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Blending Trimix October 19, 2009

Posted by Chris Sullivan in Technical Diving.
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As described in a prior post, I recently took the DSAT Gas Blender course which includes Trimix blending. No formulas were provided for trimix blending but you get a handy little program for calculating the blend for a partial pressure mix or a top-up for trimix, which is a whole lot simpler than doing it by hand. However, it’s not really difficult to calculate trimix blends using the method I came with during the class.

The most  difficult part of blending Nitrox is that the Oxygen in air comes premixed with 78% Nitrogen. If we were making Nitrox from pure Oxygen and pure Nitrogen anyone proficient in mental arithmetic could figure out the blends in his or her head. For instance, a 3000 PSI tank of EAN36 would need 1,080 (3,000 x .36) PSI of Oxygen and 1,920 PSI of Nitrogen, but as we generally top up with air, only 570 PSI of Oxygen is needed because the air already contains some.

As Helium is not a significant component of air, we can use this simple method to calculate the required partial pressure of Helium in any mix. So a 16/40 mix (40% Helium) has 40% x 3,000 or 1,200 PSI of Helium and 1,800 PSI of some combination of Oxygen and air. Similarly, a 4/80 mix has 2,400 PSI of Helium in a 3,000 PSI tank, with 600 PSI of Oxygen and Nitrogen.

So what about the rest? This is a little more complicated, but there’s a fairly easy shortcut. Let’s look at an 18/50 mix, since this is an easy example. This mix is 50% Helium, which would be 1,500 PSI in our AL80. The other 1,500 PSI is Nitrox, and our goal is to get a final blend of 18% Oxgyen. To do this, we need to top up the tank with 36% Nitrox. Why? Because ½ a tank of 36% Oxygen has the same O2 content as a whole tank of 18% Oxgyen. From there, it’s a simple matter of looking up what  pressures are needed to blend 36% Nitrox at 1,500 PSI. Nitrox blending tables should show this, and most people who have done Nitrox filling will  know that 36% into a 3,000 PSI requires 570 PSI of Oxygen, so a 1,500 PSI fill would need ½ that which is 285 PSI.

So our 18/50 mix will need 1,500 PSI of He, 285 PSI of O2, and be topped off to 3,000 with 1,115 PSI of air. I contrived this example so that the numbers would work out conveniently for me, especially with exactly ½ the tank containing Helium. What about other blends?

If we were looking for a 21/40 mix, clearly we’d need 1,200 PSI of Helium. The other 1,800 PSI would contain some Nitrox blend. To figure out the Oxygen percentage, we divide the desired percentage of 21% by one minus the Helium fraction, so in this case it’s (1-.4) or 60%. This turns out to be EAN35. So you can look up a 35% mix in a 1,800 PSI tank for the amount of Oxygen and air that needs to be added. You can also use the Nitrox blending formula of (W-.21)/.79 x PSI  (where W is the percentage of Oxygen wanted) to calculate it. This works out to 319 PSI of Oxgyen with the remaining 881 PSI topped off with air.

Note that there are some blends that can’t be made The 4/80 mix, for instance, can’t be made exactly. I chose this in my example because  it is the mix that was used by David Shaw when he died cave diving in South Africa at a depth of 270 metres. The reason it can’t be made exactly from Oxygen and air is the remaining 20% of the mix that isn’t Helium is only 20% Oxygen, and this Nitrox mix can’t be blended with a combination of Oxygen and air. I’m  sure though, that in this case his diluent was just air, and the mix was 4.2% Oxygen, rather than exactly 4%.

So what he was diving was actually “Heliair”, the simplest Trimix blend around, made from combining Helium and air with no added Oxgyen. The disadvantage of this mix is that at the optimum MOD of Oxygen, usually considered to be a partial pressure of 1.4 (although many rebreather divers use 1.3, and some open circuit divers use 1.5 or 1.6 under ideal diving conditions or in deco), has the same END (Equivalent Narcotic Depth) as air  at the same O2 partial pressure. This is true at any depth as the ratio of Nitrogen to Oxygen is the same as air for any mix of Heliair.

Deon’s END was 46 metres or about 152 feet, which is greater than many divers would care to venture, although less than many others including me have regularly dived, and his PPO2 1.12, which is well within acceptable boundaries. You can see from this though that he was getting quite exposed to narcosis while being well within the acceptable limits for Oxygen Exposure (at least from a CNS perspective, it was scheduled to be a long dive and Pulmonary exposure would undoubtedly be a concern). This points out the problem with Heliair – long before you’re at your CNS limits you will be well-narked. This extreme example has other considerations, so don’t take this as saying that for this dive his mix was incorrect or that narcosis contributed to his accident.

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