The Art of Gas Blending
By Karl Shreeves
Consider a typical tek trimix dive-you breathe various blends of helium, oxygen and nitrogen, none of which occur naturally. Helium, nitrogen and oxygen occur in nature, but not in the various mixes for the dive, so they have to be made.
There are several methods for blending synthetic gases. Industrial gas suppliers mix by weight, which is possible because gases differ in molecular weight. This works with large volumes using highly precise instruments, but not with your aluminum 80 and a bathroom scale.
A method used for making enriched air (nitrox) pushes air (79 percent nitrogen, 21 percent oxygen) through a membrane that removes some of the nitrogen, leaving a greater proportion of oxygen. The faster you send air through the separator, the more nitrogen comes out, allowing you to control the resulting blend. Dive centers that produce a lot of nitrox often have this type of system.
However, membrane blending is only efficient with large volumes. You can't use a membrane to make enriched air with more than 40 percent oxygen, nor can you make trimix simply by running air through it. For these reasons the most common blending method is partial pressure blending.
With partial pressure blending, you add gases to scuba cylinders in the desired proportions to get the mix you want. This is possible thanks to the Ideal Gas Law, which says all gases compress the same and exert the same pressure under the same conditions, irrespective of their other properties. For example, if you want 3,000 psi of EAN50, you could fill your tank with 1,500 psi of pure oxygen, then 1,500 psi of pure nitrogen; the resulting blend should be 50 percent nitrogen and 50 percent oxygen.
Not So Simple
But unfortunately, it's not really that simple. For one, you want to blend using as much air as possible because, frankly, it's cheap and available. To make the aforementioned EANx50, then, you fill your tank with 1,101 psi of oxygen and 1,899 psi of air to end up with the 50-50 blend. This works because 399 psi (21 percent) of the air is oxygen. 1,101 + 399 = 1,500 psi of oxygen. Of the 1,899, 1,500 psi (79 percent) is nitrogen.
You can make almost any EANx or TMx you want from helium, oxygen and air by mathematically determining the component gas pressures. Suppose you want 3,000 psi of TMx20/40 (20 percent oxygen, 40 percent helium, 40 percent nitrogen). This means the final blend is 600 psi oxygen, 1,200 psi helium and 1,200 psi oxygen. You put 281 psi of oxygen in the cylinder (oxygen always goes first for safety reasons), then 1,200 psi of helium and then top it off with air (1,519 psi) to 3,000. Note that 79 percent of 1,519 (the nitrogen) is 1,200 psi. The remaining 319 (the oxygen) + 281 (the oxygen you started with) = 600 psi (desired final amount).
By applying math, you can even blend a new mix into a partially filled cylinder with another blend. When you take a blender course, you learn the formulas for determining how much of each gas you need using only a standard calculator. After you're certified, you'll forget the formulas and use a laptop computer program. It's a whole lot easier and less error prone.
Besides a computer, blending calls for highly accurate gauges (your SPG won't cut it), along with oxygen compatible equipment and procedures to avoid fire/explosion hazard (I'll touch on that in a future Totally Tek-suffice it to say that it's not safe to run pure oxygen through equipment rated only for standard breathing air).
You also need an oxygen analyzer, which tells you the oxygen percentage in a blend. Although you may deal with up to three gases, you only need to track oxygen if you accurately analyze at each blending stage. With nitrox it's no sweat-anything that's not oxygen is nitrogen, so if your analyzer says 27 percent oxygen, then you can be sure the 73 percent remaining is nitrogen. Trimix calls for more precision because when you're finished, if your analyzer tells you the blend is 20 percent oxygen, you can't be sure of the helium and nitrogen proportions unless you checked at each step.
Still Not So Simple
Okay, you've got your computer program, oxygen compatible equipment, analyzer and accurate gauges. Now it's pretty simple, huh?
You wish. Although you partial pressure blend based on the Ideal Gas Law, it is not exact. That is, owing to differences in molecular shape, some gases compress more easily than they ideally should-especially oxygen. The higher the pressure, the more "out of line" the compression, so when you analyze your final blend, you find less oxygen than the Ideal Gas Law predicted.
Then there's temperature. As you fill a cylinder, it heats, causing pressures to rise. When the cylinder cools, the pressure drops. In an ideal world, you do all your filling at the same temperature by allowing the cylinders to cool completely before going to the next gas. But in the real world, you don't always have the time to do this.
This is where the "art" of blending comes in. While you can use formulas to account for oxygen's real compression properties and minor pressure variations caused by temperature differences, that's seldom how you handle it. With experience, you learn to tweak the gas amounts as you blend to account for compressibility and temperature so you don't have to correct and adjust when you're done.
This may sound imprecise, but the reality is that when you analyze the final mix, an experienced blender usually ends up within one-half percent of the desired blend on the first try (plus or minus one percent is acceptable) compression and temperature be damned. So the next time you see someone blending enriched air or trimix, show some respect. You're looking at an artist.