Nitrox END confusion

[buggeriamold]

Hi there

I have back read through most of the posts and it seem that you folks don’t go to hard on silly questions.
Having been certified in 1984, I thought it was about time to catch up with the times and do a nitrox course, which brought to me the concept of EAD. My understanding of the EAD calculation that PADI teaches says that it’s the partial pressure N2 that is important in working out the no deco limits, not the actual depth (pressure) that you are subjected to. This to me seem that there is a whole heap of fudging going on.

To illustrate my point lets look at the recommended safety stop depth of 5 meters.
Air ppn2 at 5 meters = 1.185
40/60 nitrox ppn2 at 5 meters = .9

there for if the only thing that influences nitrogen absorbs ion is ppn2 we should either be doing, if we believe that old air 5 meter stop, a stop on nitrox 40/60 at 9.75 meters … or do an air stop at 1.39 meters to use the 40/60 ppn2.

So I gues what I am asking is, is there a theretical reason that only the ppn2 influences the uptake and offgassing of nitrogen

Thanks Mark

 

[Uncle Pug]

buggeriamold once bubbled...
So I gues what I am asking is, is there a theretical reason that only the ppn2 influences the uptake and offgassing of nitrogen

1. O2 doesn't add any nitrogen to the tissues.
2. O2 that is absorbed is metabolized.

Using Nitrox you are replacing some of the N2 with O2 in your breathing gas.

While still allowing the same total absorbed gas into the tissues this results in less inert gas (N2) absorbed while the extra O2 that is absorbed is metabolized...

So... really the only thing that counts is the PPN2.

 

[MikeFerrara]

I'm not certain exactly where your confused but I'll take a stab at it. It sounds like you understand what EAD is. Is that correct? I we are going to calculate the dive using "AIR" tables we use the EAD.

We don't adjust the "safety stop" depth based on the gas. The purpose for the stop is to allow time for N2 offgassing while still under some presure. The lower the PPN2 the more efficient the offgassing.

During the stop you want a lower PPN2. In fact it is common in more advanced (technical) diving to make all stops at or above 20ft on pure O2. The pp of inert gas we are breathing does effect on/off gassing but the ambient presure has an effect also. Did I help or confuse the issue more?

 

[Uncle Pug]

Maybe I misunderstood what he was really asking...

Did I help or confuse the issue more?

Let's tell him about the oxygen window and throw in the toggling effect too

 

[MikeFerrara]

Uncle Pug once bubbled...
Maybe I misunderstood what he was really asking...

Let's tell him about the oxygen window and throw in the toggling effect too

I did the best I could? I did start out by saying I wasn't sure what the question was. I think an explaination of the O2 window would work for his question about safety stop depth as it relates to PPN2. I would leave out the toggle effect though.

 

[Uncle Pug]

actually I think you are probably closer to answering his question than I am... but we'll just have to wait until the planet spins around a bit more and he'll tell us what he meant.

 

[Rick Murchison]

buggeriamold once bubbled...
To illustrate my point lets look at the recommended safety stop depth of 5 meters.
Air ppn2 at 5 meters = 1.185
40/60 nitrox ppn2 at 5 meters = .9
there for if the only thing that influences nitrogen absorbs ion is ppn2 we should either be doing, if we believe that old air 5 meter stop, a stop on nitrox 40/60 at 9.75 meters … or do an air stop at 1.39 meters to use the 40/60 ppn2.

So I gues what I am asking is, is there a theretical reason that only the ppn2 influences the uptake and offgassing of nitrogen

Thanks Mark

In lay terms...
You're confusing the offgassing ratio with the bubbling ratio. The objective in offgassing, whether it be during ascent, or during a safety or decompression stop, is to remove excess nitrogen as quickly as possible, without forming any damaging bubbles. The offgassing rate is determined almost exclusively by the ratio of the partial pressure of disolved nitrogen to the partial pressure of nitrogen in the breathing mix, so the lower the partial pressure of nitrogen in the breathing mix the quicker you can eliminate excess nitrogen. This is good. This is why pure oxygen is a great decompression gas at shallow (20ft & shallower) depths.
The bubble forming ratio, on the other hand, is a ratio between the partial pressure of disolved nitrogen and the total absolute pressure of the surrounding environment. If we reduce the pressure around the body too fast then the bubble forming ratio will get to the point where damaging bubbles form, and we get DCS. Note that it's the ratio of the partial pressure of dissolved nitrogen to the ambient pressure that relates to bubble formation - nitrogen disolved deep in the tissues has no idea what you're breathing - all it knows is that if you reduce the pressure too fast without removing some of it it'll come out of solution right there in the form of a bubble.
So, reducing the partial pressure of nitrogen in the breathing gas while maintaining the ambient pressure allows us to remove nitrogen more quickly while maintaining a safe "no bubbles" pressure on our whole body.
Rick

 

[MikeFerrara]

Good explaination Rick.

 

[buggeriamold]

Thank for spending the time to answer my not very well asked question.
I think that my rambling about the 5-meter depth was probably a red herring, and the last post from Rick probably came the closest to it.

If you all would indulge me one more time … may be what I should have asked was, do any other factors apart from ppn2 such as “total absolute pressure of the surrounding environment” influence nitrogen absorbs ion. If so doesn’t this then render the EAD as calculated using only equivalent ppns’s inaccurate?

Like I said more confusion.

Once again thanks
mark

 

[Doppler]

buggeriamold once bubbled...
Thank for spending the time to answer my not very well asked question.
I think that my rambling about the 5-meter depth was probably a red herring, and the last post from Rick probably came the closest to it.

If you all would indulge me one more time … may be what I should have asked was, do any other factors apart from ppn2 such as “total absolute pressure of the surrounding environment” influence nitrogen absorbs ion. If so doesn’t this then render the EAD as calculated using only equivalent ppns’s inaccurate?

Like I said more confusion.

Once again thanks
mark

Hi Mark:

As briefly as possible... there are two things we need to consider: The fraction of Nitrogen in our breathing mix; the depth at which we are breathing it.

OK. Let's say we are breathing a mix that contains 50% nitrogen. At one atmosphere, the partial pressure of Nitrogen is 0.5 atmospheres, right? If we take that mix to 10 meters, the fraction of nitrogen remains unchanged -- it's still 50% -- but because the ambient pressure around us is now 2 atmospheres, the gas we are breathing has to be two times as dense to fill our lungs and therefore the partial pressure of Nitrogen will also have to be two times what it was... i.e. 1 atmosphere absolute. If we took that mix to 20 meters (3ata) we'd have a PPN2 of 1.5 ata. (If we were to breath air at that depth, the PPN2 would be greater-- 2.37 ata, OK?)

Calculating END we consider the fraction of nitrogen in our mix and then the actual depth at which we are using it. This gives us our partial pressure of nitrogen... PPN2 is a variable that is directly related to "total ambient pressure."

EAD is a ratio between the actual PPN2 that our mix delivers at X meters and the PPN2 that air would deliver at the same depth.

Hope this helps mate

DD