One of the real pleasures of homebrewing is serving your own beer from your own tap. A properly set up and maintained home dispensing system allows you to pour correctly carbonated beer that has the appropriate head and appearance for style. However, it can also be the source of frustration if things are not done right. You can end up with a glass full of foam or flat and lifeless beer, depending. Both of these pitfalls can be avoided with a little knowledge and planning.
The science behind the bubbles
During fermentation, one molecule of glucose is broken down into two molecules of ethanol and two molecules of carbon dioxide (CO2). The CO2 that is produced is soluble in beer and results in residual carbonation. All beer contains at least some dissolved CO2, and most styles are additionally carbonated, whether by fermentation of added sugar or by “force carbonation” with additional CO2.
The total amount of CO2 dissolved in the beer is measured in “volumes,” which is the volume the gas would occupy if it were removed from the beer and kept at standard temperature and pressure (STP — 32 °F/0 °C) and 1 atmosphere of pressure) divided by the volume of the beer. This is also used to describe the carbonation level of a beer. For example, American lagers generally are carbonated to about 2.6 volumes of CO2. Less carbonated styles such as many British ales can have a carbonation level as low as 1.2–1.3 volumes, while some sprightly German wheat beers may be carbonated to levels above 4.0 volumes.
The solubility of CO2 increases as the temperature decreases. There is also some decrease in solubility as the specific gravity increases, but the effect is small and can be disregarded for the gravity of beer. The solubility of CO2 also increases with increasing gas pressure. In order to achieve the correct volumes of gas in beer, it must be stored under pressure (either in bottles or kegs).
May the force (carbonation) be with you
The correct procedure for force carbonating beer to the appropriate carbonation level is outlined in an article in the Summer 2000 issue of BYO (“3 Ways to Carbonate Your Keg,” by Ashton Lewis). To summarize, you can set the regulator pressure to the appropriate level (from the formula below, a carbonation chart or brewing software) and let the beer carbonate over a period of several days. Or you can use the “rock and roll” method (which is less exact but requires less time) of setting the regulator to a high pressure and shaking the keg vigorously for several minutes and repeating several times over a period of a couple of hours. A third method is to use an airstone, which greatly increases the surface area and reduces the required time with no decrease in accuracy.
The formula for setting the regulator to the correct pressure, P (in pounds per square inch, or PSI) for the desired level of carbonation, V (in volumes of CO2) at a beer temperature of T (in °F) is:
P = -16.6999 – (0.0101059 * T) + (0.00116512 * T2) + (0.173354 * T * V) + (4.24267 * V) – (0.0684226 * V2)
Problems down the line
Assuming the beer is carbonated to the appropriate level, it still has to make its way from the keg, through the line, out the tap and into the glass — and this is where problems can occur.
If the dispensing pressure is too low, the beer will pour too slowly and excessive foaming can result, to the point where little beer and mostly foam ends up in the glass. Furthermore, over time the beer in the keg will lose carbonation as more CO2 comes out of solution as it attempts to achieve equilibrium with the headspace. At extreme underpressure the beer can become nearly flat. If the dispensing pressure is too high, it, too, can result in excessive foam from the beer pouring too quickly from the tap. With time, the beer will become overcarbonated as more CO2 goes into solution, further complicating the situation.
At lower than the correct pressure, in addition to low carbonation, the line will tend to collect bubbles and pockets of CO2 where it has come out of solution, especially just above the keg and behind the faucet, as well as in places where the temperature is warmer. These pockets will become larger the longer the time period between dispensing beers. The first beer will have a shot of foam, followed by clear beer, followed by more foam. After pouring a few beers, the problem may dissipate, only to return again after a rest. Low-pressure problems also tend to show themselves early when a keg is nearly full.
At higher than optimum pressure, there will be over-carbonation and symptoms similar to those that occur at low pressure. The difference is that they tend to appear and grow worse as the keg is emptied. If a fresh keg is foamy, the odds are that it is not an over-pressure problem.
The reason is that, as the keg empties, more CO2 occupies the larger headspace as the difference between the equilibrium pressure and dispensing pressure increases. Again this causes excessive foaming when the beer is first dispensed, until there is less CO2 and more beer in the line.
A matter of balance
Calculating the correct dispensing pressure and making changes to the system is known as “balancing” and is critical to pouring a perfect beer. Balance is not only dependent on the carbonation level and the temperature of the beer, but several other factors also enter into the equation. These include the overall height difference between the keg and the tap, the length and diameter of the dispensing line and the type of tap being used. Changes to any one of these will change the balance of the system.
Between the keg and the tap, there is resistance to the flow of the beer. Gravity (the difference in height) accounts for 0.5 PSI per foot (11.3 kilopascals per meter), a positive value if the tap is located above the keg, negative if the tap is below it. A standard beer faucet has a resistance of 2 PSI (13.8 kPa); the shank adds another 1 PSI (6.9 kPa). A picnic or “cobra” tap has a resistance of about 0.5 PSI (3.4 kPa). Additionally, the beer line itself offers the following resistance based on the inside diameter. (These figures are for flexible vinyl beverage tubing):
- 3/16 in. (4.75 mm) inside diameter (ID):
- 3.0 PSI/ft. (67.9 kPa/m)
- 1/4 in. (6.35 mm) ID:
- 0.8 PSI/ft. (18.1 kPa/m)
- 5/16 in. (7.94 mm) ID:
- 0.4 PSI/ft. (9.0 kPa/m)
- 3/8 in. (9.53 mm) ID:
- 0.2 PSI/ft. (4.5 kPa/m)
Finally, some additional pressure is necessary to achieve a proper flow rate. The generally accepted desirable pour rate for beer is considered to be 1 US gallon (3.8 L) per minute or 1 US pint (473 mL) per 7–8 seconds. For most systems, a value of 5 PSI (34.5 kPa) is sufficient for balancing calculations.
Assuming that the other values remain the same, the easiest way to balance the system is to adjust the line length so that the total resistance of the system equals the carbonation pressure minus the required 5 PSI (34.5 kPa) for a proper flow rate. Round the result to the next highest foot (0.3 meter).
For example, for a pale ale that is carbonated to 2.3 volumes of CO2 at 46 °F (8 °C), the correct carbonation pressure (from the force carbonation formula) is 13 PSI (89.6 kPa). The beer is dispensed through a standard shank and beer faucet at a height of 2 ft. (60.9 cm) above the center of the keg. Here are the calculations for the required length of 3/16 in. (4.75 mm) diameter beer line in order to balance the system:
Gravity resistance: +2 ft. (60.9 cm) * 0.5 PSI/ft (11.3 kPa/m) = 1 PSI (6.9 kPa)
Shank resistance: 1 PSI (6.9 kPa)
Faucet resistance: 2 PSI (13.8 kPa)
Fixed resistance of the system (not including the line): 2 + 1 + 1 = 4 PSI
(13.8 + 6.9 + 6.9 = 27.6 kPa)
Carbonation pressure of the beer (2.3 volumes of CO2 at 46 °F/8 °C): 13 PSI (89.6 kPa)
Pressure required to dispense beer at 1 gallon (3.78 liters)/minute: 5 PSI (34.5 kPa)
Pressure needing to be balanced: 13 – 5 = 8 PSI (89.6 – 34.5 = 55.1 kPa)
Resistance to be supplied by the line: 8 – 4 = 4 PSI (55.2 – 27.6 = 27.6 kPa)
Resistance of 3/16 in. (4.75 mm) ID beer line: 3 PSI/ft. (67.9 kPa/m)
Length of 3/16 in. (4.75 mm) ID line required to achieve 8 PSI (55.1 kPa) resistance: 4/3 = 1.33 ft. (40.5 cm)
Rounded to next highest foot (0.3 meters): 2 ft. (61 cm)
Therefore, 2 ft. (61 cm) of 3/16 in. (4.75 mm) ID diameter tubing will balance this system for the example beer.
(Note: This length seems short by homebrew standards because 5 PSI is a higher “overpressure” than most homebrewers use. Lowering the dispensing pressure to 0.5–1.0 PSI will result in a line length more in line with usual homebrew setups. Experiment with flow rates to find one you like.)
Achieving new balance
Of course you may choose to serve a variety of styles at different carbonation levels and perhaps at different temperatures. This will affect the system balancing equation somewhat. You can recalculate the new carbonation pressure and line length necessary to balance the system, and adjust the dispensing pressure and replace the line with the proper length. If the difference is small, you may choose to ignore the slight imbalance; balancing a system does not require extreme precision. Or you may use what is called a “choker,” a short length of smaller diameter line installed at the faucet shank. A more elegant solution is to purchase and install a line restrictor, which allows you to vary the flow of beer through the line. These devices are available from draft beer equipment suppliers.
Try to maintain an even temperature throughout the system. Carbon dioxide tends to come out of solution and collect in warm places, especially near the tap. This is why you may want to discard the first small amount of beer and foam that is in the line after the system has not been used for a while. If the taps are enclosed in a tower, insulate the lines or provide them with a supply of cold air.
Beer line deposits also can increase restriction and cause dispensing problems. This is another reason to regularly clean the lines and taps. And finally, keep your glassware clean, well rinsed and free of soap deposits — and store them at room temperature. Frozen glasses or mugs will cause foaming and greatly reduce beer aroma and flavor. The proverbial “frosty mug” is a gimmick that does not improve the quality of the beer.