Hydrometers are one of the most useful tools for brewers as they are used to measure the specific gravity of liquid. Basically, it compares the density of the liquid to the density of water. Brewers use hydrometers to measure the density of the wort before and after fermentation to calculate how much sugar the yeast have eaten, which gives us the Alcohol by Volume (ABV) of the beer.
The density of water is measured as 1.000 on a hydrometer. Before fermentation, while there is lots of sugar in your brew, take the first reading which is called the original gravity, or O.G. The hydrometer will read in the range of 1.040 for roughly a 4-5% beer or up to 1.080 for roughly a 9-10% beer.
After fermentation the next reading is taken, called the final gravity, or F.G. The reading is often closer to 1.010, showing that the amount of sugar is significantly reduced. From here you can calculate the approximate ABV using the following equation.
(original gravity – final gravity) x 131 = ABV (alcohol by volume)
For example: (1.042 – 1.012) x 131 = 3.93% ABV
Most hydrometers will read accurately at 18-20 degrees celsius so take your readings as close to that temperature as possible. Draw a small amount of beer out of the fermenter tap into your trial jar to get your O.G., and at the end of fermentation to find out your F.G. Spin the hydrometer when it is submerged in the wort to ensure there are no air bubbles that will give an inaccurate reading.
A refractometer is best used to check your gravity while it’s high, such as pre-boil gravity readings or for your original gravity. Refractometers only need a few drops of wort to measure the gravity, compared to the 50-100mls that is required for a hydrometer. The benefit of checking pre-boil gravity is it gives you greater control over how much liquid you need to add or boil down to reach a target gravity and volume.
Our refractometers measure in brix, but there are plenty calculators online which will help change it to gravity.