How to make a simple hydrometer

Written by susan reynolds
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How to make a simple hydrometer
Use a graduated cylinder to make a hydrometer. (cylinder image by Hubert from

A hydrometer is a type of instrument that measures the density of liquids. Making a simple hydrometer to measure the density of several types of liquids is a good science project for elementary or middle school students. Building a hydrometer takes only a few common materials that you probably have in your home. Once you have your hydrometer, you can use it over and over again for various projects. It will be easy for anyone to use, because if you know how to read a thermometer, you can read a hydrometer.

Skill level:
Moderately Easy

Things you need

  • Water
  • Graduated cylinder
  • Permanent marker
  • Clay
  • Straw
  • Ruler

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  1. 1

    Roll a small piece of clay into a ball one centimetre around.

  2. 2

    Plug one end of the straw with the clay ball.

  3. 3

    Fill a graduated cylinder with water to the 100ml line.

  4. 4

    Place the straw into the graduated cylinder, with the clay ball at the bottom and one inch of the straw sticking out of the water. Add or remove some of the clay ball to make the top of the straw stick out one inch. Use a ruler to help you.

  5. 5

    Mark the straw with a permanent marker where the top of the water hits the straw. This is line "0" representing pure water without salt.

  6. 6

    Take the straw out of the water.

  7. 7

    Add one gram of sea salt into the cylinder and stir it until it dissolves.

  8. 8

    Put the straw back in and mark it where the water and straw meet. This line is "10," because the solution has a salinity of 10 o/oo (4.54 Kilogram of salt per 454 Kilogram of water).

  9. 9

    Keep adding a gram of salt and remeasuring the straw, making marks with multiples of 10. There should be lines with numbers all up and down the straw when you're finished.

  10. 10

    Wash off your straw hydrometer. Place it into the fluids that you want to test and make a note of where the water touches the straw.

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