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What The Different Hydrometer Scales Mean

By Ed Kraus

There always seems to be a little confusion going on about the different scales you will find on various wine hydrometers for home making wine. So, I thought I would take a little time here to give some background and explain what is really going on when we take a hydrometer reading.


The Basic Idea Behind the Hydrometer

The whole premise behind the hydrometer is: "the heavier the liquid being tested, the higher the hydrometer will float" - in other words, the buoyancy of the liquid increases with the weight of the liquid. For example, if you float a hydrometer in water, it will float much lower than if you put the same hydrometer in maple syrup. This is because the maple syrup weighs much more than water.

Some people have a problem thinking in terms of a liquid having a weight. For some it is easier to think in terms of thickness. The same can be applied by saying, "the hydrometer floats higher in maple syrup because it is 'thicker' than water". This is okay too.


Why do we even care how high or low a hydrometer floats?

During a fermentation, basically thick sugars are being turned into thin alcohol. In other words, the juice is going from heavy to light. This also means that throughout a fermentation the wine hydrometer will float at different heights, giving different readings.

At the beginning of fermentation the hydrometer will float at its highest. At the end of fermentation the hydrometer will float at its lowest. And, throughout the fermentation it will float everywhere in between those two readings. It is with these various readings that we are able to monitor the progress of the fermentation and track the alcohol that has been produced.


The Hydrometer Scales 

SPECIFIC GRAVITY:
This particular scale is the most commonly used among home wine makers and is referred to in most home wine making books. It should be thought of only as a way of keeping in step with any recipes you may be using that make references to this particular scale.

The Specific Gravity scale is based on the weight of water. If you float a hydrometer in water it will read 1.000 on the Specific Gravity scale. At the beginning of fermentation a typical reading might be 1.090. This means, for example, that the juice at that point weighs 9 percent more than water, or the juice is 9 percent thicker than water.

When all the sugar is turned into alcohol you will have a reading on the Specific Gravity scale that is less than water - typically around .995. This means that the juice weighs less than water, or it is thinner than water by a half of a percent.

Again, simply think of the Specific Gravity scale as just a very common scale that allows you to follow the progress of your wine's fermentation, and to stay on track with home wine making books and wine recipes that mention it. 


POTENTIAL ALCOHOL:
This scale tells the winemaker how much alcohol can be made from the sugars that are currently in the juice. A single reading on the Potential Alcohol scale can NOT tell the winemaker how much alcohol is already in the wine. It can only tell the winemaker how much MORE alcohol can be made, based on the liquid's current weight or thickness, or in other words, how much sugar still remains in the juice.

The Potential Alcohol scale on most hydrometers usually ranges from 0 to 20 percent alcohol. So, for example if you mixed a recipe together and took a reading of 13 percent on the Potential Alcohol scale before the fermentation began, this means that there is enough sugars in that juice at that point in time to potentially produce 13 percent worth of alcohol.

Now, not all fermentations use up every bit of sugar, so when the
fermentation has finished, you can take a second reading to see how much "potential alcohol sugar" has been left over, if any. By comparing these two readings, you can then determine the current alcohol level of your wine.

Let's say for example, that a juice started out with a reading of 13 percent before the fermentation started, and had a reading of 1 percent after the fermentation stopped. By comparing the two readings you can determine that the juice now contains 12 percent alcohol. Another way to look at it is that the fermentation moved 12 points across the scale, resulting in 12% alcohol.


BRIX SCALE:
This hydrometer scale is used mostly by grape growers and commercial wineries. It is also referred to in more advanced home wine making books.

The Brix scale is based on percentage of sugar that is in the liquid by weight. Typically the scale will go from 0 to 30 on most wine making hydrometers. If you have a grape juice that reads 24 on the Brix scale, that means that the juice is made up of 24% sugar by weight.

Vineyards use this scale to determine if the sugar level of the grape is sufficient for harvest. Wineries use this scale to determine if the juice has sufficient sugar to produce the alcohol level they desire.


SUGAR SCALE:
This scale tells the winemaker how much sugar is in the juice at a given time. The readings are given in ounces per gallon. For example, you might have a reading at the start of fermentation of 35. This means that you have 35 ounces of sugar per each gallon of juice.

Now, this does not necessarily mean that you added 35 ounces of sugar for each gallon of juice. Some of the sugar comes naturally from the fruit as well. For example in the case of grape juice, you can have a reading of 35 without adding any sugar at all; the sugar comes completely from the grape itself.

Why is this important? This information in itself is not really important. But, when used in conjunction with other scales on the hydrometer it can be very, very valuable.

Let's say that you have a recipe that says to add sugar to the juice until the Specific Gravity reading is 1.072. So, you mix all the ingredients together as the recipe calls for, except for the sugar. You take a reading and find that your Specific Gravity reading is 1.046, but you need to be at 1.072.

This is where the sugar scale comes into play. By determining how many ounces of sugar is represented in each gallon of juice now, at a reading of 1.046, and determining how many ounces of sugar will be represented in the juice at a reading of 1.072, you can then determine how much sugar you need to add to a juice to take your from a reading of 1.046 to 1.072.

When looking at a hydrometer that has both the Specific Gravity scale and the Sugar Scale, you will see that a Specific Gravity reading of 1.046 also equals 20 ounces of sugar per gallon. Likewise, a reading of 1.072 equals 30 ounces of sugar per gallon. Armed with this information you can see by comparing these two readings that to raise the hydrometer reading from 1.046 to 1.072, you need to add 10 ounces of sugar for each gallon in the batch - this is the difference between 20 and 30.

The Sugar Scale can also be used in conjunction with the Potential Alcohol scale. For example, if you have a Potential Alcohol reading of 8 percent and you want it to be 12 percent, just look up the corresponding ounces on the Sugar Scale to determine the amount of sugar to add.

In this example, a potential alcohol of 8 equals 26 ounces of sugar per gallon, and a potential alcohol of 12 is equal to 38 ounces of sugar per gallon. The difference is 12 ounces of sugar per gallon. This is the amount of sugar to add for each gallon to increase your potential alcohol from 8 to 12.

If you found this useful, you may want to read our other wine making articles or maybe you are ready to start making your own wine. Then check out our huge selection wine making supplies and wine making ingredients

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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.