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You are here: Home arrow Articles arrow Cooking, preserving and home brewing arrow The Hydrometer


The Hydrometer

Written by jema

The hydrometer is an indispensable aid to those who wish to be scientific about wine and beer making.

Even if you are not interested in the finer details, it is a great tool for diagnosing problems with stuck ferments and judging whether your wine and beer has consumed all the available sugar.



The principle behind a hydrometer is quite simple. A sugar solution is denser than plain water, thus when you put a weighted hollow object in a sugar solution, it will sink less than it would in plain water. Thus you can measure the density of the water.

Alcohol on the other hand is lighter than water and so a fermented out solution, will have a density less than plain water.

A hydrometer may come with a few different scales, but the relevant one to home brewing will run from below a 1000 to maybe 1200.

Hydrometers with a built in sampler

The biggest problem using a hydrometer is getting you sample, the type of hydrometer picture here, allows you to use the simple principle of a vacuum to lift a sample and measure it, in a quick simple action. If you can get this type, it is worth the extra money.


Simple use of a hydrometer

At its simplest you can use the hydrometer at the start of wine making to check that the reading which is called the specific gravity or SG is in the range 1070-1090. It is in this range that there is enough sugar to make a dry wine 9.2% to 12% alcohol by volume. Add more sugar than this, and you may find the ferment struggles to start, less and the keeping qualities of the wine may be less.

At the end of fermentation you need to check that the sugar really has fermented out, and the ferment is not simply stuck. The hydrometer reading should be taken and checked that it has fallen below the 1000 mark.

The above usage is really very easy, and will avoid you making the basic mistakes, that lead to problem ferments or burst bottles.

Advanced use of a hydrometer

Apologies for the use of imperial scales, but we brew wine in gallons, so it makes life easier.

The primary advanced use of a hydrometer is if you want to feed your wine sugar, as it ferments, if you are trying to brew a high alcohol wine, it is better to feed the wine sugar in stages.

In this case what matters it the total drop in SG throughout the process, each time you add sugar you take before and after readings, enabling you to add up each drop in SG from the previous stage. This total is then divided by 7.36 to give alcohol by volume.

Specific gravity (SG)Potential %vol alcohol

sugar /Gallon

10100.92 oz
10151.64 oz
10202.37 oz
10253.09 oz
10303.712 oz
10354.415 oz
10405.11 lb 1 oz
10455.81 lb 3 oz
10506.51 lb 5 oz
10557.21 lb 7 oz
10607.81 lb 9 oz
10658.61 lb 14 oz
10709.22 lb 1 oz
10759.92 lb  4 oz
108010.62 lb 6 oz
108511.32 lb 9oz
109012.02 lb 12 oz
109512.72 lb 15 oz
110013.43 lb  2 oz
110514.13 lb 5 oz
111014.93 lb 8 oz
111515.63 lb 11 oz
112016.33 lb 14 oz
112517.04 lb 1 oz
113017.74 lb 4 oz

4 lb 7oz

Temperature Correction

When wine making, boiling ingredients to extract flavour is a common process and you may want to get a reading before it has returned to room temperature.

Degrees FahrenheitCorrection
40Subtract 2
50Subtract 1
70Add 1
80Add 2
90Add 4
100Add 5
110Add 7
120Add 8
130Add 10
140Add 13
150Add 15


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