Nitrox END confusion

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buggeriamold once bubbled...
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
The effect that the total environmental pressure has on nitrogen uptake is by increasing the partial pressures of each gas in the breathing mixture. Increased partial pressures mean increased rates of uptake of the individual gases.

By using a higher fraction of O2 and lowering the fraction of N2 in a breathing mix (nitrox) we end up with a lower partial pressure of nitrogen for any given depth.


1. O2 doesn't add any nitrogen to the tissues.

The O2 component of the total absolute pressure (PPO2) does not influence the way nitrogen enters the tissues (afaik)... only the N2 component (PPN2) is of concern.


2. O2 that is absorbed is metabolized.

The higher partial pressure of O2 in a nitrox mix (becasue the fraction of O2 was higher to start with) does mean that more O2 will enter the tissues... but that is not of concern since it is metabolized.
 
Dear b mold:

Pressure and Gas Dissolution

The earlier responders have done a nice job on this question. :wink:

Within the very narrow pressure range of human diving, the total pressure does not affect the amount of dissolved gas. That is determined only by the pressure (or partial pressure) of the gas itself.

If you double the pressure of helium gas, you will get more helium to dissolve, but it will not change the amount of nitrogen gas that is present in the solution.

Soda Pop Pressurizers

A gizmo that is sold in stores to help retain carbon dioxide in open bottle of carbonated beverages is a small pump that you can use to add pressurized air to the container. Will this device, which only adds air ( nitrogen and oxygen ) to the bottle, affect the amount of carbon dioxide that dissolves in the liquid? What do you think?

Soda Pop Pressurizers - answer :eek:ut:

No it will not - save your money. Likewise adding oxygen to the breathing gas will not increase the amount of nitrogen in the tissues.

Dr Deco :doctor:
 
Uncle P:

No problem. I appreciate your help since I am not always able to get to these letters as quickly as I might like.

You know my motto, "I am always prompt, no matter how long it takes."

:doctor:
 
Hi guys,

Just to get these concepts clear in my befuddled head could you run through a couple of examples for me?

Am I right in considering EAD is of no use with Trimix?

What is the EAD of Nitrox 28% at 40 metres?

What is the END of Nitrox 28% at 40 metres?

What is the END of Trimix 18/35/47 at 50 metres?

Now for some real exposure! I was a little surprised you did not jump in following my contribution to this thread, started by King Kong Matt, asking for an explanation of the oxygen window.

. . and what is toggling? :)
 
Dear Dr T:

If I understood you correctly, there was said to be some link between hemoglobin and oxygen pressure reduction. To my understanding, the "partial pressure vacancy" as it is properly termed, results solely from metabolism. "Oxygen window" is just some shorthand term.

Dr Deco:doctor:
 
Dr Paul Thomas once bubbled...
Hi guys,

Just to get these concepts clear in my befuddled head could you run through a couple of examples for me?

Am I right in considering EAD is of no use with Trimix?

What is the EAD of Nitrox 28% at 40 metres?

What is the END of Nitrox 28% at 40 metres?

What is the END of Trimix 18/35/47 at 50 metres?

Now for some real exposure! I was a little surprised you did not jump in following my contribution to this thread, started by King Kong Matt, asking for an explanation of the oxygen window.

. . and what is toggling? :)

I’ll try...
As I’m sure you know Equivilant Air Depth is used to determine the depth at which we have the same PP of inert gas using air as with the nitrox mix we are using. EAD is used to allow us to calculate decompression status for a nitrox dive using air tables. The equation is...

(Depth in feet + 33) * Fn2/0.79 - 33 = EAD

When using trimix we don’t attempt to use air tables to calculate decompression status. We use END to determine the depth at which air will yield the same level of narcosis (PP of narcotic gas).

There seems to be some cross use of terms in texts. Some don’t think of O2 as being narcotic. Therefor Equivalent Narcotic Depth has the same meaning as Equivalent Nitrogen Depth (or EAD). In this case the equation used is the same as given above. This also explains why some insist that nitrox is less narcotic than air.

However today O2 is thought to be at least as narcotic as N2. If we assume O2 to be equally narcotic the equation for END becomes...

(Depth in feet +33) * (1 - Fhe) - 33 = END

Here the only non-narcotic gas is He. Also some blending and decompression software allows the user to select whether of not O2 is to be considered narcotic.

Toggle effect....
I’m not the one to explain this but I’ll try a non technical explanation. The bottom line of the theory is that since O2 is a vasoconstrictor continued exposure to high PPO2 reduces the off gassing efficiency of the lungs. Periodic breaks to back gas are used to undo the temporary lung damage and increase off gassing efficiency and at the same time hold off O2 toxicity.

In use divers will break to back gas for part or all of a stop prior to a stop with a switch to O2 and at regular intervals thereafter. Total decompression time is not adjusted to compensate for the time spent on back gas. On dives requiring long decompressions divers are exposing themselves to many time the NOAA recommended O2 exposure limits.

This was discussed a while back in the context of the effectiveness of air breaks (if I remember right). Some of you doctors pretty much blew the theory out of the water. However the fact remains that is seems to work. The WKPP for instance are subjecting divers toO2 exposures equal to thousands of percent of the NOAA O2 exposure limits.

I hope I didn’t mess that up too bad.
 
MikeFerrara once bubbled...


I hope I didn’t mess that up too bad.

No but you just convinced me that I needed an Advanced Nitrox course.

:wink:
 
MikeFerrara once bubbled...
I’ll try...
As I’m sure you know Equivilant Air Depth is used to determine the depth at which we have the same PP of inert gas using air as with the nitrox mix we are using. EAD is used to allow us to calculate decompression status for a nitrox dive using air tables. The equation is...

(Depth in feet + 33) * Fn2/0.79 - 33 = EAD
Hi Mike,

To this semieuropean this translates to

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

In my example this works out at

(40 +10) * (1-0.28)/0.79 - 10 = 35.6 metres

Thus a dive to 40 meters using Nitrox 28 is the same for decompression purposes as a dive to 35.6 metres on air. Of course this is largely academic as most now use dedicated Nitrox tables (or Nitrox dive computers).
When using trimix we don’t attempt to use air tables to calculate decompression status. We use END to determine the depth at which air will yield the same level of narcosis (PP of narcotic gas).

There seems to be some cross use of terms in texts. Some don’t think of O2 as being narcotic. Therefore Equivalent Narcotic Depth has the same meaning as Equivalent Nitrogen Depth (or EAD). In this case the equation used is the same as given above. This also explains why some insist that nitrox is less narcotic than air.

However today O2 is thought to be at least as narcotic as N2. If we assume O2 to be equally narcotic the equation for END becomes...

(Depth in feet +33) * (1 - Fhe) - 33 = END

Translating

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

Thus for my trimix dive to 50 metres using 18/35/47 Trimix gives;

(50 +10) * (1- 0.35) -10 = 29 metres.

A mix containing 35% helium to 50 metres has the equivalent narcotic effect of diving air to only 29 metres .
Here the only non-narcotic gas is He. Also some blending and decompression software allows the user to select whether of not O2 is to be considered narcotic.
I was hoping to do a trimix course this year but that's another story. I feel the jury is still out concerning the narcotic effects of oxygen but if this could be discounted the END equation becomes

(Depth in feet +33) * (FN2)/0.79 - 33 = END

Translating

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

Thus a trimix dive to 50 metres using 18/35/47 would give;

(50 +10) * (0.47)/0.79 -10 = 25.7 metres. END, which to me seems too optimistic for a 50 metre dive.

In addition the additional factor makes this equation even more cumbersome to use.

Regardless of whether oxygen is narcotic or not, the addition of helium certainly reduces the narcotic effects of the breathing medium to a remarkable extent.

I think that's about right?
 
https://www.shearwater.com/products/teric/

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