We have all had our troubles trying to figure out how to calculate the specific gravity of our fermentation. Below is some information on how to read an SG hydrometer and how to use it to diagnose the progress of our fermentation.

What is specific gravity?

Specific gravity has been defined as the ratio between the density of a substance to that of a reference substance. So, what does that mean in laymen’s terms? Basically, when it comes to fermenting, specific gravity is the density of the fermentation, relative to the density of water. Knowing this helps us to accurately measure whether a product has been fully fermented or if fermentation has stalled. It also helps us calculate our final alcohol percentage by using our original and final SG readings.

How does an SG Hydrometer work?

Fermentation usually starts with an SG reading of 1080 or higher. Although this may vary slightly depending on what you are fermenting. The hydrometer is the only way of determining that the fermentation is complete. Fermentation is finished when the hydrometer indicates 0.980 – 0.990 or less ( right at the top of the hydrometer).

Hydrometer Instructions:

Allow the hydrometer to float freely and read from the surface of the liquid. Be careful to note the true level of the liquid, not where the meniscus curves up to meet the hydrometer.

Tip 1: Never monitor your airlock’s movement as a conclusion for your fermentation completion. This is an unreliable method because you could have a leak in your fermentation vessel or lid.

Tip 2: Hydrometers are sensitive to temperature. You will get a “true” reading at 20 degrees Celsius or as indicated on your hydrometer. If your sample temperature is significantly warmer or cooler it advisable that you consult a temperature correction chart. These are usually supplied with your hydrometer. An example of one can be found below:

Tip 3: CO2 gas can also affect the hydrometer reading. If your sample seems to be fizzy, you can dissipate some of the gas by transferring it repeatedly between two cylinders.

How to calculate alcohol percentage

You can calculate your fermentation’s alcohol percentage with a simple formula. The original gravity reading minus the final gravity reading X 131 equals the percentage of alcohol.

Example:

OG (Original Gravity) is 1080

FG (Final Gravity) is 1030

Calculation:

1080 – 1030 X 131 = 6.5%

Whether you are a seasoned distiller or just starting out, we hope this article helped you. Understanding your equipment is key to a successful distillation. Remember, a knowledgeable distiller is a good distiller!