Tuesday, May 18, 2010

Hypothesizing Zebras

So, this past Sunday Mr. Cicerone and Skip brewed up some dunkel to commemorate the wedding of their friend, Fred. Fred lives in Tanzania, where dunkel can be hard to come by. In the picture below, Fred is drinking the dread Safari lager, (possibly the worst beer Mr. Cicerone knows of,) while he helps Skip brew his first batch of African homebrew.

But this isn't a post about any of that. It's about draft beer. While we were brewing, Skip related some recent troubleshooting he'd done, mostly over the phone, with a homebrewer friend who'd recently started kegging his beer. The complaint was foamy beer. BEGIN LENGTHY DIGRESSION This is one of Mr. Cicerone's pet peeves: many homebrewers seem to believe there exist local variances to the basic laws of physics. Homebrewer forums are full of discussions about foamy beer, with assertions of "I always dispense at 4PSI." and the like. (The preceding example would only be OK if the entire keg were being served over the course of a couple hours, as it would go flat otherwise.) There are really only two rules for direct dispensing draft systems:

1) For any given serving temperature and carbonation level, there is exactly one regulator pressure that will keep that carbonation level constant through the life of the keg. CO2 solubility decreases as temperature increases, so to keep the same carbonation level if the serving temperature is increased, the PSI must also increase. Conversely, temp down, pressure down. Three variables: pressure, temperature, and v/v dissolved CO2. Many handy charts like this one illustrate the relationship. This is established physics, so accept it.

2) Rule #1 must be abided by. If the beer comes out too fast, restriction needs to be added to the dispensing line. For a given length, smaller diameter hose provides more resistance to flow, slowing the beer down. Micromatic has some great resources available on their website for understanding draft system operation. Check it out. END OF LENGTHY DIGRESSION

Anyway, (remember Alice?) As Skip related troubleshooting the problem Dave, the new-to-kegging homebrewer, was having with his foamy beer, I could see the punchline coming up. Skip asked about temperature, pressure, if time enough had been allowed for equilibrium to be reached, all of which seemed OK, before discovering that Dave was pouring the beer wrong, causing foaming. BEGIN NOT-AS-LENGTHY DIGRESSION A standard beer faucet or picnic tap has 2 positions: open, and closed. If the beer comes out too fast, see rule #2 in the previous digression. Choking, or partially opening the tap, causes turbulent flow, thus foaming. (This can be used to put a little more head on top of a full glass.) END OF NOT-AS-LENGTHY DIGRESSION There was nothing wrong with his beer or his setup, he was merely choking the tap, not opening it fully! I can't recall seeing this question asked of folks reporting problems with foaming in the forums and elsewhere, but I'd bet Dave's not the only one. Which brings us to the title of this post, inspired by the old saying, "When you hear hoofbeats, first think horses, not zebras." Of course, for Fred, you'd reverse that....

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