Do any of you guys know how long it takes to purge a serving keg with fermentation gas from the fermenting vessel?
Water purged vs. just hooking it up without doing any “pre” purge?
About as long as fermentation proceeds. Here's a link to discussion/formulae about how effective different approaches are to purging a keg. But how effective this is depends on how much CO2 is produced, which is dependent on the amount of sugar in the wort and the degree of attenuation.
http://www.********************/bre...rging-transferring-stabilizing-finished-beer/
How effective pressurized purging of a keg will be depends on the pressure; purging at 30psi will work faster/better than 10psi simply because you're increasing the ratio of CO2 to O2 in the keg at the higher pressure, then venting it.
********
Here's what I do: I fill a keg w/ Star-San, then push it out with CO2. This produces a pretty darned low-O2 environment in that keg. Then I connect it to the fermentation gas output of my fermenter to continue to purge out whatever O2 remains in that keg. Overkill? I don't know. All I know is that fermentation gas is free, I might as well use it.
And starting so low as a result of a star-san purge means I will truly get very, very close to 0 ppm O2 in the keg when I start filling it.
Sometimes I'll hook up a gas QD on the keg and run it into a blowoff jar so I can monitor using the bubbling to know when I'm about done.
Like so; the fermenter is covered up with blankets, but the ferm gas line comes out of the fermenter to the black QD, then another line from the gray QD to the blowoff jar.