EAN GAS MIX CALCULATOR HELP FILE

Gary C. Kessler

This online calculator is a utility for EAN gas blenders. It should only be used by those certified in partial pressure gas blending.

Given a tank with a known starting EAN mix (M_{S}) and pressure (P_{S}), and a desired EAN mix (M_{D}) and pressure (P_{D}), the calculator will display to what pressure you need to add your fill gas (P_{F}), followed by the pressure to add of your top-off gas (P_{T}). The calculator assumes that the fill gas is 100% oxygen (M_{F} = 100) and the top-off gas is air (M_{T} = 20.9).

The estimated fill time is based upon the assumption of filling at a rate of 1 psi (0.068 bar) per second, which is the recommended rate when filling with 100% oxygen.

In some cases, you need to bleed off some of the tank before you can hit your desired pO_{2}. In those cases, the calculator will provide an appropriate message.

The Starting and Desired Mix must be between 20.9% and 40%, and the Starting and Desired Pressure must be between 0 and 4000 psi/270 bar. Incorrect input will be flagged with an error message.

Data entry hint: If the starting tank pressure is 0 psi/bar, the starting mix value doesn't matter, as long as it is valid. Note, however, that an "empty" tank actually has a starting pressure of 14.7 psi or 1 bar, and a starting pO_{2} of 20.9.

As an aside, the mathematics behind the calculator is actually just some basic algebra. We know from physics (Dalton's Law, to be precise) that the composition of a gas is the sum of the composition of the component gases that make it up and the pressure of a gas is the sum of the pressure of the component gases (e.g., air is 79% N_{2} at 14.7 psi/1 bar and 20.9% O_{2} at 14.7 psi/1 bar).

So, after all of this, we get these two equations:

- P
_{D}xM_{D}= P_{S}xM_{S}+ P_{F}xM_{F}+ P_{T}xM_{T} - P
_{D}= P_{S}+ P_{F}+ P_{T}

We know the mix of the fill and top-off gas. We can measure the starting pressure and mix of what's in the tank. The customer will tell us the desired pressure and mix. Therefore, all we need to determine is the needed pressure of the fill and top-off gases. Thus, we have two equations and two unknowns. Simple algebra gets us the required formula.

If you'd like to read more about the math, see my paper, *The Mathematics Behind Partial Pressure Gas Blending*. If you'd like to use the spreadsheet version of the calculator, download my spreadsheet.

Feel free to direct any questions or comments to me at kumquat@garykessler.net.