nitro, again

[tschmidlin]

Hi everyone,

I've read through the nitro threads and I think I understand what works and plan to try something out. I have to say I'm a little confused about the theory though.

Whatever way you get the CO2 into the beer, everyone seems to agree that a nitro beer should be carbonated to under 2 volumes. People also seem to agree that nitro beers should be served at roughly 35 psi with a 75/25 N2/CO2 mix.

If it is 35 psi, then the partial pressure of CO2 is nearly 9 psi. At that pressure and normal serving temps, there will (eventually) be about 2.4 volumes of CO2 in the beer. Last I checked, 2.4 is not less than 2.

Does anyone have an explanation for this?

[TL Services]

The serving pressure is made up from two components:

A - The balance (equilibration) pressure to keep CO2 and N2 in solution
B - Additional pressure to move the beer to the tap whilst overcoming pressure losses in the line

To maintain an absolutely correct dissolved gas mix, the dispense gas proportions should match the dissolved levels; so, dispensing a lightly-nitrogenated beer with a gas that is primarily N2 will result - over time - in an increased N2 level in the beer and, likewise, dispensing a heavily-nitrogenated beer with a low-N2 gas will cause the N2 level to drop, over time.

Getting the two gases into solution, with stability, and maintaining those levels during dispense is not a simple matter as there is a constant interchange of gas between dissolved and gaseous states. Anything that can affect this equilibrium will do so, and the effects can be very marked.

[jwalts]

The gas blend is based on absolute pressure, not gauge pressure. So at sea level, 35 psig = 35 + 14.7 = 49.7 psia. 25% of that is 12.4 psia, which becomes 12.4 - 14.7 = -2.3 psig. That gauge pressure will result in about 1.2 volumes of CO2 at 38 degf, which is pretty typical for nitro beers.

Joe

[tschmidlin]

Thanks Joe, that makes sense. I suppose if gauge pressure vs. absolute pressure can trip up Neil deGrasse Tyson I can forgive myself!

I appreciate the help.

Tom