This guide will aim to give you a brief understanding about how to use a hydrometer in order to determine the alcohol content and how far along you are in fermentation.
This guide is for a beer and wine hydrometer, a spirit hydrometer is even simpler to use, just add straight to finished spirit and read off the alcohol content.
What a hydrometer does
The hydrometer is a handy little device that works by comparing the Specific Gravity (SG) of a liquid to that of pure water (think of this as the density of a liquid). As beer and wine use sugars for the yeast to convert to alcohol, the sugars increase the density of the water so it becomes 'heavier' and as the sugars are converted the liquid become less dense or 'lighter'.
If your still with me, you can use the hydrometer to work out how much of the sugar has been converted so therefore can calculate the alcohol content or how far through the fermentation process you are, which is handy if you may think your yeast isn’t working or if you want to create a low alcohol beer or a sweeter wine.
Reading it properly
First you need to get a sample of the liquid you are measuring, this can be done by using a Wine Thief and Trial Jar, pouring some into a pint glass or just sterilising it and putting it straight into the fermentation bucket with the beer or wine.
Then all you need to do is take the reading at the water level as seen in the picture;
Calculating alcohol content
There are many methods of determining the alcohol content using a hydrometer that can get quite detailed, but for a quick close estimate use the following equation;
(Original Gravity Reading – Final Gravity Reading) * 131 = ABV%
Where the Original Gravity is the reading before you add the yeast and the Final Gravity is the reading after fermentation is finished.
(1.045 – 1.010) * 131 = 4.5% ABV