From time to time we receive calls regarding the use of the wine making hydrometer: about how a reading is actually taken; what the readings actually mean and so on. Let's face it, unless you've used a hydrometer before it is somewhat of a foreign concept. I will attempt to clear up some of the confusion here.
What the hydrometer can do for you is two-fold. First, it allows you to track the progress of your fermentation by taking readings throughout the fermentation. Secondly, it can tell you what the alcohol percentage of your finished product is by taking a reading before fermentation and one after fermentation and comparing the two.
HOW TO TAKE A HYDROMETER READING
To take a reading with the wine hydrometer it is required that you get the hydrometer to float in the juice to be tested. Whether it is floating in the plastic tube it came in, in a hydrometer test jar you purchased, or floating in the entire batch makes no difference, as long as it is floating you will be able to take a reading.
Once the wine hydrometer is floating, you take the actual reading by seeing where the surface of the liquid crosses the scale on the hydrometer.
DIFFERENT SCALES ON THE HYDROMETER
Typically, there are two scales on the hydrometer that you are concerned with: the "Specific Gravity" scale (normally labeled as S.G. or SP GR) and the "Potential Alcohol" scale.
Specific Gravity is referred to by most wine making books and recipes. It is simply a scale based on the weight of water. The "Potential Alcohol" scale is used to easily determine how much alcohol was made or can be made.
LEARNING ABOUT THE "SPECIFIC GRAVITY" SCALE
Just for fun, if you float the wine hydrometer in water and read the Specific Gravity scale, the surface of the liquid would cut across the hydrometer at the 1.000 mark, towards the top of the scale. Most hydrometers are calibrated to be most accurate at 60 degrees Fahrenheit. So, if the water is warmer or cooler, the reading may be off just a hair.
When you take a reading in fruit juice or water that has had sugars added to it, the hydrometer will float higher than before. This is because the liquid is now heavier than water which in turn increases the buoyancy of that liquid.
For example, it you completely dissolve 2 pounds of cane sugar into 1 gallon of water, you will have a Specific Gravity reading of 1.068. You will find this on the Specific Gravity scale by going down from the 1.000 to the 60. The 60 represents 1.060. In between the 60 and 70 you will see several tick marks. Each one represents two points. So, the fourth tick would be the 8. This is the point on the hydrometer that represents 1.068.
LEARNING ABOUT THE "POTENTIAL ALCOHOL" SCALE
If you put your thumbnail at the 1.068 mark mentioned above and roll the hydrometer around to the Potential Alcohol scale you will see it matches a reading of 9%. What this means is that if you were somehow able to get this liquid to ferment all of the sugars it contained into alcohol, the result would be a liquid with 9% alcohol by volume.
As the wine ferments you will see the Potential Alcohol reading becoming lower and lower. What this means is that as the sugars in the juice are turned into alcohol the potential for more alcohol is reduced.
To find out what the actual alcohol percentage of a finished wine is, you would subtract the ending fermentation reading from the beginning fermentation reading.
TYPICAL EXAMPLE OF HYDROMETER USE
For example, lets say you have 5 gallons of freshly pressed grape juice. You take a beginning reading of 12 percent on the Potential Alcohol scale of your hydrometer. When the fermentation is complete, you take another reading with the hydrometer that indicates a Potential Alcohol of 1 percent. You then take the 1 and subtract it from the 12, giving you a total of 11 percent alcohol. This means that the fermented juice now has 11 percent alcohol by volume.
If you are not currently using a wine hydrometer, I would suggest that you might consider using one in the future. While it may be a little intimidating at first, once you use one you'll soon discover that there is not much to it. The benefit is the hydrometer allows you to have much more control over your batches. With the hydrometer you can control the alcohol content of your wines, monitor the fermentation's progress, as well as determine when the wine is actually done fermenting.
Once you have finished learning about what a Hydrometer does, it is time to see Adventures in Homebrewing's wide range of Hydrometers available for sale.
If you find this article useful, you may want to read “What the Different Hydrometer Scales Mean”. Want to start making wine? Check out our selection of quality wine making supplies and wine making ingredients.
Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.