It's actually not that hard to get to grips with, but I think people often tend to overcomplicate it with awe and bad explanations. So here, for the benefit of future generations of AHBers, possibly, is my best attempt at de-mystifying the 'balanced' draught beer system:
The law of fizzyness
1 CO2 dissolves in beer. It is called dissolved CO2. CO2 also comes 'out of solution', in the form of bubbles. The amount of CO2 that stays dissolved at any time is a direct consequence of temperature and pressure.
2 Higher pressure increases dissolved CO2. Lower pressure decreases it. Higher temperatures decrease dissolved CO2, lower temperatures increase it. The effect is predictable for any known temperature and pressure.
3 We usually describe dissolved CO2 in 'volumes'. A litre of beer that contains a litre of dissolved CO2 is said to contain one volume. This isn't very fizzy. A litre of beer that contains 3 litres of CO2 is said to contain 3 volumes. This is very fizzy. Most average commercial lagers contain around 2.5 volumes of dissolved CO2.
4 It takes a few days for the beer to absorb (or release) its CO2, but once it has reached its particular equilibrium for its temperature and pressure, it stays there.
The law of foaminess
1 If the temperature increases, or the pressure decreases, then the CO2 will try to come out of solution. When it does this, the head-creating proteins in the beer form lots of bubbles, and the result is foam.
2 Pouring a beer will usually result in some decreased pressure, and some increased temperature. So you will usually get some foam on it.
3 If the changes are too great, then you will get too much foam.
The two commandments (of draught beer systems)
1 Thou shalt not screw with the regulator.
2 Thou shalt not screw with the thermostat.
Zen and the art of balancing draught systems
1 We decide what temperature and pressure we want to store our beer at, to give us the desired fizzyness.
2 We cunningly create enough resistance in our beer line so that we get a nice pour, at that temperature and pressure.
3 Different types of beer line have different resistance characteristics. There are charts that try to predict the effect of length, diameter, material, pressure, tap height etc, but they are usually for commercial installers who need to design a large installation. At the homebrewer level they are usually inaccurate and are no substitute for trial and error. The basic rule is get thin line, and cut it too long, so that your pour is too slow. Live with it for a week or so, and if you are sure that your beer has settled and the pour really is too slow, then shorten it a bit, and so on.
4 Relax, don't worry, have a home brew. :beer:
