Using 65/35 Beer Gas for Nitro Faucet?

UncleRusty said:

Hey guys,

I'm looking to setup my first beer on Nitro draft, and have everything setup except for buying the blended gas/cylinder itself. My local gas/welding supply store only offers beer gas in a 65/35 N to CO2 ratio, and I'm wondering how well this will work for serving stouts and porters on a Micromatic Nitro Faucet?

I typically run my kegerator at around 42-45F, and most people I see using a 75/25 gas ratio. My concern is that this may be too much CO2 and result in the beer overcarbing.

Should I try to track down a different supplier who offers a more standard 75/25 blend, or will this be just fine so long as I lower the serving PSI slightly?

Click to expand...

doug293cz said:

@ 43.5°F (midpoint in your temp range), 1.1 volumes requires 21.4 psi of 35/65 beer gas to maintain steady carbonation, and 1.2 volumes requires 24.7 psi (assuming you're not too high above sea level.) So, let's just say: set it to 23 psi.

Brew on

Click to expand...

UncleRusty said:

Thanks, I appreciate the help!

Click to expand...

doug293cz said:

U R welcome. If you are interested in understanding the math and science behind the answer, feel free to ask. Just be warned it will be a technical response.

Brew on

Click to expand...

UncleRusty said:

Absolutely, if you don't mind that would be awesome. If I can run the calculations myself that would make the whole process much easier.

Click to expand...

william_shakes_beer said:

If I set my (co2 only) gage at a serving pressure of 23 PSI I'd have beer spraying half way across the room. Why is the pressure setting for a co2/nitrogen mix so much higher than co2 alone? Is nitrogen more dense? I accept your explanation of how to do the calculations. I'd be interesed in understanding the concept as well.

Click to expand...

doug293cz said:

The first thing you need to understand is the concept of partial pressure. You can read about that here: https://en.wikipedia.org/wiki/Partial_pressure. Then you need to know that the equilibrium carbonation level is determined by the temperature and the partial pressure of CO2 in the headspace. In a normal CO2 only case, the CO2 partial pressure is equal to the total pressure, since there are no other gases in the headspace. In a beer gas system, the CO2 partial pressure is equal to the total pressure times the fraction of the gas that is CO2 (0.35 in your case.) The last piece of the puzzle is knowing that the carbonation tables and calculators work with gauge pressure, but the total pressure in the headspace is atmospheric pressure plus the gauge pressure.

So here's how the math works for your case (1.15 volumes @ 43.5°F, 35/65 CO2/N2.):

Step 1: Look up, or calculate the gauge pressure corresponding the equilibrium partial pressure for your temp & carb level. Since the temp & carb tables don't include the combination you are after, you need to use a calculator like this one: http://www.brewersfriend.com/keg-carbonation-calculator/. This turns out to be -1.5 psi (yes a negative gauge pressure.)

Step 2: Convert the gauge pressure to total pressure by adding the atmospheric pressure to it. At sea level, atmospheric pressure is 14.7 psi. Thus the total required CO2 pressure in the headspace is:

-1.5 psi + 14.7 psi = 13.2 psi​

Step 3: Calculate the total pressure (CO2 + N2 pressure) needed to have a CO2 partial pressure of 13.2 psi, when the gas mix is only 0.35 CO2:

Total pressure = 13.2 psi / 0.35 = 37.8 psi​

Step 4: Convert the total mixed gas pressure back to gauge pressure, so you know how to set your regulator. Do this by subtracting atmospheric pressure from the total pressure:

Gauge pressure = 37.8 psi - 14.7 psi = 23.1 psi​

Since you can't read the pressure gauge to less than +/- 1 psi anyway, 23 psi is your target pressure.

Brew on

Click to expand...