A Hydrometer is used to measure a specific gravity in order to tell what the alcohol content is in your brew. To find the percentage, two readings are needed. The first will be the original gravity, that's measured after everything is added, but before adding the yeast. This is labeled O.G. The Final gravity is taken after everything has finished and before you're getting ready to bottle or keg the hard cider. This is labeled. F.G.
The Hydrometer works by measuring the density of the cider against the density of water. Which basically means the liquid is being measured after all the sugar is fermented out of the cider.
There are three readings on a hydrometer:
- Specific gravity – Measures the relative density of liquids.
- Approximate alcohol by volume – This will give a guesstimate of the Final%
- Brix – To measure the sugar content based on the beverage temperature of 68 degrees Fahrenheit.
To find the alcohol content by volume the first thing that you want to do is check the original gravity by putting the unfermented cider into a separate container or jar. This reduces the possibility of contamination. I'm using a plastic graduated cylinder, which makes it nice since the diameter is small and it has a pretty good height on it. This makes it easier for the hydrometer to float.
Place the hydrometer gently into the liquid until it floats freely. Now give it a spin to eliminate any air bubbles attached to the side. Take a reading of the Specific O.G. and note of the approximate alcohol by volume.
For the F.G. reading use the exact steps, write down the specific gravity and also note the approximate alcohol by volume.
On the chart below follow the SP. GR. across column to the APPROX. %VOL column and make a write down the APPROX %VOL readings from both O.G. and F.G.
O.G. of 1.090 = 11.7 APPROX%VOL
F.G. of 1.010 = 1.3 APPROX%VOL
Now to determine the alcohol % subtract the O.G. APPROX%VOL form
the F.G. APPROX%VOL.
11.7 – 1.3 = 10.4%
So your brew would end up with a 10.4% Alcohol by volume
Quick side note: If the cider that you want to take a F.G. with has carbonation in it, try pouring it from glass to glass to remove the fizziness before the reading. This will give a more accurate percentage.