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How does a hydrometer measure alcohol content?

Published in Alcohol Measurement 3 mins read

A hydrometer measures alcohol content by determining the density of a liquid before and after fermentation.

Here's a more detailed explanation:

How a Hydrometer Works: Density and Alcohol

A hydrometer is a tool designed to measure the specific gravity (relative density) of a liquid. It works based on Archimedes' principle: an object floats in a liquid by displacing a weight of liquid equal to its own weight. The denser the liquid, the higher the hydrometer floats.

The Process of Alcohol Measurement

  1. Initial Measurement (Original Gravity - OG): Before fermentation, the liquid (typically a sugary solution like wort for beer or must for wine) is measured. This reading, called the Original Gravity (OG), indicates the sugar content. Since sugar is denser than water and alcohol, a higher OG reading means a denser solution. The hydrometer floats higher in this denser solution.

  2. Fermentation: Yeast consumes the sugars, converting them into alcohol and carbon dioxide.

  3. Final Measurement (Final Gravity - FG): After fermentation is complete, another reading is taken, called the Final Gravity (FG). Because much of the sugar has been converted into alcohol (which is less dense than sugar and water), the liquid's density decreases. The hydrometer will sink further down in the less dense solution.

  4. Calculating Alcohol Content (ABV): The difference between the OG and FG readings is used to calculate the Alcohol By Volume (ABV). The formula typically used is:

    ABV = (OG - FG) * 131.25

    Where:

    • ABV is the Alcohol By Volume (percentage)
    • OG is the Original Gravity
    • FG is the Final Gravity
    • 131.25 is a constant to convert specific gravity to ABV

    Therefore, a larger difference between OG and FG indicates a higher alcohol content.

Why This Works

The key is understanding the change in density. The hydrometer doesn't directly measure alcohol; it measures density. The assumption is that the decrease in density is primarily due to the conversion of sugars into alcohol. While other factors can influence density, the sugar-to-alcohol conversion is the dominant factor in most fermentation processes.

Example

Let's say the OG reading is 1.050, and the FG reading is 1.010.

ABV = (1.050 - 1.010) 131.25 = 0.040 131.25 = 5.25%

Therefore, the alcohol content is approximately 5.25%.

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