Calculating proper PSI

[DannyBoy21]

What are the variables that go into calculating the proper PSI? Is there an equation out there, or is it mostly trial and error? I figure the following must be taken into account: type of beer, beer line length, temperature of beer, altitude.

I'm trying to get my 1/2 barrel bud light keg right. I know 12-14 psi is the range. I've got the standard 5 feet of beer line from the tower to the keg which resides about a foot below (chest freezer). I'm pretty close to sea level (So Cal). If my temperature is lower, does that mean I need more psi? Likewise, if my temperature is higher, does that mean I need less psi?

[Scott Zuhse]

Carbonation level of the beer is required along with exact beer temperature and elevation. The beer line length is secondary and has no impact on the PSIG (pounds per square inch gauge) to the keg. The longer the line, the slower the flow - shorter, faster.

Bud Light is a 2.6 v/v (volume) carbonated beer and requires 14 PSIG @ 38 F @ sea level (1-1000'). Increase one PSIG for every 2000' elevation change above 1000'.

Adjust PSIG one pound for every two degree temperature change from 38 F. Up for warmer, down for colder. Before attempting to tune your system, you must have a handle on beer temperature using a calibrated thermometer. Beer temperature must be constant 24/7 from the keg to the faucet to be most successful with your PSIG setting.

Before adjusting PSIG based on your temperature, be absolutely certain that you do not have any pressure leaks and your system is clean with no obstructions, sharp edges, etc. which can cause the beer's carbonation to "rake" out of the beer.

[DannyBoy21]

Thanks for the thorough answer, Scott!

[psychodad]

Here is a IMHO really good link discussing c02 and temperature.
Home draught dispensing page!

Note this illustratation

Illustration "A" represents CO2 pressure adjusted properly for the storage temperature and carbonation level of the beer. There is an equal amount of CO2 dissolving into and breaking out of the beer.

Illustration "B" represents too little CO2 pressure. Gas breaks out of the beer into the head space of the keg and in the lines. Pockets of gas will accumulate in the lines creating pouring problems as well as flat tasting beer.

Illustration "C" represents too much CO2 pressure, more CO2 is dissolving into the beer than is breaking out. The carbonation level of the beer is increasing. Pouring problems and "buzzy" tasting beer will result when the keg is low and dispenses the over carbonated beer. Over carbonated is often perceived as tasting extra bitter.

[cubby_swans]

I use this page to determine the pressure to apply to a keg.

Maintaining Draft Beer Equilibrium