Nitrox END confusion

[Uncle Pug]

Dr Paul Thomas once bubbled...
Of course this is largely academic as most now use dedicated Nitrox tables (or Nitrox dive computers).

just take 15% off the depth for EAN30, 20% for EAN32 and 25% for EAN36.

 

[MonkSeal]

Dr Paul Thomas once bubbled...
Am I right in considering EAD is of no use with Trimix?

No, you're not. At least for the mixes containing different % of O2 then 21%.

 

[The Iceni]

Dr Paul Thomas once bubbled...
Am I right in considering EAD is of no use with Trimix?

and

MonkSeal once bubbled...
No, you're not. At least for the mixes containing different % of O2 than 21%.

Now we are getting to the crux of the matter.

There is a lot of confusion over the use of these terms as I see it, and I can well understand why many, like buggeriamold, repeatedly pose the same question. This is why I posted the examples I did. Until you use examples and work them through confusion must remain.

Please do correct me if I am wrong but I believe MonkSeal has illistrated exactly why so many find these terms confusing.

EAD

The concept of an Equivalent Air Depth arose when Nitrox was first employed as a diving gas. It allowed Nitrox divers to use air tables to determine no-stop limits and decompression requirements before Nitrox tables and PC-derived Nitrox decompression profiles were widely available.

The equation allowed Nitrox divers to treat their dives as if they were more shallow than they actually were because Nitrox has a lower ppN2 at any particular depth than air. Thus in my worked example a dive to 40 metres using Nitrox 28 can be treated the same for the purposes of decompression as a dive to 35.6 metres on air and EAD follows UPs percentages pretty well.

Helium has very different physical properties to Nitrogen and a trimix containing 21% oxygen (such as 21/35/44) can never be treated as air on any dive, MonkSeal. The concept of EAD (using air tables for decompression purposes) obviously cannot apply to such a dive. Trimix tables must be used.

END

The concepts of Equivalent Nitrogen Depth or Equivalent Narcosis Depth to my mind are interchangeable and give an indication of the narcotic potential of the gas mix. As I tried to make clear an added confusion arises depending on whether the diver considers oxygen to be equally as narcotic as nitrogen. To my mind this seems very unlikely. Oxygen may indeed be narcotic but there exists very little evidence to indicate that is equally as narcotic as nitrogen if it is narcotic in practice. However see below.

It is the narcotic fraction of the mix that determines its narcotic potential.

Oxygen as a narcotic gas

Let us assume, as many do that oxygen and nitrogen are equally narcotic. In this case all Nitrox mixes will be just as narcotic as air, whatever the depth of the dive. A 40 metre dive on EAN 32 will have an END of 40 meters with a narcotic gas pressure of 5 bar.

The pressure of the narcotic gasses in air or Nitrox will be the same as ambient pressure, which equals, in bar (depth in feet +33)/33 (or (depth in metres +10)/10).

When non-narcotic helium is added the narcotic potential of the mix is reduced, in proportion to the fHe of the mix. If the fHe is 35%, the narcotic potential of the mix is reduced to 65% of air since it is the partial pressure of the narcotic gasses (other than helium) that counts.

Thus the pressure of the narcotic gasses in trimix or heliox is ambient pressure multiplied by the narcotic fraction or 0.65 in this case. The END is the depth at which the ambient pressure of air (or Nitrox) will give the same value.

Thus if oxygen is narcotic the pressure of the narcotic gasses in trimix or heliox is the ambient pressure multiplied by the fraction of the narcotic gasses in the mix (1 - fHe) or 0.65 in this case, and the END is the depth at which air (or Nitrox) will give this same pressure.

At 163 fsw pp narcotic gasses = (163+33) * 0.65/33 = 3.9 bar.

At 50 msw pp narcotic gasses = (50+10) * 0.65 /10 = 3.9 bar.

If these equations are manipulated to give the depth at which the ambient pressure is 3.9 bar - by subtracting one bar and dividing by 33 (for fsw) or 10 (for msw) they simplify to the equivalent narcotic depth.

Hence the equation;

(Depth in fsw +33) * (1 - fHe) - 33 = END

(163 + 33) * 0.65 - 33 = 94 fsw

or

(Depth in msw +10) * (1-fHe) - 10 = END

(50 + 10) * 0.65 - 10 = 29 msw


Let us now assume oxygen is not at all narcotic

Now Nitrox mixes must become less narcotic than air, which has a standard narcotic potential of 79% because this is the fraction of narcotic nitrogen in air. Thus whatever the mix the narcotic potential will be referenced to air and in proportion to nitrogen alone in the mix (now the only narcotic gas).

Since EAN 32 contains 65% nitrogen its relative narcotic potentail with respect to air would be 0.65/0.79.

(Depth in feet +33) * (fN2)/0.79 - 33 = END

(Depth in metres +10) * (fN2)/0.79 - 10 = END

Coincidentally and confusingly this is exactly the same formular used for EAD but it should not be confused with EAD as the application is different.

For EAN 28 at 133 fsw or 40 msw

(133 +33) * (0.72)/0.79 - 33 = END = 118 fsw.

(40 +10) * (0.72)/0.79 - 10 = END = 36 msw.

When non-narcotic helium is added the narcotic potential of the mix is further reduced, in inverse proportion to the total of the fHe and fO2 of the mix. If the fHe is 35% and fO2 is 18% the narcotic gases in the mix are reduced to 47% or 0.47/0.79 of air.

Thus the pressure of the narcotic gasses in trimix is ambient pressure multiplied by the narcotic nitrogen fraction (0.47 in this case) divided by the narcotic fraction of N2 in air 0.79. The END is the depth at which the fN2 in air gives the same pp N2.

At 163 fsw ppN2 = (163+33) * 0.47/33 = 2.8 bar.

At 50 msw ppN2 = (50+10) * 0.47 /10 = 2.8 bar.

If these equations are manipulated to give the depth at which the ppN2 in air is 2.8 bar - by dividing by 0.79, subtracting one bar and dividing by 33 (for fsw) or 10 (for msw) they simplify to the equivalent narcotic depth.

Hence the equation;

(Depth in fsw +33) * fN2/0.79 - 33 = END

(163 + 33) * 0.47/0.79 - 33 = 84 fsw

or

(Depth in msw +10) * fN2/0.79 - 10 = END

(50 + 10) * 0.47/0.79 - 10 = 25.7 msw

To simplify

The aim of the concept of END is to estimate the narcotic potential of the mix used and it has no role in calculating deco schedules. In my own case I tended to suffer narcosis at depths over 40 msw on air so seldom used it. I found EAN 28 to be less narcotic at this depth so, from my own experience considered oxygen must be less narcotic than nitrogen.

If one looks at the results of the second equation at 40 msw EAN 28 has an END of 35 msw, a difference of 5 msw which is hardly major and, indeed, Nitrox is hardly the mix to use at depths where narcosis becomes a problem.

In respect of trimix 18/35/47 at 50 msw the first approach gives an END of 29 msw, while the second gives approximately 26 msw, again the difference is marginal.

I consider it is entirely appropriate to err on the side of caution and to consider oxygen to be narcotic in practice and to use the first, much simpler equation.

I hope that is about right.

 

[MikeFerrara]

Dr Paul Thomas,

No argument from me and a very elegant presentation BTW.

 

[Genesis]

I find that I first "notice" narcosis at about 110' some of the time. It is not really impairment, but it definitely is noticable for me at that depth. Above that I don't notice it at all.

If I dive air at 110', it seems to be a bit worse than diving 30% EANx (which is what I use), but not much. On the other hand I'm not down as long on air either! The difference, for me, is probably not statistically significant, as there are days when I don't notice narcosis at all at 110', and there are days when I do, and as a consequence I'm not confident enough in how it works "for me" to say that O2 is not narcotic.

However, I accept that for some people it is not; we're all individuals and within broad limits we do have minor differences in how things work for each of us when it comes to diving.

Thanks Dr. Paul for a good description of the differences....

 

[MonkSeal]

Dr Paul Thomas once bubbled...
Please do correct me if I am wrong but I believe MonkSeal has illistrated exactly why so many find these terms confusing.

You're right (at least in my case). I'm familiar with equations and physical (and practical) background of the problems but I wasn't sure about terminolgy and its meaning.

Helium has very different physical properties to Nitrogen and a trimix containing 21% oxygen (such as 21/35/44) can never be treated as air on any dive, MonkSeal. The concept of EAD (using air tables for decompression purposes) obviously cannot apply to such a dive. Trimix tables must be used.

Of course.

Thanks for clearing this.

 

[Uncle Pug]

Dr Paul Thomas once bubbled...
Helium has very different physical properties to Nitrogen and a trimix containing 21% oxygen (such as 21/35/44) can never be treated as air on any dive, MonkSeal. The concept of EAD (using air tables for decompression purposes) obviously cannot apply to such a dive.

not to poke a stick in your spokes... but we do.

 

[MikeFerrara]

If you run some schedules and compare "air" to a mix like 21/30 or 21/35 you will see they are almost identical. This is true at least for the profile we're diving. I realize that sume just plain believe the current deco models are wrong and they might be right but I would suggest running your profile and comparing before diving it that way. For lots of the dives we do, using "air" actually actually calls for slightly more decompression.

Once the He content get's above a certain point you start to see some difference. Likewise reducing FO2 has an effect.

 

[The Iceni]

Uncle Pug once bubbled...
not to poke a stick in your spokes... but we do.

Now you've gone and spoilt it UP!

Without any information on what dives you are referring to it is a little difficult to reconcile what you posted above with any explanation of EAD and END.

Can you clarify? Do you use the EAD formula, which follows to cut a table for every trimix dive and then use air tables?

(Depth in feet +33) * (fN2)/0.79 - 33 = EAD

If this is the case I take it you assume that what you lose on the swings you gain of the roundabouts which is what Mike appears to confirm for certain mixes for certain dives. However, as nitrogen half times are 2.6 shorter than nitrogen half times I do not really understand the theoretical reasoning behind this for every trimix dive.

I am not trying to score points. Honest.

As you know I am not even qualified to dive trimix.

Could you please explain?

 

[Uncle Pug]

Dr Paul Thomas once bubbled...
I do not really understand the theoretical reasoning behind this for every trimix dive.

Paul... I was specifically speaking to your post where you wrote:

"a trimix containing 21% oxygen (such as 21/35/44) can never be treated as air on any dive"

On all the dives we use 21/35 (120'-165') we use it as though it were air. The helium goes in fast... but it comes out fast too. We do our deco stops just as though we had been diving air. All I can say is that it works for us.