A hydrometer measures the relative density of a liquids relative to the density of water. This is referred to as Specific Gravity. Sugar water will have a density that is greater than pure water. Ethanol has a density that is less than water. The hydrometer will sink further into a solution of alcohol and not sink as far into a sugar solution. Specific Gravity is unitless. It defines the ratio of the density of a solution to the density of water. Specific Gravity is calculated like so, Specific Gravity = (Density g/cm)/(1 g/cm).
Generally a hydrometer is used to determine the sugar content of something that will be fermented. This could be wine must, beer or whiskey mash and even things that do not have anything to do with making alcoholic products. Hydrometers are scaled and calibrated for each distinct purpose. For example, a hydrometer used to determine the sugar content and potential alcohol in a whiskey mash will generally measure the Specific Gravity in a range between ~0.990 – 1.180. When you are measuring distilled spirits you’ll need a liquor hydrometer which will have a proof range from 0-200 or a tralle range from 0-100. Here is some example data that shows the Specific Gravity of some well-known solutions.
Pure Water: Specific Gravity = 1.0
Pure Ethanol: Specific Gravity = 0.785
50% Ethanol: Specific Gravity = 0.8925
To calculate the sugar content and potential alcohol of a mash you will need to use a beer and wine hydrometer. Simply take a sample of the mash and pour it into a tall graduated cylinder. The graduated cylinder might be a jar or anything that is narrow and tall. However, make sure that the cylinder is as tall or taller compared to the hydrometer. Next, lower the hydrometer into the graduated cylinder until it’s floating. Give it a little spin similar to a top to shake loose any air bubbles that might adhere to the hydrometer. Now you are ready to read the hydrometer. Examine the number on the hydrometer that is at the same level as the liquid. You can now determine sugar content and potential alcohol assuming that both measurements are on the hydrometer.
To learn more about using a hydrometer take a look at the article “What is a Hydrometer?” If you are curious about how to make whiskey, educating yourself about hydrometers will be critical to your success.
