Could you give some tips or advice on how to best calculate the final gravity of a beer? I often nail the original gravity (OG) down by knowing my system efficiency, but occasionally I am let down by my final gravity differing from brewing software predictions.Clinton Fischer
Adelaide, South Australia
Calculating the final gravity of a beer is simply a rough approximation because there are too many things affecting final gravity to produce a very accurate estimate. A common estimate is to multiply your gravity points by 0.25 (1.056 has 56 gravity points so the predicted final gravity (FG) would be 14 points or 1.014). Wort fermentability, wort original gravity (i.e., high gravity worts), yeast strain, wort nutrients, wort aeration, and fermentation temperature can all influence a beer’s final gravity. For many beers, the final gravity is something that is discovered during fermentation.
A batch began as 1.056 wort and ended up as 1.018 beer, and after three gravity checks at 1.018 the beer was bottled. Sounds pretty familiar, right? But 1.018 is still pretty high, and when 18 (FG points) is compared to 56 (OG points) we see that the apparent degree of fermentation (ADF) is 68%. ADF can be calculated by the following equation:
ADF% = (OG-FG)/(OG)x100.
ADF is a good number to calculate because beers of similar OG, fermented with the same yeast strain, under “normal” conditions tend to have similar ADF values. This value is often published by yeast suppliers as a property of yeast, and the same strain is not always described the same by different suppliers. Wyeast shows the attenuation of their strain 1056 (American Ale) between 73–77%, and White Labs shows the attenuation of their strain WLP001 (California Ale) between 73–80%; similar, but not the same (most brewers believe these are the same yeast strain).
The differences in these ranges does not mean that one of these suppliers is incorrect, but has more to do with what ADF really is. ADF is a property of wort, not yeast, but since nothing is known about the wort that various brewers may produce sometime in the future, we often look to yeast properties when trying to predict what may happen. There is nothing wrong with this in most cases because the actual ADF/FG is simply a property of the beer, and most brewers don’t really care if their beer finishes at 1.014 versus 1.018. Brewers do care if their beer finished at 1.018, but somehow woke up when bottled and started fermenting again because this usually results in over-carbonated bottles, and may end up producing bottle bombs.
The best way to predict the FG of the next batch you brew is by using a time machine instead of a calculator. A time machine allows you to know the FG of your current batch before the batch finishes. Brilliant! The great news is that this time machine has a name, and is called the Forced Fermentation Test (FFT). The equipment you need to perform this test by the books includes a 500 mL Erlenmeyer flask, a magnetic stir bar, a stir plate, and a large wad of cotton. What you put into the flask is wort and yeast (about 5–10 times more than you typically use for fermentation). The FFT is easy to perform; simply put about 200 mL of wort, 1 gram of dried yeast or equivalent liquid yeast, and the stir bar in your flask, stuff the wad of cotton in the opening of the flask, put the assembly on the stir plate for 36-48 hours, and measure the FG at the end of the test.
The results of the FFT are much more reliable than calculating FG based on numerous assumptions and is a great indicator. One of the challenges of this test is having sufficient yeast for the test. If you use dried yeast, things are very easy. For those brewers using liquid slurries, you can either grow up some extra yeast for the test or use a dried yeast with similar properties as the strain you plan on using.I use a keezer, and my kegs are standard ball lock corny kegs with the top of the keg being in-line with the taps themselves. I have 4.5 feet (1.4 m) of 3⁄16-inch line, three taps, and I am getting foamy beer out of my taps. I see CO2 bubbles on the start of the line at the keg and at the shank and beer in the middle. I suspect my lines are too long. However, I have posed this question to other homebrewers and have been advised that the lines be a minimum 4.5 feet (1.4 m) long, although none of these replies have shown any math to explain why. Can you please help clarify?
Elkford, British Columbia
Draft beer dispense problems involving gas breakout, the little bubbles you see in your beer lines, relate to properly balancing your draft set up. All too often, troubleshooting begins by adjusting the gas pressure applied to the keg. A strict brewmaster would remove adjustable gas regulators from carbon dioxide tanks to prevent bartenders and poorly trained draft technicians from touching the gauge because the keg pressure will change beer carbonation if the pressure is not adjusted for the beer in the keg. Hands off that knob!
Most draft beer is served at about 38 ˚F/4˚C, and these beers typically contain about 2.6 volumes of carbon dioxide; the equilibrium pressure of this beer is 12.5 psi. If the solution to a dispense problem includes changing the applied pressure, the character of the beer will also change within a couple of days. The real solution is to determine the conditions that need to be created to allow the beer to be poured without changing the beer.
There are many articles written about balancing a draft system, and most of these articles have sections discussing long-draw (tap located more than about 50 feet from the keg) and high lift (tap located one building floor higher than the keg) scenarios. Very few homebrewers have these sorts of set-ups, so I am going to simplify this discussion by omitting reference to these outliers.
There are only three things you really need to know to balance your draft system: 1) the equilibrium keg pressure at your storage temperature, 2) the distance from the center of the keg to the tap, and 3) the type of tap being used. Beer carbonation level and storage temperature define the keg pressure. You can refer to gas tables or calculators for this value, but for this example we will use 12.5 psi (2.6 volumes at 38 ˚F/4 °C).
The distance from the center of the keg to the tap is the average hydrostatic head (liquid weight) that must be lifted by the keg pressure to get the beer to the tap. This value is dependent upon liquid density, but since beer has a density similar to water we can use 0.45 psi per foot of lift. For this example, we will assume that the beer is lifted 3 feet (0.91 m) from the center of the keg to the tap, so that the hydrostatic head is equal to 3 feet x 0.45 psi/foot or 1.35 psi.
A key point to note here is that I am using the center of the keg and calculating the average hydrostatic head; there is a difference between dispensing a full keg and a nearly empty keg because the full keg requires less pressure to lift the beer to the tap. Using the average is a common method used to recognize this fact without too much fuss. This detail becomes very real when considering dispensing beer in a brewpub from vertical tanks that may be taller than 10 feet (3 m), where the liquid head varies between 0 and 4.3 psi.
The last piece of information for this problem relates to your tap and tower. This value ranges from 1.0 psi on the low end for a simple Cobra head tap (picnic type) attached to a Cornelius keg, up to 3.0 psi for tap towers with stainless steel extensions running down the tower. A value of 1.5 psi can be used for the typical set-up at home using a Cornelius keg with a tap tower 3⁄16-inch barbed elbow connected to the back of the tap.
Now we have the information required to balance the system. The 12.5 psi applied to the keg in this example must be balanced by the 1.35 psi of lift, the 1.5 psi associated with the tap equipment (including the tube and valve of the keg), plus the draft line. The resistance of the draft line needs to be 12.5-1.35-1.5, or 9.65 psi. Home systems can use 3⁄16-inch polyethylene tubing for almost all draft applications because the runs are usually short. This type of tubing has 2.2 psi of resistance per foot; so we need 9.65 psi/2.2 psi per foot (0.3 m) or 4.4 feet (1.3 m) of 3⁄16-inch tube to balance this system.
The result of this example is very similar to the information you found online (but with little explanation
of why). So it sounds like you have the right set up, but are still experiencing gas breakout. Gas breakout, when a system is correctly installed, is usually caused by one or more of the following problems:
1. The keg pressure and the equilibrium pressure required for the beer are different (can happen if the carbon dioxide tank empties without being noticed).
2. The beer is warming up as it flows from keg to tap (tap towers should be insulated, and can be cooled with fans that move cold air from the cooler into the tower).
3. The draft line is dirty, and the roughness of the line is causing gas breakout during dispense (this is fairly common, and can be confusing since dirty draft lines do not always appear dirty).
4. There is more pressure drop associated with your draft system than assumed, and the draft line is too long.Does it matter if the CO2 tank is inside vs. outside of the kegerator?
This question has a very direct answer; it makes no difference at all if you put your CO2 tank inside or outside of your kegerator. Locating the CO2 inside of the kegerator can make for a tidier installation. And if your kegerator is portable or has wheels for easy positioning, having the tank inside of the unit makes movement easier. Since refrigerators have limited space and because there is no functional benefit to storing the CO2 tank in a cold environment, most folks keep the CO2 tank outside of therefrigerated compartment.