In home brewing and winemaking, a hydrometer is used to work out when fermentation has finished, or how much alcohol is in your new brew. They aren’t essential if you’re brewing from a kit, but they do take a lot of the guesswork out of things which means fewer accidents with exploding bottles.
Technically, hydrometers measure how dense a liquid is, compared to water. Hydrometers have various units on the neck which should be explained in the leaflet that comes with them, but ALL hydrometers should have Specific Gravity (SG). This is the ratio of the density of the liquid to the density of water: so water has an SG of 1.000. Sugars make the reading higher, and alcohol makes it lower. Hydrometers are often sold with a specially designed beaker called a trial jar (or test jar), which is essential to get an accurate reading.
How to read a hydrometer
- Sterilise your hydrometer, test jar and the syphon or ladle that you’re going to use to take a sample of your brew. Dry the neck of the hydrometer, as any clinging drops of water will affect your reading.
- Take the sample and transfer it to the trial jar. Avoid having any floating bits and bobs, and leave the sample until it is at room temperature.
- Gently lower the hydrometer into the liquid until it floats in the middle of the jar. If you notice bubbles forming on the sides, give it a gentle spin to dislodge them.
- Give the hydrometer a little nudge so that it sinks a couple of millimeters below the surface and bobs back to find its own level.
- Bend or kneel down so that your eye is level with the surface of the liquid, and read the number off the scale. The point to read is the flat surface of the liquid – not where it curls up to meet the side of the trial jar. You can see the difference in the image on the right. If you find it difficult to see in practice, try putting something dark behind and just below the level of the liquid.
- Write the result down – now! You’ll be amazed how quickly the numbers get muddled up. Was it 1.010 or 1.001?
- Throw the sample away, or drink it. Don’t risk putting it back into the fermentation vessel in case it’s been contaminated.
How to tell when fermentation is finished
Knowing how far farmentation has gone is the most important use for a hydrometer in home brewing and winemaking. If it hasn’t finished and you bottle your brew, you’re looking at exploding bottles. Many home brewers will tell you horror stories about kitchens awash with sticky liquid and broken glass… don’t be one of them!
Working out if fermentation is finished is very simple: test your brew and record the reading. Test it again three days later: if the results are the same, the fermentation has finished and you’re ready to go on to the next step in your recipe.
How to work out how much alcohol is in your brew
If such things are important to you, you can also use your hydrometer to work out the alcohol by volume (ABV) of your brew. Don’t confuse this with the ‘potential ABV’ given on some hydrometers – that’s just a measure of how much sugar there was in there to start off with. Working out the real ABV means knowing the SG at the start (just before the yeast went in) and again at the end (when fermentation had finished). All you do is subtract the smaller number from the larger one and multiply by 131 – and the result is your percentage of alcohol.
(SGstart – SGend) x 131 = % alcohol
Things that can affect a hydrometer reading
Your hydrometer is calibrated to read at a particular temperature, usually 20C. A degree or two either way doesn’t make much difference, but big temperature differences certainly do – so don’t try to measure the SG of hot or cold liquids. Allow your sample to come to room temperature before using the hydrometer. If that’s not possible, use the temperature correction chart that came with your hydrometer.
Fizzy liquids such as sparkling wines are difficult to read because little bubbles tend to form on the sides of the hydrometer. This makes the hydrometer sit higher in the liquid than it should, giving you a false reading. Allow your sample to go flat (preferably overnight) before using the hydrometer.