The hydrometer is made up of a glass tube with a bulb at one end and a graduated scale in the stem. When placed in a liquid, it floats with the stem protruding out of the liquid surface. The specific gravity (S.G.) of the liquid can then he read off the scale. But what does this mean? 

The S.G. of a liquid is in fact a measure of its thickness or density; pure water weighs one gram per cubic centimetre, so its S.G. is said to be 1.000. But as you dissolve sugar in water, the water, or rather the sugar solution, becomes thicker: it now has a density greater than that of pure water, so the S.G. will be greater than 1.000. Thus a hydrometer placed in sugar solution will float with the scale showing more than 1.000 at the liquid surface. The S.G. of a wine can be measured in exactly the same way, and the S.G. can then be used to determine the amount of sugar present in the must. The table also shows the per cent of alcohol by volume which is produced for a given weight of sugar in a must, provided that the yeast ferments all the sugar to alcohol. This figure is shown in the "potential alcohol" column.

Table 1 - The Hydrometer - UK and Metric measurements


Table 2 - US measurements

The instructions for using a hydrometer are always included with it when you buy it. They need not, therefore, be repeated in full here. To summarize: 

1) The hydrometer must be spun when it is floating in the liquid and before taking the reading to dispel any air bubbles which are clinging to the sides of the glass - otherwise they would add extra buoyancy to the hydrometer and produce a false reading.

2) The hydrometer must be floating freely away from the sides of the jar or trial tube.

3) Take the reading at the bottom of the meniscus where the liquid actually touches the hydrometer. The hydrometer cannot differentiate between sugar solution and any other material such as fruit pulp in a must. The liquid or must being tested should strained through a fine mesh sieve before its S.G. is measured.

Example of the use of the hydrometer in making wine at home

Suppose that a wine of 15% alcohol is required. Six pints (3.4 litres) of must have been prepared from the ingredients. How much sugar will be needed to produce the desired level of alcohol? The hydrometer is the only way you can tell how much sugar has been extracted form the ingredients and how much you need to add. 


1 Check the S.G. of the must. Suppose this is 1.030 

2 Refer to Table 1. This shows that S.G. 1.030 equals 376 grams of sugar in 4.5 litres.

3 Adjust this figure to give the weight of sugar in 6 pints (3.4 litres) of must: 3.4/4.5 *376 = 248 grams

4 Refer to Table 1. Check the weight of sugar required for a wine of 15% alcohol : 1332 grams per 4.5 litres.

5 Subtract weight of sugar already present in the must to determine the amount to be added:  1048 grams. Note: Most of the recipes on this website refer to 1 gallon (4.5 litres) of wine. However, a demijohn actually holds 8.5 pints (4.8 litres) up to the base of the neck. This can produce a slightly weaker wine than the figures indicate for a given amount of sugar. Generally the error is insignificant; but if you want to be really precise, you can add an extra 1 oz (28 g) of sugar for each 1 lb (450 g) specified in the table. 


Some authors talk in terms of gravity rather than specific gravity. In fact gravity refers to nothing more than those figures of the specific gravity which fall after the decimal point. Thus, for example, S.G. 1.020 becomes gravity 20; S.G. 1. 115 becomes gravity 115. For specific gravities less than 1.000, the gravity becomes negative: for example, S.G. 0.995 equals gravity - 5. 


Yeast is unable to work efficiently much sugar is dissolved In a must. If you have a hydrometer, aim not to exceed S.G. 1.100 at any time. Extra sugar is best added when the conversion of sugar to alcohol during fermentation has reduced the S.G. to 1.010 or less. 

Checking the progress of fermentation and producing strong wines

While a wine is fermenting samples of wine can be checked at intervals. As the fermentation proceeds the sugar is fermented to alcohol and the S.G. falls. The rate of fall decreases as fermentation nears completion, and when no change in readings occurs over a period of two or three weeks, the fermentation will have ended. The hydrometer is especially useful in producing strong wines. When producing a strong and sweet wine, it is best to add half the sugar at the start of fermentation and the remainder in 4 oz (110 g) lots each time the S.G. falIs to 1.010 or less. This will avoid producing an over-sweet wine. If you want a strong dry wine, allow the S.G. to fall to 1.000 before each addition of sugar, and add it in 2-oz (55 g) lots when nearing the point at which the yeast cannot work because of the high level of alcohol. 

When fermentation is complete

The ideal winemaking practice is to ferment a wine to dryness at the desired level of alcohol and the n to sweeten to taste. Clearly sugar cannot be added until the wine is perfectly clear and all traces of yeast have been removed from it, otherwise fermentation will restart. (This does not apply if the unfermentable sugar lactose is used.)

When a wine has fermented to dryness, those yeast cells which have been in suspension will fall to the bottom and add to the sediment or "lees". As the sediment thickens, the wine will begin to clear noticeably. This is the time to rack, i.e. to siphon the wine off the yeast sediment.

Occasionally one sees books which recommend racking as soon as one month after the wine has been placed in the demijohn - whether or not the fermentation is complete. This is bad winemaking practice. The bulk of the yeast colony forms a light sediment on the bottom of the fermentation vessel soon after it has been set up, and racking before the fermentation has ended will clearly leave only those yeast cells in suspension in the wine. This could deplete the colony to a point where it would almost certainly be inadequate to carry fermentation through to completion. So why do some books recommend such early racking? The answer seems to be that if fruit pulp and debris have passed into the demijohn, they may impart unpleasant flavors if they are left In the wine; hence the need for early racking. In my view, however, this problem should not arise if the pulp has been strained correctly. Therefore in general, once a fermenting must has been put under airlock, it should remain there until the fermentation has finished.

The simplest way to tell when fermentation is complete is to see whether any bubbles are rising to the surface of the wine or passing through the air-lock. Other useful ways to judge the point at which to rack include:

  • The change in appearance of a wine: Although a wine will not be completely clear, a firm sediment will have formed and it will look less murky than it did when it was fermenting vigorously.
  • The taste of the wine: There should be a high level of alcohol and little, if any, sweetness.
  • The rate of fall of the specific gravity, if you are using a hydrometer.

Next: racking the wine in detail; the finished wine


Original posting date
01 January 2018