Gas Blending in Metric is a lot more intuitive as well. . .
Starting from an empty tank or set of tanks (5 .5L, same as AL40 deco cylinder; and 11L, same as AL80 in doubles/twinset backgas Cylinders):
To get Nitrox 50 add 37% O2:
That is, for every 100 bar of Eanx50 deco mix, you need 37 bar of pure O2 and top off the remainder with [hyper-filtered clean] Air;
Nitrox 32 (add 14% O2):
For every 100 bar of Eanx32 mix, you need 14 bar of pure O2 and top off the remainder with Air;
20/20 Trimix (add 4% O2 & 20% Helium):
For every 100 bar of 20/20 Trimix, you need 4 bar of pure O2, 20 bar of He and top off remainder with Air;
25/25 Triox (add 12% O2 & 25% Helium):
For every 100 bar of 25/25 Triox, you need 12 bar of pure O2, 25 bar of He and top off remainder with Air;
30/30 Triox (add 19% O2 & 30% Helium):
For every 100 bar of 30/30 Triox, you need 19 bar of pure O2, 30 bar of He and top off remainder with Air;
21/35 Trimix (add 9% O2 & 35% Helium):
For every 100 bar of 21/35 Trimix, you need 9 bar of pure O2, 35 bar of He and top off remainder with Air;
18/45 Trimix (add 8% O2 & 45% Helium):
For every 100 bar of 18/45 Trimix, you need 8 bar of pure O2, 45 bar of He and top off remainder with Air;
15/55 Trimix (add 7% O2 & 55% Helium):
For every 100 bar of 15/55 Trimix, you need 7 bar of pure O2, 55 bar of He and top off remainder with Air;
12/60 Trimix (add 5% O2 & 60% Helium):
For every 100 bar of 12/60 Trimix, you need 5 bar of pure O2, 60 bar of He and top off remainder with Air;
10/70 Trimix (add 4.5% O2 & 70% Helium):
For every 100 bar of 10/70 Trimix, you need 4.5 bar of pure O2, 70 bar of He and top off remainder with Air.
Using a 11L (AL80) cylinder, or set of twin 11L doubles (double AL80's) for a total of 22L, a full tank or set of tanks is 200 bar:
-->Therefore, all you need is 2 times the amount in bar for O2 (& He for Trimix) from above recipes for a particular mixture.
Example)
21/35 Trimix requires blending 9% Oxygen and and 35% Helium; Therefore a full 11L (Al80) cylinder(s) at 200 bar total pressure needs 18 bar of O2 (2 times 9 bar equals 18 bar), and 70 bar of Helium (2 times 35 bar equals 70 bar); and top off remainder to 200 bar with [hyper-filtered clean] Air.
Gas Blending in Metric (cont.):
Now how did we get the "cookbook recipes" for blending the particular mixtures above? And why for instance with Nitrox 50, can't we use 50 bar of Oxygen and 50 bar of Nitrogen for every 100 bar of Eanx50 to blend a seemingly real intuitive "half & half" mixture?
The answer is YES! You can blend 50 bar of O2 and 50 bar of N2 -->If you are mixing pure O2 AND PURE N2 TOGETHER!!!
But for practical means, why do you need pure N2 when you can just use Air with its natural constituent contribution of 21% Oxygen & 79% Nitrogen?
Using Air this time, let's blend 100 bar of deco mix Nitrox 50:
We know empirically that for every 100 bar of Eanx50, 50 bar must be O2 and 50 bar must be N2;
Quantitatively then, how much Air do we need to add in order to give us a N2 amount of 50 bar?
Algebraically and by Dalton's Law, you divide 50 bar N2 by 79% (the %age of Nitrogen in Air), and this yields approximately 63 bar of Air needed. And of this 63 bar of Air, 21% of it (the %age of Oxygen in Air) contributes to the amount needed for O2: approx 13 bar of Oxygen.
Therefore, instead of initially mixing in a full 50 bar of O2, and since we're using Air instead pure N2, you only need 50 bar minus the 13 bar Oxygen contribution from Air, which equals 37 bar. So hence the cookbook recipe of for every 100 bar of Nitrox 50, add 37 bar of pure O2, and fill the remainder with Air to 100 bar (100 bar total minus 37 bar of O2 equals 63 bar --the amount of Air needed for N2 which we calculated above).
Now to fill an empty 5.5L/bar deco tank (same as an AL40) with Nitrox 50, we need 200 bar of the above recipe (i.e. "For every 100 bar of Eanx50, add 37 bar of pure O2, and then fill the remainder with Air"). Therefore just multiply 37 bar by 2 which equals 74 bar of O2 needed, and then fill the remainder with Air to the total fill pressure of 200 bar.
Similarly, it all applies also to the Trimix recipes --the only difference is accounting for the exact percentage and amount in bar of the Helium constituent in the total mixture . . .
