How To Use a Hydrometer

Many starter kits contain a hydrometer and many beer, wine and cider kits refer to this device, so what is it and what does it do? A hydrometer measures the density of a liquid (called the specific gravity). Water has a specific gravity of 1 (or 1.000 on a hydrometer). Liquids that are more dense than water will have a higher reading, while liquids that are less dense than water will have a lower reading.

When you add sugar to water, it increases the density. Unfermented beer, wine, cider and mead contain a lot of sugar, therefore the specific gravity is higher than 1.000. As fermentation proceeds, yeast convert the sugar to alcohol, which is less dense than water. The result is that as fermentation progresses, the specific gravity will gradually decrease. 

So, why is gravity important and why do you need to use a hydrometer? There are two reasons. Firstly and most importantly, taking a hydrometer reading is the only sure way to show that fermentation has stopped. Secondly, by taking hydrometer readings both before fermentation has started and after it has finished, we can calculate the alcohol level of our beer, wine, cider or mead.

First of all though, let's look at how you read the hydrometer. The hydrometer is a weighted glass instrument, which needs to be floated in a liquid. Ideally you should use a trial jar for this, as it means you don't put your hydrometer into your brew, which carries the risk of introducing an infection. To fill the trial jar, use a sterilised wine pipette or turkey baster. Float the hydrometer in the trial jar and it should look like this:

As you can see, the reading is above 1.000, in fact it is actually measuring 1.036. You need to take the reading from the surface of the liquid, rather than where it curves up the glass of they hydrometer. This hydrometer is sitting in unfermented beer, if it was in wine, then the reading would be much higher, in the region of 1.075 to 1.085, depending on the expected end alcohol content.

So, how do you tell if fermentation has stopped? By taking a hydrometer reading, then waiting for 24-48 hours and taking a second reading. If the gravity has decreased, then fermentation has not stopped. If the gravity reading remains the same, then fermentation has completed. Let's look at another beer sample:

In this picture, we can see that the gravity reading is now 1.014. Although this is a little high for beer, may kits will finish around this point, but again, check the reading over several days to make sure it hasn't dropped any further. Once you are sure that fermentation has stopped, you can proceed to bottling.

Calculating the alcohol content of beer, wine, cider or mead

If you took a hydrometer reading before you added the yeast (Original Gravity or OG), and another one at the end of fermentation (Finishing Gravity or FG), you can calculate the alcohol content of your wine, beer, cider or mead. To do this, you use the following equation:

(OG-FG) x 131.25 = ABV

So for a typical kit beer, we might have an OG of 1.038 and a FG of 1.010. This would give us 0.028 x 131.25, which equals 3.7% ABV.

For a wine, the OG might be 1.086 and the FG 0.995 as an example. This would give us 11.9% ABV (0.091 x 131.25) = 11.9

Typical hydrometer readings for beer, wine, cider and mead

Each brew will be different and depending on the types of sugars present, hydrometer readings may vary, but here are some typical finishing gravities for beer, wine, cider and mead. If your FG is substantially higher, then your fermentation may have stopped prematurely and will need to be restarted using a restart yeast.

Wine and mead have a higher range of final gravities, depending on whether you want a dry or sweet finish:  

Hydrometer dos and don'ts

Do use a trial jar to contain your sample in which the hydrometer floats.
Do sterilise your pipette or turkey baster before taking the sample.

Do keep your hydrometer in its protective case, it is fragile and will always break when you need it most!

Do drink the sample you take after fermentation has stopped - this will tell how well your brew has fermented.

Do not float they hydrometer in your demijohn or fermenting vessel. It could infect your brew and you may not be able to retrieve it, especially from a demijohn.

Do not put the sample back into the brew - you could contaminate it.

Do not bottle your brew if the hydrometer reading is higher than expected of still changing, you could end up with exploding bottles.