Using The Hydrometer

A hydrometer and test cylinder are among the most important tools for any winemaker. Used to measure specific gravity to monitor fermentation and sugar levels, these two items are very inexpensive (about $15.00 for the hydrometer and test cylinder) and there is no good reason that any winemaker should be without them.

Although it is not absolutely necessary to measure the specific gravity (S.G.) in order to make wine, knowing the starting S.G. provides an indication of the potential alcohol content of the juice or liquid that is going to be fermented. As well, the hydrometer will assist in tracking the fermentation process. For wines that are not meant to be dry, knowing the S.G. of the wine will provide an indication of when to stop the fermentation process.

We can also use a hydrometer to find out if there is enough sugar in the liquid to make wine with an alcohol content above 10%. Grape wines require about 10% alcohol by volume in order for it to preserve, while some fruit wines require a higher volume of alcohol. If we’re making a wine from a fruit where the
juice’s S.G. is 1.092, we’ll want to add sugar to increase the potential alcohol.

Using a hydrometer and test cylinder together is very easy. A wine thief is used to draw a small sample of the juice or wine from the fermenting vessel. With the hydrometer in the test cylinder, the wine or juice is then transferred into the cylinder from the wine thief. I like to have the hydrometer in the test cylinder prior to adding the liquid to ensure there is no spillage. If you add liquid to the cylinder first, and then put in the hydrometer, you risk raising the level of the liquid and overflowing the cylinder.

Once you have enough of the liquid that you are measuring the specific gravity of in the cylinder so that the hydrometer begins to float, you can stop adding to the cylinder – however I like to nearly fill the cylinder as I find it easier to read the hydrometer the higher it is.

A decent wine and beer hydrometer will have three different measurement scales on it: Specific Gravity, Potential Alcohol By Volume, and Percent Sugar (Balling). These are referred to as “Triple Scale Hydrometers.” Most often it is the S.G. that is recorded but the Potential Alcohol By Volume may also be of interest.

Specific Gravity is actually a measurement of the density of a liquid. A hydrometer is usually calibrated against distilled water at a temperature of 60°F for a S.G. of 1.000. For the purposes of winemaking, the majority of the density above 1.000 is sugar in the juice. It is sugar that the yeast feeds on and turns into alcohol and carbon dioxide. As the sugar is consumed, the alcohol content increases. The Specific Gravity of ethyl alcohol is 0.787. Therefore, if the juice has fermented and all the sugar used, the S.G. of the final wine should be below 1.000 – probably closer to 0.990.

Although you cannot determine the actual alcohol content using a hydrometer, you can get pretty close to calculating the final alcohol content as long as you have an accurate reading of the S.G. of the juice prior to fermentation and the S.G. after the fermentation has stopped. If the S.G. of the juice was 1.094, and the final S.G. is 0.990, the alcohol content will be about 12%.

There are a couple of caveats:

1. Beer and wine hydrometers, as noted above, are usually calibrated for 60F. As the temperature warms up, the liquid thins so the actual S.G. will be a bit higher than what is being shown on the hydrometer.

2. You may recall in your Junior High science classes that liquids in a container form a crescent shape on the surface, with the liquid either “creeping” up the side of the container at the edge if it is a light liquid such as water and creating a concave shape, or “creeping” down the side for heavier liquids creating a convex shape at the surface. When measuring the liquid, for the purposes of accuracy, it is important to measure at the meniscus – that part of the liquid that hasn’t “creeped.” This is the level of the liquid at it’s centre, and that is where you should take your reading.

When you use your hydrometer and test cylinder, don’t forget to sanitize them both first! You’ll want to be able to pour the juice or wine back into the fermenting vessel.