Ask Mr. Wizard

Carbonating in a Unitank


James Broadbent — Wakefield, England asks,

I’ve been brewing now for around 2 years and always bottled the beer out of my plastic buckets. Recently, I stepped my game up a bit and plunged into the realms of Ss Brewtech’s 7 gallon (26.5 L) Chronical. Great bits of kit, really impressed. So, I now have a unitank and a glycol chiller. My question is how do I go about carbonation using the unitank and carbonation stone?

Can’t seem to find an awful lot online regarding the actual method and how long to carbonate. I’ve just done a trial on a lager that I ruined due to leaving it on the yeast for too long so didn’t mind over-carbonating or  wasting it. My steps were as follows:

• Temperature set to 3 °C (37 °F)
• Crank the CO2 pressure up to 20 psi (138 kPa) and left it for approximately 1 hour
• Being scared of over-carbonating, reduced the pressure down to desired 10 psi (69 kPa) to give around the 2.4 mark, and left it for a few days in the unitank at this pressure.

Came back to it at the weekend and it felt under-carbonated. Any advice on this? I like doing my research and would like a tried and tested method using a carbonation stone.


There a few ways to go about carbonating beer using a stone, and I think your approach is sound, but needs a few changes. I will review the method that has worked for us at the Springfield Brewing Company in Springfield, Missouri for the last 20 years because what we have is not too different from the set up you describe. Our nominal batch size is 20 hL (2,000 liters or 528 gallons), so our tanks are not so large to be totally different beasts. The beers that we carbonate or nitrogenate with a stone are adjusted in the bright beer tank, and then dispensed directly to our taps. Just giving a bit of background to establish that I will be describing something that is routinely used for this process.

There are really two key pieces of data that you need to know about your system. One is the pressure required to push gas through your carbonation stone; in engineering lingo this is called the pressure drop, or ΔP, across the stone. Pressure drop is important because it lets you know the pressure after the gas flows through the stone. The easiest way to determine the ΔP is to connect your stone to a carbon dioxide line with a gas regulator controlling the pressure to the stone, submerge the stone in a bucket of water, and slowly increase the pressure while watching the stone for bubbles. When you see gas bubbles flowing from the stone, you have determined the ΔP across the stone. It may be a good idea to check this a few times to verify your findings. A typical value is about 2 psi (14 kPa). Beer deposits, like beer stone or calcium oxalate, will increase the ΔP across the stone, so this is a good reason to soak stones in acid after use. I will come back to ΔP in a moment.

The second value you need to determine is the hydrostatic pressure exerted by the beer when your tank is full. In small tanks this value is negligible, but it exists, so let’s not ignore it. A medium-sized tank in a pub may be 10 feet tall (3 m) and the beer level at 8 feet (2.4 m) above the tank bottom when the tank is full. A one foot (30.5 cm) column of beer exerts approximately 0.43 psi (3 kPa) of pressure, so 8 feet (2.4 m) of beer is equal to 3.4 psi (24 kPa). This value is often called liquid head by engineers. This is where things get a little fuzzy; serving tanks and kegs change level when used, so the liquid head changes over time. I figure the average height between full and empty is about as exacting as one can be, so I would use 1.7 psi (12 kPa) for the liquid head value in this example. This beer height in your 7 gallon (26.5 L) tank is about 3 feet (0.9 m), so your average liquid head is about 0.7 psi (5 kPa).

Now that we have the ΔP across your stone and the average liquid head of your beer, we need to know the carbon dioxide pressure required to hit the desired carbonation level. Your target of 2.4 volumes is actually a bit low, so your test beer may have seemed a bit low in CO2 because of your target value. For the purpose of this example, I am going to bump this value to 2.5 volumes. Referring to a gas chart, for example, the pressure at 3 °C (37 °F) required for 2.5 volumes can be determined, and that pressure is about 11.5 psi (79 kPa).
Now it’s time to use the pressure drop and static head information. The pressure drop must be overcome by adding more pressure, so the 11.5 psi (79 kPa) required for carbonation needs to be increased, and the liquid head increases gas solubility by adding pressure to the system, so this value is subtracted. The resulting math looks like this:

Target pressure delivered to the inlet of the stone = 11.5 psi (79 kPa) + 2 psi (14 kPa) – 0.7 psi (5 kPa) = 12.8 psi (88 kPa).

The easiest and most robust method is to adjust your gas pressure to 12.8 psi (88 kPa), open the valve to the stone, and leave things be for 24-36 hours. This should be plenty of time for the system come to equilibrium. The stone size does factor into this because the size is related to area, and insufficient area will slow down the gas transfer rate (as will a dirty stone), but this is unlikely with a 7-gallon (26.5-L) tank.

A variation on this theme that can really speed things up is to use an adjustable pressure relief valve on the top of your tank, and adjust this pressure to the pressure fetched from the chart minus your liquid head; 11.5 psi (79 kPa) – 0.7 psi (5 kPa) = 10.8 psi (74 kPa). Then bump the 12.8 psi (88 kPa) in the example above by about 0.2 (1.4 kPa) to 13 psi (90 kPa). What will happen is that a slow flow of carbon dioxide will escape from the relief valve as an excess of gas constantly flows into the system because we have set the conditions up to force a flow of gas through the beer. Depending on the size of your stone, this method will carbonate your beer in about an hour. It is important not to get too carried away with the inlet pressure because this method will cause foaming if too much gas flows through the beer. It also may result in aroma loss, beer loss, and reduced foam stability, not to mention a potential mess if foam spits out of the pressure relief valve.

Response by Ashton Lewis.