How To Use a Hydrometer – All You’ll Need To Know!

Converted temp. in °F
Converted temp. in °F

Use our calculator to calculate the actual specific gravity of your beer to correct the reading for the wort sample temperature and the hydrometer’s calibration.

You can also choose which measurement of Gravity you want to use – Brix, Plato, or Specific Gravity.

Most homebrewers will work in Specific Gravity and, conversely, most hydrometers have a Specific Gravity scale but some double or triple-scale hydrometers do exist.

Hydrometer Temperature Correction: What You Need to Know

If you brew beer, you know the importance of using hydrometers to measure the amount of sugar in the wort. But if the liquid you’re measuring isn’t at the temperature the hydrometer was calibrated for, the hydrometer reading won’t be accurate.

This is why hydrometer temperature correction is so important.

To understand why hydrometer temperature correction is necessary, it’s important to know how a hydrometer works.

A hydrometer measures the density of a liquid. The denser the liquid, the more sugar it contains. Therefore, the more sugar content present in the liquid, the higher the hydrometer will float.

However, the density of liquid changes with temperature. So if the liquid temperature is not at the same temperature the hydrometer was calibrated for, the reading will be inaccurate.

This is why it’s important to correct the reading for temperature.

The process of hydrometer temperature correction is relatively simple. The first step is to measure the temperature of the liquid being tested. Then, use our hydrometer temperature correction calculator to determine a more accurate measurement.

It’s also important to remember that hydrometers are not always accurate even when they are calibrated for the correct temperature.

Therefore, it’s important to take multiple readings and average them to ensure accuracy over time.

The Temperature Calibration of a Hydrometer

A hydrometer is perhaps the most essential measuring tool any home brewery can have and can be used at every stage of the brewing process to keep up with the health and progress of your home brew.

However, not all hydrometers are calibrated to the same temperature so a handy calculator will help you get a more accurate reading.

Older hydrometers tend to be calibrated to 59ºF (15ºC) while newer models will be calibrated to 68ºF (20ºC) – closer to the ambient temperature of most rooms.

The temperature range of newer brewing hydrometers has also been increased to measure sample liquids at higher temperatures with a temperature range of 32 – 159 ºF (0 – 71ºC).

However, please be careful when dealing with hot wort. I always recommend allowing your wort sample to cool down to below 100ºF (38ºC) before handling it.

Not only will it be closer to the hydrometer’s calibration temperature, but any glassware such as the hydrometer jar or even the hydrometer itself is less likely to crack at lower temperatures.

Cracked glass can be a real bummer!

If you are unsure of the calibration temperature of your hydrometer kit, you will normally find it marked on the side hydrometer label or by the scale on hydrometers.

If it has rubbed off over time, look for the original documentation that came with your hydrometer kit, or the original manufacturer’s website product details may list it.

If you still can’t find the calibration temperature, you may need to calibrate the hydrometer yourself, which is never a bad thing to do as the accuracy over time can often change with some of the less expensive gravity hydrometers.

The Science Behind Homebrew Hydrometers

A hydrometer works on the basic Archimedes principle that a solid body or object will displace its own weight within a liquid when it floats.

Most hydrometers will be thin glass or plastic tubes sealed at both ends. The hydrometer bulb at one end will be weighted with a ballast of either fine lead shot or steel shot.

This weight enables the hydrometer to float upright in the sample jar, a bit like a fishing bobber weight.

The second glass or plastic cylinder, commonly known as a hydrometer tube, should be filled with the sample liquid. When filled with pure water the hydrometer should read 1.000 if at the calibration temperature of the hydrometer.

The specific gravity of the sample liquid or your wort is indicated when the top of the jar aligns with a point on the hydrometer scale.

In brewing, the difference in the original gravity of the wort before fermentation and the final gravity reading after fermentation will allow you to work out the alcohol percentage contained in the liquid, and the ABV of your beer.

For an amateur brewer who is keen to know the alcohol by volume of a beer, especially when entering competitions, a correct hydrometer reading is crucial.

Online calculators like our handy ABV Calculator (Link to ABV Calculator) can help work out the potential alcohol content of a beer from the gravity values without the need for complicated mathematical formulas on the part of the beer maker.

As more and more sugar gets converted to ethanol (which is less dense than water), the hydrometer sinks further and further into the fluid.

Water has a specific gravity of 1.0. Beers usually have a final gravity of between 1.005 and 1.015 (although there are many that finish higher). The higher the beer’s finishing gravity, the more sugars are left in the beer, and the sweeter it will taste.

Beers that finish around 1.005 will be perceived as being “dry” because they have very little residual sugars left.

This is just one tool you can use. Use it to find problems throughout the entire brewing process in the mashing, boiling, and fermentation processes.

The Risks of Inaccurate Hydrometer Readings

Using a hydrometer does, however, come with risks.

If a hydrometer reading is not correct it can be more challenging to know the actual state of the fermentation. Incorrect hydrometer measurements will impact the final results of the beer, which may not have been left in the fermentation process long enough.

If the initial reading of the hydrometer is not correct when it comes to working out the potential alcohol reading, the final product may have an unknown volume or an ABV which is higher than it should be.

And finally, the most significant impact of an inaccurate hydrometer measurement is the risk of contamination of the wort.

If the hydrometer has to be recalibrated and then another sample is taken from the fermenter, there is a strong likelihood the wort will be exposed to potential contamination.

How to Use a Hydrometer

So how do you use a hydrometer for a correct gravity reading without risking contamination of the beer?

As with most things, if not everything in homebrewing, it all starts with sanitation. Sanitize all your equipment thoroughly before even taking a sample.

That includes the hydrometer itself, the hydrometer jar, and any device you may be using for taking the sample such as a wine thief (like a big turkey baster but designed for taking beer or wine samples in homebrewing!).

Then follow these six steps:

Step 1:

Take the sample from your fermenter when it is cool enough to handle without the risk of scalding yourself or shattering any glass equipment such as the hydrometer jar of an all-glass hydrometer set. I personally prefer to use a plastic hydrometer tube for less risk of breakage.

Step 2:

Using the sanitized beer thief or beer sample device, fill the hydrometer jar or tube with wort, which may sometimes require as much as 3/4 cups from the fermenter. Remember the temperature will affect the reading so ensure you take the temperature of the sample once it is in the jar and also ensure that it is within the range of your specific hydrometer.

Be sure the sample is well mixed prior to testing.

Step 3:

Place the hydrometer in the jar, weighted end first, of course, and it is advised you do this over a sink or other easy-to-clean area in case the jar overflows as you drop in the hydrometer.

Spin the hydrometer to free it from any clinging bubbles but also to guarantee it’s not touching the walls of the jar. Any contact with the sides of the jar when taking a reading can give inaccurate results. You don’t want any extra buoyancy from those little clingy bubbles or resistance from the sides of the jar.

Step 4:

Take the hydrometer reading. When the hydrometer has stopped bobbing up and down the specific gravity of the sample liquid is shown by the point on the hydrometer where it aligns with the level of the liquid. take the reading for the lowest level of the liquid, also known as the bottom of the meniscus curve.

Step 5:

Using our temperature correction calculator, input the measurement you have just read, the temperature of the sample taken, and the calibration temperature of the hydrometer for an automatic temperature-corrected gravity value. In an ideal world, the temperature of the sample would be the same as the calibration temperature of the hydrometer, but this rarely happens, so make sure you use a calculator as above or check out a hydrometer correction chart.

Step 6:

Discard the sample. The sample you removed from the fermenter to take the specific gravity is no longer suitable for fermentation. Returning the sample back to the wort in the fermenter will most likely infect your entire batch of beer. DON’T DO IT!! You can taste the sample if you really want but please don’t tip it back into the fermenter.

When To Use a Hydrometer?

As mentioned before, the purpose of using a hydrometer is to determine the gravity of your beer or wort, or the density of water in the beer solution. There are a few times during the brewing process when you may need to check the gravity of the wort:

  • Post-mash/Pre-Boil – Knowing the gravity or volume of your wort going into the boil can be important, as it can tell you just how effective the mash has been in converting those starches into sugar (if a whole grain brewer). Ensure your wort is well stirred so you get an accurate measurement of the liquid density, especially if using extracts where often the sugars can sink to the bottom of the liquid.
  • Post-Boil – Also known as your original gravity, this reading is taken before the yeast is pitched and before fermentation begins. The original Gravity is used in conjunction with a Final gravity (FG) to calculate the ABV of a beer.
  • Towards the end of fermentation – A hydrometer reading can let you know how close you are to a beer’s desired final gravity. When two individual readings taken at different times are both the same with no change in gravity levels, it is safe to assume fermentation has finished. A lot more accurate method than just watching to see if the beer is still releasing gas bubbles in the airlock.
  • Post-fermentation – After the readings have remained the same for subsequent hydrometer measurements, you can, a couple of days later, take another (hopefully the FINAL) reading just to ensure the gravity has not changed. This reading will give you the Final Specific Gravity (FG) which, when used with the original Specific gravity reading of pre-fermentation, will enable you to calculate the ABV.

In conclusion, hydrometer temperature correction is an important process for brewers.

By making the necessary adjustments for temperature, you can ensure that your readings are accurate and that your beer turns out just the way you want it.

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