Correct Use of a Hydrometer

A Hydrometer is an essential piece of equipment for wine and beer making. Careful use of a hydrometer is very helpful in ensuring that the wine is started with the correct amount of sugar. It can also be used to help monitor the progress of fermentation and to determine the final sweetness level. Using your readings you can easily calculate the alcohol level of your finished wine.

Keeping good records of specific gravities during your wine making will enable you to give essential information to your local retailer if you have experienced any problems.

When sugar is dissolved in water the density (or specific gravity) of the liquid increases in proportion to the amount of sugar dissolved. In simple terms the hydrometer is a balanced device which floats. The denser the liquid the higher hydrometer floats and therefore the higher the specific gravity reading. Most hydrometers are calibrated to give a reading of 1.000° when placed in distilled water at 20°C. Any reading higher than 1.000° therefore indicates that the liquid is denser than simple water and therefore suggests that there is sugar present in the liquid.

As sugar is fermented by yeast, alcohol and carbon dioxide are produced. Alcohol is much less dense than sugar and water and therefore the specific gravity reading will decrease. The hydrometer can be seen to be floating lower in the water.

By measuring specific gravity (SG) at the start and finish of your wine fermentation you can calculate the percentage alcohol produced. The final specific gravity reading after completion of fermentation will also give you a good guide to the dryness or sweetness of your wine:

Less than 1.000°     dry to medium dry wine
1.000° - 1.006°        medium wine
Greater than 1.006° sweet wine





How to Use an Hydrometer

It is useful to use the hydrometer in conjunction with a trial jar rather than try to float the hydrometer in the wine itself. Sterilise the hydrometer and trial jar and drain. Fill the trial jar to within 1cm (1/2”) of the top and place the hydrometer into the jar. It is important to stand the trial jar on a sink drainer as there may be some spillage. If any froth is present gently blow this away from the hydrometer. Spin the hydrometer gently to remove any air bubbles and look at the hydrometer reading with your eye at the same level as the hydrometer. Take the reading from the gravity scale on the hydrometer at the point where the hydrometer stem enters the liquid surface (see diagram). After taking the hydrometer reading the wine may be returned to the fermenter.

If your wine is too cloudy to take a reading from the correct liquid level easily then take the reading where the liquid touches the hydrometer and add 0.002° to your reading to compensate.

Most hydrometers are calibrated at 20°C so your wine should be at about this temperature. If it is 5° higher then add 0.001 to the reading. If it is 5° lower then deduct 0.001 from the reading.

Calculating the Alcohol

The difference between the initial starting gravity and the final gravity, known as the gravity drop, can be used to calculate the approximate alcohol level. The alcohol content can be determined by dividing the gravity drop by 7.45. This will provide an answer in alcohol percentage by volume. For example, a wine with a starting gravity of 1.085, finishing at 1.005, produces an alcohol drop of 80.80 divided by 7.45 gives a level of 10.7% alcohol by volume.

General Comments

The hydrometer can be used to achieve the correct sugar level when starting a country wine. Simply take an initial gravity reading and then add sugar or grape concentrate as necessary to achieve the gravity required.

It is good practice to taste your wines at all stages of fermentation to enable you to correlate sweetness and dryness with specific gravity.

Finally, remember that a hydrometer is a delicate instrument as it has to be thin and light enough to float. Treat it gently. It is worthwhile keeping a spare!