance, it would make a difference in those tissues less likely to get filled up anyway. That’s why as mentioned above, fighter pilots and astronauts have to pre-breathe oxygen. They’re doing an infinitely long “dive” here on the surface, and are ascending at the end of their infinitely long “dive”. But really, their pre-breathing of oxygen is like us using oxygen on a deco stop before ascending. It’s not really pre-breathing. It’s decompressing in advance!

Plus, that fighter pilot or astronaut is ascending a lot faster than 30 feet a minute…

 

[GJC]

This suggests another question.
Consider a diver who lives on an high mountain, 2000 m or more.
His body contains much less Nitrogen.
He comes down to the sea and immediately dives.
Does the computer take into account his initial lower Nitrogen content?
And, after surfacing, does the computer warn him if he climbs up the mountain back home too early?

Actually, I live at 2000 m and could be home 90 min after a shore dive, but I have always stayed at sea level overnight before going home. I would use the USN Ascent to Altitude Table if I ever went home the same day.

This brings up another thing about dive computers. I really wish they would tell me what my residual nitrogen or repetitive group was at the end of a dive. The only thing I can get out of the few that I have experience with is the NDLs using planning mode. I guess I could use those numbers to guesstimate my repetitive group.

 

[GJC]

Serious, but admittedly ignorant question:

Why isn’t this covered in nitrox class?

In a related discussion of an advanced nitrox class, it seems that my 13-year-old nitrox class is different than modern nitrox classes. All the math seems to have been eliminated, and it is now computer focused. All of that math burden has now been shifted to the advanced class.

I actually have no problem with any of that: you don’t need to know the math behind an internal combustion engine in order to get a drivers license. But, if they don’t teach you how a computer handles your decompression limit in a modern computer-based nitrox class, then exactly what do they teach?

Is it literally 15 minutes of: don’t exceed MOD, and here’s how you change the numbers on your computer screen?

I’m really trying not to be grumpy old guy. I don’t care if no one has to do another math formula ever again. But if they aren’t explaining what your computer is doing for you in a computer-based class, what are you paying for?

And just to make this doubly clear: this is in no way any type of shot at the original poster. I think they asked what is a brilliant question. Just one that I would’ve been expected to cover in the first five minutes of the class! Because the answer really is: exactly the same way the computer kept track of air, just using a slightly smaller number for the percentage of nitrogen.... Which is the whole reason for nitrox!

Agree entirely.

My Nitox class did all the math to convert EAD, calculate MOD, and oxygen time accumulation. There was nothing in it about computers and I only took the class 6 years ago when there were a lot of computers being used.

I am the person that reads the computer manual front to back and can get to any menu item without getting lost. But other than mentioning the deco model used, none explain the math they use, and they all have made proprietary changes to the model that they keep secret (except Shearwater).

I would love to take a class that explains what the computer does (not how to use it). I would also be interested in a class comparing each deco model used in dive computers and how they compare when used within recreational NDLs.