A hydrometer is a device thats used to measure the gravity (density) of a liquid compared to water (which has a gravity of 1). The more dense a liquid is, the more the hydrometer will float. Knowing the gravity of our cider means we can approximate how much sugar is in our cider. Hydrometers are calibrated at a certain temperature, so you may need to use a calculator to figure out your actual gravity if your cider is at a different temperature. Mine is calibrated for 60 degrees F, but my brew area is 71 degrees. So I’ll be using a hydrometer temperature calculator to adjust my gravity.
Usually, our final product will end up with a density of 1.000. Knowing the starting and finishing density can help us calculate the percent alcohol of our cider!
I grabbed this hydrometer for about $5.
So how do we actually use it? Its a ~10 inch long glass tube with a weight at one end. The weighted end will sink, and the other end will point upwards, so we need a container with enough depth to allow the hydrometer to float.
Some hydrometers come with a container that holds liquid and can be used to take a reading. Mine didn’t. So I also picked up a 250ml graduated cylinder. The graduated cylinder is ~14 inches tall, which ensures that we have enough room to allow the hydrometer to float. There are similar glass tubes, however a polypropylene tube is going to be much harder to break/crack.
Like always, the first step is going to be to sanitize everything. Throw your hydrometer, tube, and funnel into the star san to get ready. You may want to do this some time in advance to allow things to air dry.
We’re then going to pour enough cider into the graduated cylinder to mostly fill it up to allow for a reading. Stick your hydrometer in the tube, weight side down, and wait for it to settle. Then think back to your high school chemistry class to try and remember how to take a measurement. Put your eyes level with the cider and find where the surface of the cider meets the hydrometer.
The angle of the camera is a bit off from where you want your eyes to be, but you can see that it is approximately at the first line below 1.050, so the reading here is 1.052. But remember that I’m using a different temperature! Plugging my info into the calculator from before gives me an actual original gravity of 1.053. We can then either pour the cider back into the carboy (especially if you let your graduated cylinder air dry the sanitizer), or you can leave it out to give your cider a little more headspace.
You can continue to take the gravity of the cider throughout the fermenting process. When you have the same reading for a few days in a row, then fermentation has stopped and you’re good to move onto the next stage. Alternatively, you can stop it at a specific gravity if you’re aiming to bottle it at a certain % alcohol or sweetness. If you don’t want to measure every day, then you can simply wait a few weeks to ensure it finishes fermenting, and then take 1 measurement at the end to check.
Your final gravity is likely going to be pretty close to 1.000. Again, my picture isn’t the greatest, but its right about 1.000 – perhaps 0.999. Going back to our temperature calculator, the actual final gravity is going to be 1.000 – exactly what we expected!
So now we have our two gravity readings. My hydrometer has approximate % alcohol on it for the starting gravities, so I’ve got an idea of what my final product has, but lets plug it into an ABV calculator to make sure. Looks like my cider is going to have a 7.22% ABV content, wooh! A standard 1.050 (which is what my starting juice with no added sugar comes out to be) ends up right around 6.5%, which is like a typical beer.
Now you should be good to use a hydrometer – and have a much more accurate picture of how alcoholic your cider actually is!
As a final note, it may seem like a good idea to skip the graduated cylinder and simply pop the hydrometer into the carboy itself. While the carboy is certainly deep enough, getting the hydrometer out can be quite difficult. If you do want to try this, grab some unflavored dental floss and tie it to the top of the non-weighted end. This should hopefully let you pull it back up when you’re done.