I follow most of that math but am not sure I follow the last bit.
If we are comparing chart to chart or reg pressure to reg pressure wouldn't I be applying 7.5 psi of co2 at 30 psi in a 75:25 mix?
Regardless of sea level or other factors, the gauge is reading pressure applied to the keg isn't it?
Another question, if the gauge on the regulator reads 30 psi, should any gauge read 30psi (ie tire gauge etc) or should they be reading the 30 plus atmospheric. I think just 30 also but want to be sure. This thing has been frustrating the hell out of me so I want to be darn sure I'm understanding this and get things right this time before I say f it.
Thanks for the help
Pressure is a measure of how much gas is present in a given volume. The total number of gas molecules in a given volume is equal to the absolute pressure times the volume divided by a constant and temperature.
Number of Gas Molecules = (Abs_Pressure * Volume) / (Constant * Abs_Temperature)
Usually written as PV = nRT
Temperature is absolute temperature measured in °Kelvin. °Kelvin = °Celsius + 273.16°. Atmospheric gas fills all spaces at 14.7 psi absolute, unless special effort taken to remove it.
Almost all common use pressure gauges measure the difference between atmospheric pressure and the pressure in the vessel of interest. These include compressed gas gauges and tire gauges. So, they all read 0 psi when the actual absolute pressure is 14.7 psi. If you add gas to a keg so that the gauge reads 10 psi, then you have the original 14.7 psi that was there before you did anything, and the additional 10 psi you added, so the total absolute pressure in the keg is 24.7 psi. When you vent a keg completely, the pressure drops back to 14.7 psi absolute. If you pulled a complete vacuum on the keg prior to pressurizing (you can't actually do this if there is liquid in the keg), the gauge pressure in the keg would be -14.7 psi (even though the gauge can't read below 0, usually), and the absolute pressure would be 0 psi.
The partial pressure of a gas is equal to the total absolute pressure times the volume fraction of that gas. So for example the partial pressure of O2 in sea level air is 0.21 * 14.7 = 3.09 psi.
Partial pressures are always absolute pressure. You cannot measure partial pressure with a gauge (unless the space contains a single gas species, and it is an absolute pressure gauge.) So, partial pressures need to be calculated from absolute pressure and a knowledge of the total gas composition.
The equilibrium between any gas in the local atmosphere (like the headspace of a keg) and the gas dissolved in a liquid in contact with the local atmosphere, is determined by the temperature and the partial pressure of the gas. So, what matters for determining the equilibrium volumes of CO2 in beer is the
partial pressure of CO2 and the temperature.
When the keg headspace is 100% CO2, then the CO2 partial pressure is equal to the total absolute pressure. And since absolute pressure is just gauge pressure + 14.7 psi (at sea level), it's easy to "fudge" the carbonation charts and calculators, so that you can work with gauge pressure. However, the common charts and calculators are not designed for use with beer gas, so you cannot use them. It is possible to make calculators that work for different gas mixtures (I have one), and charts that work for a particular gas mixture (but the charts won't work for other gas mixtures.)
When dealing with beer gas, you need to do all the math with partial pressures, not gauge pressures, and then calculate what gauge pressure will give you the desired results. Here's a screen capture from my calculator that handles various beer gas mixtures showing results for a carb level typical of stouts, etc.:
If you want to put something on nitro at a higher carb level, things get out of hand.
Finally, the best way to carb a keg for use on nitro, is to carb it with pure CO2 initially at the CO2 volumes you desire. Then, purge and repressurize a couple of times with beer gas at the appropriate serving pressure. The reason for this is if you put it on beer gas for initial carb, the beer will preferentially absorb the CO2 from the headspace, and the make up gas for the head space will be mostly N2. Thus as the beer carbs, the partial pressure of CO2 in the headspace decreases, and you end up with less carbonation than you intended. (However, since a lot of ales are going to be at almost 1 volume of carbonation at the end of fermentation, putting on beer gas immediately will probably be acceptable.)
Who knew beer gas (nitro) was so complicated?
Brew on
