MikeFerrara once bubbled...
Ill try...
As Im 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 dont 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 dont 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?