System and beer
The temperature in the keg, not the one at the tap. With an inline cooler this is the temperature before the cooler – so often cellar temperature, not serving temperature.
The brewery states this value. Rules of thumb from the internet differ widely for wheat beers and bottom-fermented beers – ask instead of guessing.
Resistance values after braumagazin.de. The jump from 7 to 4 mm is the reason why short thin hoses work and long ones do not.
Positive if the tap stands higher than the keg. 0.1 bar per metre.
Result
- Saturation pressure at –
- Friction loss in the line–
- Static head–
- Safety margin–
- Dispense pressure–
The saturation pressure alone is the pressure that holds the beer in the keg without it losing or taking up CO2. Everything above that serves only to get it through the line.
Saturation pressure against temperature
At a constant CO2 content. The dot marks your setting.
Why this is the most common source of error
Too low a dispense pressure is not noticed at all at first. The beer flows, it tastes fine, all good. Over hours, though, CO2 escapes from the beer into the headspace, because the applied pressure is below the saturation pressure. On the second day the keg is flat, on the third it foams while pouring – because the expelled gas breaks out in the line.
Too high a pressure does the same in reverse: the beer takes up gas, becomes sparkling to sharp, and the foam stands in the glass before there is any beer underneath it. Both faults look the same in the glass. They can only be told apart over time – and with the calculation above.
If the calculation gives a dispense pressure above about 3 bar, the system is designed wrongly, not the beer. The permissible operating overpressure is stamped on the keg and is at most 3.5 bar for beer kegs; the common spears are designed for a working pressure of up to around 3 bar.
The solution is then never "more pressure", but a shorter or thicker line, colder beer or a compensator tap. Anyone who turns up the pressure instead over-carbonates the keg and puts strain on the seal and the spear.
The formulas used
| Quantity | Calculation | Origin |
|---|---|---|
| Saturation pressure | CO2 [g/l] = (p + 1.013) · e(−10.73797 + 2617.25/(T + 273.15)) · 10, solved for p | Empirical fit to carbonation tables (Braukaiser), widely used in German brewing tools |
| Friction loss | Length × resistance per metre | 0.72 / 0.05 / 0.01 bar/m for 4 / 7 / 10 mm, braumagazin.de |
| Static head | Height × 0.1 bar/m | braumagazin.de |
| Margin | 0.1 to 0.2 bar | braumagazin.de, empirical value |
The two sources are independent of each other and agree at the point where this can be checked: for 5 g/l at 10 °C braumagazin.de gives "a little over 1.2 bar", the formula delivers 1.22 bar.
A simple piston tap works really well, according to braumagazin.de, only at pressures between about 0.4 and 0.6 bar at the tap. If the pressure there is considerably higher, the beer foams because of the sudden pressure drop. The remedy: a choker coil that reduces the excess pressure through friction, or a compensator tap that does this inside the spear itself.
What this tool cannot do
It calculates the equilibrium state. How quickly that is reached depends on the surface between gas and beer and on the movement in the keg – a freshly connected keg needs hours, no freshly set pressure works immediately.
Also not taken into account are spears, bends, compensator taps, couplers and cooling coils. Each of these parts has its own resistance. Designing a commercial dispense system takes a specialist company, not a calculator in a browser.