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How and When to Use a Hydrometer

What is a Hydrometer?

The word Hydrometer literally means ‘water measurer’. It is a glass tube with a weighted bulb at one end and a scale at the other that measures the amount of sugar in a liquid.

The Hydrometer is placed in the liquid and the depth to which it sinks is measured on a scale inside the hydrometer. The more sugar there is in the liquid then the thicker, or heavier, it becomes and the higher the Hydrometer floats in the liquid.

Why is it Important to Know How Much Sugar is in a Liquid?

In wine making, yeast converts sugar into alcohol. The sugar can be natural sugar (from the fruit’s juice) or can be sugar that you’ve added (the stuff you buy from supermarkets). The more sugar that the yeast converts to alcohol then the stronger the wine.

There are basically 4 times you would measure the sugar content:

At the start of your fermentation you measure the amount of sugar naturally occurring in the juice or must.
To determine how much extra sugar you need to add to the juice to produce a wine of the desired alcohol content
To check the progress of the wine during fermentation.
To make a final check on the strength of the finished wine.

How to Take a Reading

Sterilise your Hydrometer and Test Jar.
The Hydrometer is only accurate at room temperature so let the liquid you are testing warm up first.
Pour the liquid into the test jar - down the sides to avoid forming bubbles.
Gently stir the liquid, being careful not to form bubbles.
Slowly lower the hydrometer into the liquid by holding it at the top of the stem. Let go when it just begins to float.
Gently push the hydrometer into the liquid by a few millimeters, let go and let it settle.

hydrometer reading

Take the reading as shown in the diagram. Dont let the hydrometer touch the side of the jar or you will get an incorrect reading.

How to Use the Reading

The scale on each hydrometer is slightly different but should cover the range 0.990 to 1.120 or there abouts. Sometimes the decimal point is left out giving a scale of 990 to 1120 in steps of 10. To add to the confusion the scale may be marked 990, 1.000, 10, 20, 30 etc. meaning 990, 1000, 1010, 1020, 1030 etc.

So what do these readings mean? If you put your Hydrometer into pure water it will float at a level of 1.000. If you add sugar to the liquid it will become heavier and the hydrometer will float higher. The table below shows the readings you will get for different amounts of sugar in the liquid. For example if you take one gallon of water and add 2.1lbs (935 grams) of sugar then the hydrometer will give a reading of 1080.

During fermentation the yeast converts the sugar into alcohol. If all the sugar was converted then the one gallon would then contain 10.6% alcohol by volume.

A Worked Example

Let's suppose that you have extracted some fruit juice and added water as required by your recipe. You take a reading with your hydrometer and find it to be 1055. Reading off the above table, this means there is 1lb 7oz (652 grams) of sugar per gallon (this is the natural sugar that was in the fruit). If all this sugar is converted to alcohol then the final strength of your wine will be 7.2%.

This is a little weak for most wines which are generally around 12%. You therefore need to add some extra sugar. Looking at the table, 12% alcohol is produced from a starting sugar content of 2lb 6 oz (1077 grams) per gallon so you need to add an extra 13oz (368 grams) of sugar per gallon. So if you had 3 gallons of liquid you need to add 3 times this amount i.e. 39oz (1105 grams).

Note: It is important that the sugar is fully disolved in the fuit juice so add a little at a time and stir thoroughly.

After you have added the sugar, check again with the hydrometer to confirm that the SG is now 1090 i.e. the starting SG for a 12% wine. If it's too low then add a little more sugar, if it's too high then either accept it (and you will get a slightly stronger wine) or add a little water to dilute the liquid.

Testing Your Wine at the End of Fermentation

When fermentation ceases you need to check the sugar content of the wine. There are two reasons to do this: