How To Use A Hydrometer

Using the Hydrometer

Many people go through their wine-making lives without using the hydrometer and this is quite understandable, for the truth is that it is not an essential in making good wines. But it does perform a very useful service and it is known that many people would use it if they could get to understand it. For some reason many people find it confusing and I think I can explain the reason for this.

How To Use A HydrometerWe use a thermometer to measure temperature whether it be of air, hot liquids or our own when we are ill. But when we use a thermometer, we use nothing by way of comparison. When using the hydrometer we use water as the comparison factor, but this need not confuse anybody. Water has what we call the gravity of 1000. It has been given this figure because many liquids — especially spirits such as whisky or methylated spirits — are thinner (less dense) than water, so they, compared with water, have a gravity below 1000, and the reading would appear as 0999 or whatever it happened to be. Conversely, many liquids, especially those we prepared for wine-making, have a greater density (or are thicker) than water. Therefore compared with water, with its gravity of 1000, the liquids we prepare for wine-making have a reading above 1000; how much above this figure depends upon the amount of sugar in the mixture. And let me say at once that all we are really concerned with is the figure above the 1000.

Just to help you get used to reading the table the specific gravity of 1030 will make 3.6% of alcohol by volume, while the reading of 1050 will make 6.4% by volume.

Specific gravity

All this means is ‘the gravity as compared with water’. Therefore the figures 20, 30 and so on, up to 120, in the table are all we are really concerned with.

The importance of the hydrometer in most people’s minds is that it enables them to make a wine of a given percentage of alcohol and at the same time allows them to check if they have actually made it. This is most important to people making dry wines when 10% or 11% of alcohol by volume is plenty. These people of course do not add sugar as given in a recipe. They calculate that 65 g (2 ¼ oz) of sugar will raise the gravity by 5 on the hydrometer in one gallon of must while 130 g (5 oz) will raise the reading by 10.

They calculate how much sugar to add and then check the reading. This is done simply by filling a trial jar with some of the prepared liquid and then sliding the hydrometer into the sample, which must be standing on a level surface.

The hydrometer must float clear of all parts of the flask. The reading is taken where the liquid cuts across the stem. This then, is the specific gravity. It can be increased by adding more sugar, bearing in mind that 65 g (2 ¼ oz) will raise the reading by 5, or it can be reduced by adding water or fruit juice.

If you begin making wine with specific gravity of 1110 you should finish up with a reading of 1000 or perhaps a fraction under this, say 0999. This means that the yeast has used up all the sugar and you have a dry wine of 14.5% by volume. But as mentioned, dry wines are best for being in the region of 10% or 11% by volume so it is best to start with a specific gravity of 1080 or 1090 for dry wines, unless you want dry wines higher in alcohol. It is for you to decide.

The sweeter wines must of course be higher in alcohol, and because all the sugar up to 1110 on the hydrometer will be used up during fermentation, some sugar will have to be added for sweetening purposes. How much depends on personal tastes but on the whole 100-150 g (4-6 oz) of sugar unfermented in one gallon of wine is usually ample.

I have quoted the figure 1120 as giving a potential alcohol content of 16% by volume. And if fermentation is as satisfactory as it should be this amount of alcohol will be made. But, and it is often a big but, this is not always the case. So it is often wise to assume that you will not make more than 15%, which is represented by a specific gravity of 1115. Indeed many people, for some reason or other, never obtain an alcohol content of above 14.5%. This is not calamitous because 14% or just over is a nice percentage that ensures that the wine will keep well.

Now let us see what happens to a must prepared as described, after fermentation has begun until it Ceases. As most of you will know, fermentation may last for several weeks, or it may be over in a few weeks. I have tried to give a reasonably representative picture of the progress of fermentation. But do not expect your ferments to progress in an identical fashion. This is merely an illustration of what might happen under ordinary home conditions.

As soon as fermentation begins, there is some natural warming of the must because of yeast action. This can lead you to think that an appreciable drop in sugar content has taken place in the first day or so. What happens is this. The must expands slightly as it gets warmer, and this lowers its density and therefore a hydrometer reading, if one is taken. So do not take a reading of the must until a week after fermentation has begun. By that time the temperature of the must will be reasonably normal. If you are using a fermentation cupboard, where the temperature of the must will be around 18°c (65°F), the warming effect will not be noticeable.

Alcohol is lighter than water, and therefore it could very well be that a well-fermented dry wine would have specific gravity lower than that of water. In this case your hydrometer would sink to just below the 1000 mark.

When you get a drop of 110 it means that you have made the amount of alcohol you wanted. But if you find that fermentation has stopped and that the reading is above 1000, it means that you have not made the amount of alcohol you wanted, and you have not got the bone-dry wine you aimed at. You should therefore try to get fermentation going again in order to use up the little sugar that is left.

However, if you are satisfied with the wine as it is, there will be no need to do this. In any case, if there is only, say, 5 registering on the hydrometer at this stage, I doubt whether you will be able to get fermentation started again. You can, of course, leave the lock in place, keep the wine warm and hope that fermentation recommences on its own, as it will often do, or regard the wine as a finished product ready for putting away to clear.

I do urge you to take weekly readings to see just how matters are progressing, for this can be quite fascinating and can add a useful measure of background knowledge for the future.

Having explained the use of the hydrometer in various aspects of wine-making, I want to bring home to those who simply must insist upon absolute accuracy, that certain points must be considered. The majority of wine-makers will be content to go ahead on the readings they obtain when finding the specific gravity of their musts and be quite content with the result of the fermentation and wine resulting from it. But it would be unfair if I were not to let others know that there are small factors which should be taken into account if they insist upon accuracy.

The illustration below shows an enlarged section of the hydrometer in a sample must. In this is illustrated what we call surface tension. What we mean by this is that sugar solutions, and even water to lesser extent, tend to climb a little way up the stem of the hydrometer. The greater the sugar content, the higher is the surface tension.

So when taking readings, take care to read the hydrometer strictly at eye level, and watch carefully for surface tension. Another factor is that, as most of you know already, a must is not made up merely of sugar and water. Neither are fruit juices. There are other constituents which have a small effect upon the hydrometer readings.

For example, the acids and pectin in fruits tend to increase the reading slightly. To allow for this, it is wise to regard the sugar content as being 3 lower than the reading states. This would be ample for musts containing the usual 1.5-2.5 kg of fruit to 5 ltr (about 4-6 lb per gall). But when, say, 3.5 kg (8 lb) or more are used and when grapes are being fermented, it would be wise to allow as much as 5 and deduct this from the reading.

Where dried fruits alone are being used, this allowance is not necessary, but if equal quantities of dried and fresh fruits are being used, half the figures quoted would be enough, depending on whether you are using ordinary English garden or wild fruits or grapes with the dried fruits.

The Hydrometer and Sweet Wines

Making sweet wines is more successful when the hydrometer is used. I say this because it is always best to start off with a specific gravity of 1110. And by using the hydrometer we are able to start with the right amount of sugar in the must. How much sugar the must contains before you add any depends on the sugar content of the fruit and the amount of fruit used. With English garden and wild fruits the amount will be very little, but it is worth knowing how much.

When making sweet wines we do exactly the same as described for dry wines to start with. We start fermentation in the normal way and watch its progress by taking readings with the hydrometer. These need not be frequent; weekly or fortnightly will do. Meanwhile, we must try to decide how much sugar left unfermented will make a wine medium sweet or sweet. This will depend a very great deal on personal taste. A wine which is medium dry to medium sweet to one person may be sweet to another, and so on. Some people like a sweet wine with only 10° of sugar left registering as unfermented on the hydrometer. As we have seen, this 10 represents 130 g (4 ½ oz) of sugar. But others would want more than this.

So it is a matter of trial and error according to personal taste how much sugar to add in addition to the amount that will be used in making the alcohol we want. So we begin the ferment with a reading of 1110 knowing that this amount of sugar will be fermented out in making the alcohol we want, and, as some sugar is used up, we add a little more. This extra will be left unfermented and will register on the hydrometer when all fermentation has ceased.

Now it is a fact that by adding sugar bit by bit we seem to be able to induce the yeast to make a little more alcohol than it otherwise might. But this is not something that you should count on, although it often happens. So you should always work on the assumption that you will not make more than 15% of alcohol by volume. If you do make more alcohol, and more sugar is used up in consequence, so much the better. But then you will have to add a little more sugar to give the wine the sweetness you want.

The Hydrometer and Pulp Ferments

Pulp ferments as we call them are fruit musts fermented on the pulp, or where the fruits are crushed and the skins and pips fermented for a time.

To take an accurate reading (to obtain the correct specific gravity) you will have to crush the fruit well, and mix it with water, as in the many recipes and methods describing this type of ferment. Add he initial 900 g (2 lb) of sugar with the remainder of the water, and then take a sample of the must. Strain out the solids through several thicknesses of muslin. You only need enough liquid to fill the trial jar. You can then make any adjustment required merely by consulting the other parts of this article.

The above applies to all garden and wild fruits. Musts made from grapes only are handled a little differently. This is because they contain a great deal of sugar, so much so, in fact, that it is always best to find the specific gravity before adding any sugar at all. It is a fact that some grapes, especially in a good season, will contain enough sugar to make an excellent dry wine without adding any sugar at all. But we cannot rely on this, so we must find out the specific gravity of the juice before we begin. And here a hydrometer which reads from 1000 to 1100 will suffice. Grapes are usually used alone and the juice left undiluted. Grapes bought from a greengrocer would make the cost of wine prohibitive — though some people seem able to afford it.

At this point I can let you into a little dodge that often works; I have done it late on a Saturday evening in certain London markets. Street traders packing up for the weekend will often sell off the remainder of their grapes at much less than the quoted price because they know that by the Monday — a day on which there is little demand for such fruit — the grapes will be past their best and by the Tuesday quite unsaleable. I have bought boxes of Cape grapes containing more than 14 lb, marked up at 50p a pound, for as little as £2.00. But you will have to bargain with them; otherwise they will diddle you with a smile and a conscience as clear as brilliant wine.

It is impossible to say with any certainty how many grapes are required to make 5 ltr (1 gall) of wine. This is because the juice content will vary with each sort. All we are concerned with in this section is finding the specific gravity of the juice of the grapes.

The first thing to do, therefore, is to crush them well by hand and wring them out by the handful in order to get all the sugar into the juice. The fact that the pulp is not strained from the juice does not matter. When the grapes are thoroughly crushed, take a small sample of the juice and strain it through several thicknesses of muslin. Put this into the hydrometer jar, and take the reading as for other juices.

You are then able to work out how much sugar you need to add to bring the specific gravity up to 1110 (or 1100 if this suits you), simply by looking up the details for other wines on this website. If you want to make sweet wines from grapes, you will tell at once by reading the appropriate section how adding sugar during fermentation affects the readings, and how to calculate how much sugar is being used and the amount of alcohol you have made when all fermentation has ceased.

The Hydrometer and Concentrated Grape Juices

When using the Hidalgo range of concentrated grape juices there is no need to use the hydrometer unless you want to. This is because the cans they are supplied in hold 1.2 1 (2 pt) which make 5 ltr (1 gall) of wine, or a 2.4 1 (4 pt) which make 10 ltr (2 gall), or 5 ltr (4 gall) which makes 20 ltr (4 gall); and all have a specific gravity of 1100. When we make 1.2 1 into 5 ltr (2 pt into 1 gal!), or the larger sizes into 10 or 20 ltr (2 or 4 gall), we reduce the figure 400 to 100. This is because we have made the original amount into four times as much. When we dilute as we do, it must be borne in mind that all we are concerned with is the figure above the 1000. This is because it represents the sugar content, while the figure 1000 only refers to the water. To make it abundantly clear we have:

  • water 1000
  • Sugar 100
  • Specific gravity of diluted juice 1100

This reading will produce a wine of roughly 12% of alcohol by volume and the wine will be dry. If you want more alcohol than this you will have to add sugar to give a reading of 1110. In other words, raise the gravity by 10 on the hydrometer. As we have seen, 65 g (2 ¼ oz) of sugar will raise the reading by 5. So to achieve the maximum alcohol all that need be done is to add 130 g (4 ½ oz) of sugar and this is best done by dissolving it in some of the water to be used for diluting the concentrate. Just warm a little of this, dissolve the sugar in it, stir it into the concentrate and then make the whole up to 5 ltr (1 gall). The above example refers to making 1.2 1 into 5 ltr (2 pt into 1 gall). If you were making 2.41 into 101 (4 pt into 1 gall) you would need 260 g (9 oz) of sugar, or 520 g (18 oz) if you were making 5ltr into 20 (1 gall into 4 galls). When you have added the sugar to give the maximum alcohol the wine will still be dry. So if you want a sweet wine you will have to add a little more so that it remains unfermented. Generally all that is needed in 5 ltr (1 gall) is an additional 50 g (2 oz) for a medium and 100 g (4 oz) for a fully sweet wine, but much depends on personal tastes.

So much for the Hidalgo range. However, there is a very wide range of other excellent concentrates on the market and most of these come in cans or sachets containing 1 kg (2 lb 3 oz or 27 fl oz) as against the 1.2 1 (40 fl oz or 2 pt) of the Hidalgo range. I have found that when one of these is diluted to make 5 ltr (1 gall) of wine the specific gravity is only around 1060, to give an alcohol content of around 7.7% by volume which is only just enough for a low alcohol dry wine. But this is not necessarily the case with all the concentrates available. Since it is important both for deciding how much alcohol you want your wine to contain and whether you want a dry, medium or sweet wine, you will have to use the hydrometer to find the specific gravity when one of these cans has been made into 5 ltr (1 gall) of must with water. My method for making wines from concentrated grape juices is quite simple. The directions on some of the cans make it all so complicated and involved that I devised the following simple method for all the concentrates.

My method with concentrated grape juice It very often happens that when concentrates have been standing for some time the sugar settles to the bottom of the can and becomes quite solid. So the first thing to do is to wipe the top of the can clean with a damp cloth, then open it and stand it in hot water over gentle heat, stirring occasionally. In a few minutes the juice will be ready to pour into the fermenting pail. When you have done this, make up to 5 ltr (1 gall) with boiled water that has cooled. Make .sure all the sugar is dissolved and that the concentrate is mixed thoroughly with the water, otherwise you will obtain an inaccurate hydrometer reading. When you are ready, take a sample of the mixture into your trial flask, stand it on a level surface and gently slide the hydrometer into it. The hydrometer must float clear of all parts of the flask — in other words it must not touch the bottom or sides. Add more juice to the flask if the hydrometer touches bottom. As the hydrometer floats, take the reading where the juice cuts across the stem.

From this you will see how much you need to raise the gravity to whatever alcohol content you wish. Bear in mind that all the sugar will be fermented out with a starting reading of 1110 or less, so all the wines made with this reading or below this will be dry.

Having decided how much you have to raise the gravity, bear in mind that 65 g (2 ¼ oz) of sugar will raise the reading by 5 and work out how much sugar you need to add. This will have to be dissolved in some of the diluted concentrate, so put the sugar in a pyrex or polythene jug, pour over it some of the mixture, stand the jug in a saucepan of hot water over gentle heat and stir until the sugar is dissolved. Then pour this into the mixture, add the yeast and continue as for any other wine.

Note that you do not use additional water to dissolve the sugar otherwise you will not get the type of wine you intend. And do not take an hydrometer reading after adding the sugar; you will not get the exact figure you aimed at because the sugar will have increased the overall volume of the liquid to some extent. For example 900 g (2 lb) of sugar occupies the space of 6 dl (1 pt), and if you try to rectify matters you will be at it for hours on end because each small sugar addition will increase the overall volume of the liquid. So when you have added the sugar to give you the ‘reading you want, leave it at that.

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