Dynamics of tank carbonation
To understand the dynamics of CO2 dissolved in beer, we must first understand the solubility of CO2 in beer and the terms and measurements used to describe it. CO2 is very soluble in beer, and its solubility increases with pressure and decreases with temperature. The amount of CO2 dissolved in beer is most often referred to in terms of volumes. Volumes of CO2 are defined as the volume the CO2 gas would occupy if it were removed from the beer at atmospheric pressure and 0o C, compared to the original volume of beer. Thus if a quart of beer were carbonated to 2.5 volumes and all the CO2 were removed from the beer, it would occupy 2.5 quarts. Most packaged beers are considered normally carbonated with 2.45 to 2.85 volumes of dissolved CO2. Generally speaking during the bottling and keging process 0.1-0.15 volumes will be lost and should be added in the tank above the desired packaged content.
Determining the volumes of CO2 in beer is easily obtained by using the temperature and pressure conditions of the beer at equilibrium conditions and reading the volumes directly from a chart. Equilibrium means the same amount of CO2 is diffusing out of the beer as is being dissolved back into solution. It is critical that the readings used for determining CO2 volumes are taken under equilibrium conditions and the instruments used are accurate. The impact of false readings on determining the volumes of CO2 can easily be demonstrated by referring to the chart on the next page. For example the largest errors often come from pressure readings taken from gauges which are often plus or minus as much as 1-7 psi. If we have a container of beer at 35oF and our faulty gauge reading is 10 psi, we see from the chart our beer is 2.52 volumes, but the actual pressure is 15 psi, so in reality we have 3.02 volumes. Not only is the beer beyond normal gas levels, but excessively high pressures can be dangerous due to container over pressurization.
Other factors can often give false volume readings from the chart even if you have good instruments. One example of this might be a gas leak from a manway or pressure relief valve and you are using your tank readings in determining volumes. Your volumes determination will be incorrect because the tank does not represent equilibrium conditions. To be certain tank carbonation is correct a sample should be properly taken from the tank and tested. The tester will be a device which seals to the sample container and is equipped with a thermometer and pressure gauge for reading the equilibrium conditions in the sample container. The tester must be shaken vigorously several times before the readings are taken. This is to ensure that as much CO2 is coming out of solution as is being dissolved back into solution and equilibrium conditions are obtained. If you do not have a tester and tank conditions are used in determining CO2 levels, always be certain your tank is under equilibrium conditions and you have accurate instruments.
Tank carbonation is most often accomplished by kraeusen or CO2 introduction into the tank through a carbonating stone. For the purpose of this discussion we will only be concerned with carbonation using a stone. At the end of normal fermentation, beer contains about 1 volume of CO2. The carbonating stone will be used to introduce the remaining CO2 into the beer and bring it to the carbonation proper level.
Beer carbonation at various temperatures and pressures
To use this chart: First find the beer temperature along the left hand vertical edge. Then read the pressure across the top and where the two cross, read the volumes of CO2.
Carbonating stones are made of ceramic or sintered stainless steel with very small openings which produce very small bubbles when CO2 is forced through them. Very small bubbles have a large surface area of exposure to the beer and are easily dissolved in to the beer. Stones are placed in the bottom of the tank and off center to produce a rolling action of the beer. Since carbonating stones have minute pores, the capillary resistance of the stone must be overcome before any bubbles are produced in the beer. This capillary resistance is often referred to as wetting pressure, which can be between 1-8 psi. Additionally the liquid head pressure above the stone also affects the total internal pressure required for the stone to produce bubbles. Every 28" of liquid height is the equivalent of approximately 1psi, consequently the higher the liquid above the stone, the higher the internal pressure to overcome the wetting pressure of the stone.
Beer carbonated in the tank using a stone can be carbonated from a few hours to several days. Generally the best results are achieved using a relatively slow carbonation. It is also highly desirable at the beginning of the carbonating process to use a relatively low differential pressure between the stone and the head space in the tank while bleeding gas from the top of the tank. This will scrub unwanted dissolved air out of the beer which was picked up during transfer or the brewing process.
The following is an example of carbonating beer at 34oF, using a carbonating stone with a wetting pressure of 5 psi. First determine the desired volumes of CO2 you want in the finished product, such as 2.58 volumes. From the CO2 chart we read 32oF on the left side and follow it to the right until we read 2.58, then move to the top of the chart and the corresponding pressure is
10 psi. The next step is to determine the head pressure of the beer above the stone. In this example the beer is 84" above the stone, divide this by 28"/psi and we obtain 3 psi head pressure. It is important to mention at this point that 28"/psi is an approximation for water and you may wish a more accurate figure, especially for higher gravity beers. To obtain a more accurate pressure multiply the total inches of liquid above the stone by the specific gravity of the finished beer then divide by 27.684"/psi. Generally speaking this sort of accuracy is not warranted for most conditions.
To obtain the total pressure needed for the carbonating stone to begin producing bubbles, add the wetting pressure of the stone (5 psi) to the liquid head pressure above the stone (3 psi) = 8 psi. This means that with 8 psi pressure applied to the carbonating stone and the head pressure of the tank at atmospheric pressure the stone will begin releasing bubbles into the beer. Since we want to carbonate slowly, the differential pressure should be kept low. 9 psi at the stone would give a differential of 1 psi above the bubble break over pressure with atmospheric pressure in the head space of the tank. As the pressure in the head space in the tank increases it is necessary to increase the pressure to the carbonating stone. Since the target carbonation level in this example is 2.58 volumes at 34oF the head pressure gauge on the tank should be 10 psi when the beer is carbonated and the pressure on the stone will the wetting pressure, plus liquid head pressure, plus the final equilibrium pressure (5+3+10=18).
Carbonating in the tank in this manner requires accurate pressure gauges and a great deal of attention. One option to reduce the amount of labor required for tank carbonating is a commercial Unit Tank Carbonator which maintains a constant differential pressure for you during the carbonating process. Even though you have carefully carbonated the beer and have accurate pressure gauges, it is still a good idea to check the beer using a good carbonation tester.