How to Build a Hydrometer

Co-authored by Bess Ruff, MA

Last Updated: November 19, 2022 Approved

Hydrometers are graduated floating devices that measure the densities of liquids. They are commonly used in the making of wine, beer, and spirits to monitor the fermentation process, and are also used often in school projects. Commercial hydrometers are tubes of glass weighted with metal, but you can make a simple hydrometer with materials purchased from grocery and hardware stores. Afterward, you can use it to test the density of water with various salt concentrations as well as other liquids.

Part 1

Part 1 of 3:

Creating Your Hydrometer

1
Gather your bulb, hydrometer, weight, and calibration chart. The bulb can be a graduated cylinder or narrow jar; the weight should be a piece of steel, other metal, or clay; the hydrometer is a standard straw. You also need a hydrometer calibration chart, which can be printed from numerous online sources, such as here: http://www.potteryatoldtoolijooaschool.com/floating_straw_hydrometer.pdf.^{[1]
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- Your bulb should be small enough to float freely in your graduated cylinder. Standard hydrometers are 20.32 centimetres (8.00 in) in overall length, including both the stem and the bulb.
- Metal weights can be purchased at home hardware stores and should be 4 grams (0.14 oz) and small enough to fit into a straw. Clay can be purchased from craft stores or online suppliers.
- Purchase a 250 millilitre (1.1 c) or 500 millilitre (2.1 c) graduated cylinder (online or from an office supply store) so that you don't risk it overflowing. For glass jars, purchase a clear 16 ounce (473 mL) clear glass type from a big-box store.
2
Fill the narrow jar or graduated cylinder with 100 millilitres (0.42 c) of water. If you're using a graduated cylinder, fill it to the 100 millilitre (0.42 c) line. If you're using a narrow jar (like an olive jar), fill it with about ¹⁄₂ cup (120 mL) of water minus 1 tablespoon (15 mL) and mark a reference line on the outside of the jar with a permanent marker.^{[2]
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- You can use either distilled or tap water. Whichever you use, be sure to use the same type of water from the same source throughout the project.
- When reading the water level on a graduated cylinder or marking your jar, always measure from the bottom of the meniscus—curved arc created on the water's surface.
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3
Create a 4 gram (0.14 oz) weight using metal, steel, or true clay. Steel rods should be about 20 millimetres (0.79 in) long and 0.64 centimetres (0.25 in) thick. Sheet lead should be a bit thicker than 1 millimetre (0.039 in) and 15 by 20 millimetres (0.59 by 0.79 in) in size, and rolled up into a cylindrical shape. Clay balls should be free of voids or pits that could potentially trap air.^{[3]
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- You may be able to find steel parts such as steel disc blanks or washers that match these specifications without having to cut the metal yourself. Look at your local hardware store or check online.
- Since sheet lead is very thin, you can usually bend it with your hands or with a pair of pliers. Be sure to wear protective gloves to prevent any accidental cuts when bending.
- Use a gram scale for measuring weight. These can be purchased from convenience stores, big-box stores, and online suppliers.
4
Seal off one end of the straw with your metal or steel weight if you don't have clay. If you're using metal or steel weights, push them into one end of the straw. For rods, slide them in. Regardless of whether you use metal or steel, seal it using silicone sealant. Snip the tube at a 45-degree angle, and hold the tip (using a silicone gun or your hands) at the same angle to the straw hole. Gently pull the trigger of the gun or squeeze the tube to apply a dab of silicone sealant underneath to seal the opening as evenly as possible.
- Test your silicone on a piece of scrap to get a feel for the flow of the sealant before applying it to your straw.
- Use as little silicone as possible to minimize weight.
5
Attach a clay ball to the bottom of the straw if you're not using metal or steel. For clay weights, place the ball on a flat surface and hold the straw vertically over it. Gently press the straw into the clay until it is secured. Always be sure that there are no holes in the clay that will let water into the straw.^{[4]
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- After attaching the clay, hold the straw up in the air vertically and make sure that the ball doesn't fall off.

Part 2

Part 2 of 3:

Calibrating Your Hydrometer

1
Float your hydrometer in water sealed side down and mark the water level. Once you place the straw into the water, the weight should pull it down until the water is a bit over its halfway point. After it's floating upright, use a waterproof permanent marker to draw a horizontal line at water level.^{[5]
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- The open end of the straw shouldn't extend more than 1 inch (2.5 cm) from the top of the container of water. If it pokes out more than this, add weight to the straw to keep it from floating up too high and flipping onto its side.
- For additional weight, you can stick clay to the outside of the straw at the sealed end, or drop split shot sinkers, small nails, or pebbles down the inside of the straw until it floats at the right level.
2
Remove the hydrometer from the water and add measurement lines. After removing the straw from water and drying it, it is calibrated for pure, unsalted water (either distilled or regular). From here, mark measurement lines on the straw above and below the horizontal calibration line. Each line should be about 1 millimetre (0.039 in) apart. Use a ruler to help keep your lines accurate.^{[6]
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- Use multiple colors for your lines, such as red for the zero and 10 millimetre (0.39 in) lines, and black for the lines between them.
- If you can't mark the lines 1 millimetre (0.039 in) apart, aim for them to be at least as close as the width of the point your marker will allow. Take note of how far apart each line is and make sure all of them are equal distances apart.
3
Align your straw onto the straw image on the printed calibration chart. Place the straw onto the chart so that the bottom of the straw is on the bottom line. The calibration line on your straw should align with the diagonal chart line that is marked 1.0.^{[7]
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- If the straw doesn't align with the chart properly, wipe down the straw, attach another weight, and create another calibration line. Be sure that no water is in the straw to throw off the calibration.
- Print the calibration chart here: http://www.potteryatoldtoolijooaschool.com/floating_straw_hydrometer.pdf
4
Draw colored marks at each of the other sloping lines on your calibration chart. The straw intersects with seven other lines on the calibration chart, each corresponding to 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, and 1.7. Create horizontal lines on the straw at each of these intersection points using colored permanent marker.^{[8]
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- If you don't have a calibration chart, measure the distance from the bottom of your straw to the water-level marking in millimeters—this length will be called "x." Using a calculator, calculate "x/1.1", which will tell you, in millimeters, where the 1.1 calibration mark should go. Use the measurement lines you drew onto the straw to help guide your markings and repeat the "x/number" calculation for each calibration mark.^{[9]
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Part 3

Part 3 of 3:

Testing Your Hydrometer

1
Add 10 grams (2.5 teaspoons) of salt to 50 millilitres (0.21 c) of water. If you can't measure 10 grams of salt exactly using a scale, use a teaspoon to do so. Stir the water using a spare straw to ensure that the salt dissolves completely and then continue adding water until it reaches 100 millilitres (0.42 c). The solution is now 10 percent salt.^{[10]
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2
Put the hydrometer into the diluted the water and record the water level. Observe where the water level meets the straw. Record the measurement of the line that aligns with the water's surface.^{[11]
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3
Remove the hydrometer and add more salt to increase the solution's density. Add the same amount of salt each time, repeating the above steps and recording the level to which the hydrometer now floats. Your readings should increase as you add more salt.
- Adding 10 grams (0.35 oz) of salt at a time is standard.
- If you want to dilute your solution and decrease its density, add more water.
4
Measure the density of other substances. Test out other liquids in your hydrometer and see how dense they are. Most beers fall into the 1.030 to 1.0700 range, while pottery glazes measure at around 1.3 to 1.5.
- Always clean out your container after filling it with substances other than water to remove any residue.
- Hydrometers measure specific gravity. Since specific gravity is dimensionless, there is no unit of measurement.

Expert Q&A

Question

Can I use homeade play dough?

Bess Ruff, MA
Environmental Scientist
Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group.

Bess Ruff, MA

Environmental Scientist

Expert Answer

Play dough is not as hardy as modeling clay and, depending on how you make it, is usually water soluble, meaning it will start to disintegrate when in contact with water. This would make for an ineffective weight for your straw, so play dough is not recommended.
Question

How do you mark the hydrometer?

Community Answer

Put permanent ink markings on the straw. Calibrate with liquids of known s.g.
Question

Can you explain why a straw and clay can function as a hydrometer?

Community Answer

A hydrometer measures the density of liquids. The more salt there is in water, the better an object would float. The clay at the bottom of the straw keeps the straw upright, while the straw has markings on it to show how much salt is in the solution.