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Make a Hygrometer to Measure Humidity

Summary

Active Time
20-30 minutes
Total Project Time
45 minutes to 1 hour
Key Concepts
Humidity
Credits
Sabine De Brabandere, PhD, Science Buddies
Make a Hygrometer to Measure Humidity – STEM activity

Introduction

Does your hair go frizzy during "April showers," when the weather turns damp? Strands of hair can relax and lengthen when the humidity (or the amount of water vapor in the air) increases, and then contract again when the humidity decreases. In fact, the rate of change in the length of hair strands is so dependable that they can actually be used as the basis for a hygrometer, a device that measures the humidity level in the air.

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Materials

  • Rubbing alcohol, 70% isopropyl alcohol works well
  • Measuring spoons
  • Small bowl
  • Water
  • Three long strands of human hair (about 20 centimeters long)
  • Cotton swab
  • Drinking straw, preferably a wide, non-bending straw
  • Scissors
  • Metric ruler
  • Glue, fast drying glue or a glue gun work best
  • Scrap piece of wood, about 25 cm long and 15 cm wide
  • Two small nails
  • Hammer
  • Marker, pencil, or pen
  • Hair dryer
  • Ziplock bag, large enough to fit the wood inside and have a little room to spare
  • Wet sponge or small towel (either paper or cloth)

Prep Work

  1. If you would like to paint the wood, do it before you start. Final decorations can be added once your hygrometer is built.
  2. Make a solution of 25% rubbing alcohol and 75% water by mixing one tablespoon of water with one teaspoon of rubbing alcohol in a small bowl.
  3. Dab a cotton swab in the diluted rubbing alcohol solution and wipe down each strand of hair. This will remove any residual oils and allow water to permeate the hair more easily. Set the hair strands aside somewhere you will not lose them.
  4. If your straw has a bendable part, trim it off.
  5. Lay the straw next to the long side of the wooden panel. The panel should be at least a few centimeters longer than the straw. Trim the straw if needed.

  6. Cut a triangle on one end of the drinking straw so it becomes a pointer. Cut a slit about 1 cm away from the other end of the straw about halfway through from the top.

  7. Take a clean strand of hair and slip one end through the slit you just cut. Glue that end of the hair to the straw.
  8. Wrap the other end of the hair twice around a nail. Fasten this end with glue as well.
  9. Make sure the glue has dried before assembling your hygrometer.

Instructions

  1. You are now ready to assemble a hygrometer, an instrument that indicates how much water vapor the air contains.
  2. The picture below shows the hygrometer you are building. Keep this figure in mind as you follow the directions. A yellow line is drawn on the board to show where the hair is strung.

  3. Start by placing the wooden panel before you in landscape orientation (so that it is wider than it is tall). Position the straw on the wooden panel so it points to the right, about 5 cm below and parallel to the top border. Align the left side of the straw with or near the left side of the wooden panel. Check that the strand of hair is not wrapped around the straw, but hangs down from it.
  4. Poke the unused nail through the center of the straw, about 2 cm from the left side. Be careful when using the hammer on the nails. Be sure to only use the hammer on a surface that can safely withstand the force and is resistant to damage. Make sure the nail is securely attached to the wood.
  5. Wiggle the straw up and down until the pointer moves snugly, but without friction around the nail.
    Think about:
    What happens to your pointer if you move the left side of the straw (where the hair is attached) slightly up and down? Why does this happen? How will this help us make an accurate hygrometer?
  6. Ask a helper to hold the pointer so it stays pointed to the right. You can also place something heavy above and below the pointer to prevent it from moving up and down.
  7. Take the nail with the hair strand attached and move it towards the lower right corner of the wooden panel until the hair is taut. Hammer the nail in the wood at that spot. The hair needs to be stretched taut when the straw is approximately horizontal. It is important that the nail is securely attached to the wood. Take your time—this step is not easy! If you notice the hair hangs a little loose when your nail is halfway in, try to hammer your nail a little sideways so the hair is stretched taut.
  8. Your helper can now release the pointer, or you can remove the objects that kept the pointer in place. Put your hygrometer upright. This is your hygrometer!
    Think about:
    What keeps the pointer from falling down?
  9. We call the air humid when it contains a lot of water vapor. We say it is dry when it contains very little water vapor. Hair strands expand in humid air and contract in dry air.
    Think about:
    What will happen to your instrument if the hair expands (becomes longer) or contract (becomes shorter)? How can this help us measure humidity?
  10. You will next want to calibrate your hydrometer by indicating two humidity extremes: dry air and very humid air. Dry the area around the hair gently with a hair dryer. Take care not to touch the pointer or blow it upwards.
    Think about:
    Do you see the pointer move? Why would this happen?

  11. When the pointer stops moving, mark the pointer location on the wood.
    Think about:
    Do you think this point indicates moist air, or dry air?
  12. The hair dryer removes moisture from the air. Label the point you just marked "Dry."

  13. To place your hygrometer in humid conditions, enclose it in the plastic bag with a small wet (not dripping) sponge or towel. Close the bag airtight. Make sure there is room for the pointer to move inside the bag.
    Think about:
    What do you expect will happen?

  14. Wait 10 minutes.
    Think about:
    Do you expect the humidity of the air in the bag to change? Why do you think that? How will you be able to see if your expectation is correct?
  15. Observe the bag and the hygrometer inside.
    Think about:
    Does anything change? Is this what you expected? Why does this happen?
  16. After the 10 minutes are up, open the bag, mark the pointer location lightly, and reseal the bag. Wait another 10 minutes before repeating this step. Keep doing this until the pointer location stops changing.
  17. Clearly mark the final pointer location and label it "Very Humid."
  18. Decorate your hygrometer any way you want. As you do so, make sure not to spill any paint on the hair.
  19. Now you can use your hygrometer to measure different humidity levels! Start measuring around your home. Place the hygrometer in a room and give it some time to adjust to its surroundings before reading the level.
    Think about:
    How humid does the hygrometer say it is in different rooms in your home? How does this compare to the humidity in the bathroom after a shower?

  20. Take your hygrometer outside. Place it in an area sheltered from rain and wind.
    Think about:
    How humid does the hygrometer say it is outside, and how does this change depending on what the weather is like?

What Happened?

The humidity can be very high in the bathroom after a shower, especially if the room is small and the door stays closed while the shower is running. This happens because water from the shower evaporates and stays trapped in the room. Under these conditions, the hair in your hygrometer expands and your hygrometer probably points to the "Very Humid" label. The humidity is most often much lower than this in other rooms of the house. In those conditions, the hair in your hygrometer is slightly shorter and your hygrometer probably points to a spot somewhere between "Very Humid" and "Dry." How humid it is in your house depends on several factors, such as the local climate, whether a humidifier or aquarium is being run, if the air conditioning or heater is on, etc.

When the hygrometer is used outside, it probably falls towards the "Very Humid" label on humid days, such as a muggy, foggy or rainy day and shifts up on a dry, sunny day.

Hair is made of proteins called keratins. In a strand of hair, keratin particles are mainly held together by chemical cross-links and weaker hydrogen bonds. The keratins in hair have many such bonds, making a strand strong and flexible. The hydrogen bonds can be disrupted by water vapor in the air, with each keratin particle binding to water in the air instead of each other. This disruption is what causes hair strands to expand and contract in humid and dry air, respectively.

Digging Deeper

To understand how a hygrometer made with hair strands works, it is important to understand what a human hair is made of. The predominant proteins in hair are from the family of keratins, the same family of proteins that make your fingernails. Protein molecules are built from amino acids. In a hair strand, the keratin molecules are held together by two main bonds: chemical cross-links between these amino acids, and weaker hydrogen bonds. The hydrogen bonds between the keratin molecules can be disrupted by water, including water vapor in the air, with individual keratin molecules binding to water instead of each other. This is what causes hair strands to expand in moist air and to contract in dry air. The change in length is small—a few percent at its maximum—but the change is very reliable.

The specific humidity is defined as the mass of water vapor in one kilogram of air. Because warm air can hold more water vapor than cold air, meteorologist prefer to use the relative humidity, or the amount of water vapor in the air compared to the maximum amount the air can hold at that temperature. Relative humidity is expressed in percentages; a relative humidity of 100% means the air cannot hold any more water vapor, so any additional water condenses. The hair hygrometer you just made measures relative humidity.

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For Further Exploration

  • Calibrate your hair hygrometer by comparing it with another instrument, such as a psychrometer or an electric hygrometer. How similar are the readings between your hair hygrometer and these precise instruments? If they differ, why do you think this is?
  • Put your hygrometer outdoors in an area that is sheltered from rain. Monitor it regularly throughout the day over the course of several days. How much does the humidity change during the course of a "normal" day? Why do you think this happens? How does a storm system moving through your area affect the humidity? Do you see patterns that help you predict the weather?

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