Hydrometer: Definition, Scales & Uses

Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

Want to know if your milk is pure? You can use a hydrometer! Want to know if your salt water is salty enough for your fish? Use a hydrometer! Learn more about this useful little tool in this lesson.


Did you know that there is a tool that measures how salty saltwater is and how much alcohol your wine has? This little tool is called the hydrometer, which measures the density of a liquid.

Hydrometers look a lot like thermometers, except they don't have an internal liquid that rises and falls. Instead the hydrometer has a weight at the bottom that is calibrated to a specific level. This hydrometer then sinks or rises depending on the density of the liquid. Lighter liquids will make the hydrometer sink and heavier liquids will make the hydrometer rise.

A hydrometer. A shows the scale by which the density of the liquid can be read, B is the weight.

The scales on hydrometers will look backwards to you if you are used to a thermometer scale. A thermometer scale increases as you go up the tube but a hydrometer's scale increases as you go down the tube. This is because heavier liquids make your hydrometer rise thus lowering the location where the top part of the liquid reaches.


Hydrometers have a lot of uses. Some are even calibrated for specific applications.

Want to keep saltwater fish as pets? Or maybe you work in marine biology using saltwater tanks? Hydrometers can be used to measure the density of saltwater. This is important because marine fish require a specific amount of salt in their water. So fish-keepers who mix their own saltwater must use a hydrometer to make sure that their mixture is just right for their fish.

Winemakers use hydrometers to check for the amount of sugars in juices and also to see how much alcohol can potentially be made from a juice.

Milk producers also use hydrometers to check for the density of milk. Pure milk that's not diluted will have a density of 1.03 grams per milliliter at 25 degrees Celsius or a specific gravity of 1.035.

With all these uses, there are also several different scales that hydrometers can have.

Differing Scales

The most common and most versatile scale is called the specific gravity scale. This scale has no units since it compares the mass of a liquid to the mass of pure water. Most hydrometers are calibrated to water at 60 degrees Fahrenheit.

When you stick a hydrometer into water at 60 degrees Fahrenheit, it will float at the 1.000 mark. Warmer temperatures will decrease your hydrometer reading and cooler temperatures will increase your reading. For example, when measuring the density of salt water, the hydrometer will read 1.023 at 79 degrees Fahrenheit for a specific gravity of 1.0253 at 60 degrees Fahrenheit.

Another common scale is that used in the wine industry. This scale is called the Brix scale, which measures the amount of sugar in the liquid. The scale goes from 0 to 30, which stands for the percentage of sugar in the liquid. So 13 means the liquid is made up of 13 percent sugar. Wineries use this information to determine whether a certain juice has enough sugar to make wine out of it. This type of hydrometer is also referred to as a saccharometer.

Another scale used in the wine industry is that of potential alcohol. This scale tells you how much alcohol you can make with the amount of sugar still in the liquid. This scale does not tell you how much alcohol is already in your liquid though. To calculate this, you'll need to take a potential alcohol reading before your fermentation and another reading after fermentation. The difference is your alcohol content of your liquid.

For example, if you get a reading of 17 percent before fermentation and 2 percent after fermentation, then your alcohol content is 17 - 2 = 15 percent. Alcohol hydrometers are also referred to as alcoholmeters.

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