What is a Hydrometer used for and do I need one?

A hydrometer is a simple device that largely resembles a thermometer. It is used to measure the specific gravity of liquids, most often beer or wine, and therefore shows you the degree to which the yeast is turning sugar into alcohol. Hydrometers are usually made of glass and consist of a cylindrical stem and bulb weighted with mercury or lead to ensure it floats upright. At the fermentation stage, the hydrometer becomes particularly important. It’s the device that will give you an insight into how well the fermentation process is coming along, and whether your cider, beer or wine is ready. A hydrometer can also alert you of any potential issues that arise, allowing you to make any adjustments if need be. It could be the difference between a spoilt and delicious product.

Using a hydrometer is an easy, four-step process.

1. Take a sample and insert the hydrometer

Prior to pitching yeast, you’ll need to take your first measurement. The reading you get is referred to as the OG, or original gravity.

To obtain the OG, retrieve a sample of the liquid and transfer it to a testing jar or cylinder. The testing container needs to have enough of the liquid in it to support the hydrometer. Place the hydrometer in the liquid and allow it to settle. Wait until all air has escaped the liquid or gravitated upward. It is recommended to centre your hydrometer and position it vertically to get an accurate reading.

2. Obtain OG Reading

The increments of your hydrometer are there to show you specific gravity points. The gravity reading depends on where the liquid reaches; this is where it should occur. Record the number that is being crossed by the liquid.

3. Calculate with Temperature

Hydrometer readings are stated assuming a standard temperature of 15 degrees Celsius. Knowing the temperature of your liquid is crucial for an accurate reading. There are many online tutorials, tables and graphs to help you calculate the gravity reading, like this one here.

4. Repeat process

Once the fermentation process is complete, take another gravity reading. This will be the final gravity reading, which is also known as FG. Don’t test your liquid too often as you are exposing your cider, beer or wine to harmful bacteria and air, which can lead to a spoilt batch.