[flbrewer]

Can you explain to me how an already carbonated keg and a CO2 tank set to serving pressure (~10 PSI) react?

Is the CO2 tank constantly supplying CO2 to the keg or only when you bleed off and pour a pint?

Could you back down the pressure to say 4-5 PSI to an already carbonated keg to simply push the beer out?

Finally, how long would a carbonated keg stay carbonated without constant CO2?

[hang5lngbd]

The CO2 tank IS constantly supplying CO2 at the regulator's set psi. If you leave you keg at force keg pressure 10-15psi, then you will end up with over carbonation and foam. I force carb at 13psi, then after 2 weeks move to serving kegerator at 5psi. 5psi is perfect for a slow pour especially if a beer comes out a bit foamy.
Once a keg is force carbed, I leave it at cellar temp with no gas hooked up. Has stayed carbonated for 3 months in there. Haven't tried longer.

[morticaixavier]

when the co2 bottle is connected to a keg and all connections are open the system between the regulator and the keg are a single vessel. if you disconnect the keg then it is an independent system. it will stay carbonated and at whatever pressure you set it at (assuming no leaks) more or less forever. If you pour a pint the pressure will drop in the headspace first but over a relatively short time co2 will come out of solution and everything will equalize again. Think of a glass of soda on the counter. it will lose co2 until it comes into equlibrium with the atmosphere.

if your keg is where you want it carbonation wise at 10 PSI and you serve at 5 and leave it there at first no co2 will flow from the bottle to the keg. Once you've served enough beer to drop the pressure in the headspace to below 5 more co2 will be supplied from the bottle. over time however the beer will lose co2 until it is carbonated at 5psi which will seem flat.

I generally turn the gas back up to carbonation pressure at the end of a day/weekend and leave it there, adjusting to serving pressure while I drink.

[JT]

There's a reason the set it and forget method is named as such.  If you have correctly calculated your psi, temp, tap line length and height, your beer will not require a separate serving pressure and no, it will not overcarb. 

[flbrewer]

^i believe I read that setting as 10-12 PSI, not 5.