How to Read and Use a Hydrometer

A hydrometer although not an absolute must is an important tool that most brewers should have on hand.  This inexpensive tool (typically less than $15) measures the sugar content of the liquid, allows the brewer to be able to track how fermentation is progressing as well as measures the ABV of the beer, wine or spirit.

In this post, we are going to take a look at exactly what this tool is, as well as discuss how to use a hydrometer and provide you with a step-by-step explanation on how to read a hydrometer.

What is a Hydrometer?

There are two cylinders that make up a hydrometer.  The actual hydrometer itself and the jar it floats in.  A hydrometer can be made out of either very thin glass or plastic.  The bottom of it is shaped somewhat like a bulb and is weighted down with either steel or lead shot, which acts as a ballast of sorts keeping the bottom end pointed downwards and the whole unit floating upright.  Very similar to how a bobber works when fishing.  The jar it floats in is also typically made out of glass or plastic and is filled with the liquid that you want to measure.

Although there are different types of hydrometers, the majority of them will usually have three scales on the side of them.  Specific gravity, Balling/Brix, and potential alcohol.

What Does a Hydrometer Measure? 
A hydrometer filled with liquid.
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A hydrometer measures the gravity or weight of the liquid it is floating in, in relation to the weight of water.  Due to the fact that the relation of gravity to water is specified, they call the measure that is taken specific gravity.

Specific Gravity is the measurement of the density of a certain liquid at a specific temperature & pressure.

When it comes to brewing it is used to measure the number of sugars that are dissolved in the wort and then in the final product i.e. the beer; as compared to water.

Water as long as it is pure will have a specific gravity of 1.000. The gravity of the liquid increases as sugars are added to it during the brewing process.

Before yeast is added you will take a gravity ready. This is called the original gravity or OG. Most recipes you use will tell you what your original gravity should be.

Besides being important to help you calculate the ABV at the end of fermentation, the OG will also allow you to determine how successful your brew day was. Did you reach the gravity reading of your recipe? Taking the measurement allows you to fine-tune your brewing setup and practices. If you did not reach your original gravity then you know you need to make some changes for next time.

Most beers will have an original gravity of somewhere between 1.030 – 1.070. With some stronger beers going even higher.

After active fermentation has finished a brewer then takes another reading.  He then waits another 24 hours and takes another reading. If the two readings remain the same then you can feel confident that the fermentation is over. This last reading is referred to as the final gravity or the FG.

The original gravity and the final gravity are then compared to determine the ABV (alcohol by volume) or the alcohol percentage of the beer.

How to Use a Hydrometer

When it comes to brewing there are three tools that can be used to take gravity readings. A saccharometer and a refractometer; however, the hydrometer is by far the more popular of the three and the easiest to use.

Using a hydrometer simply involves filling the jar with the liquid you want to measure and then placing the hydrometer itself into the jar.  Where the surface of the liquid meets the readings on the side of the hydrometer is the measurement you will use.

You take your original gravity reading at the end of brew day just before adding the yeast to the fermenter and then your final gravity reading after all signs of active fermentation has stopped.

Once you have filled the hydrometer jar with the liquid, then gently placed the hydrometer into it give it a little spin. This will allow it to oscillate and then eventually slow down and settle.  Once it has settled into the liquid you can take your gravity reading.

Be sure to take your reading where the surface of the liquid touches the markings on its scale.  It is important to also ensure that the hydrometer is floating freely and is not resting against the side of the container.

How to Read a Hydrometer

Many people take an inaccurate reading because they record where they think the surface of the liquid meets with the scale on the hydrometer.

If you are taking your reading here you are actually taking your reading at the top of what is referred to as a meniscus. Which is basically a bubble in the liquid. It looks like it is hugging the shaft of the hydrometer and if you look closely you will notice that the top of it is actually higher than the surface of the liquid in the vial. This meniscus forming is common in containers like a test tube.

It is important that you locate the true surface of the liquid. Look down the meniscus towards the bottom of it and take your reading from there.  If you look carefully you will see that the reading from the top of the meniscus is a few marks higher than the reading at the bottom; which can throw your readings off and cause you to miscalculate.

How to Calibrate Your Hydrometer

Most hydrometers when you purchase them are quite accurate and you typically will not have to be concerned with the calibration being off.

However, it is important to understand how to calibrate one in case it happens to be out of calibration and the readings are off. The instructions that come with your hydrometer will explain how to do this. When you calibrate your hydrometer you will do so in a certain temperature liquid. Older models are typically calibrated at around 60 degrees F whereas the new ones are done so at around 65 – 70 degrees F.

When you are taking a measurement of your liquid the goal is to strive to have the liquid at the calibration temperature in order to get an accurate reading, alternatively, you can use a chart designed to correct for the temperature. These come with most hydrometers or you can do a search online for these types of charts.

When you first buy your hydrometer you can check the calibration of it to make sure it is accurate by taking distilled water, heating or cooling it to the temperature that your hydrometer calls for it to be, pouring the liquid into the jar, and placing the hydrometer in it and taking a measurement.

If it reads at 1.000 then it is correctly calibrated and you are good to go. If it does not read 1.000 then it is out of calibration and it is recommended to dispose of it and get a new one.  Some people do attempt to correct the calibration by using a file to shave off some of the glass from the bottom of it.  However due to how inexpensive these tools are we recommend simply getting a new one.

If you have any questions on anything we have covered here feel free to reach out to me. Drop a question in the comment section below and I will be sure to do my best to help you out.

Cheers, Big Robb is Out!

Big Robb with a pint of home brewed beer
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P.S. Interested in getting the recipes to my top 5 best-selling beers from my brewpub?  Sign up to get them now on the side of the blog or at the bottom on your smart device.  Enjoy!

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