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Make a Hygrometer with Strands of Hair

Difficulty
Time Required Short (2-5 days)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety No issues

Abstract

Does your hair go crazy when the weather turns damp? Did you know that strands of hair can relax and lengthen when the humidity increases and then contract again when the humidity decreases? In fact, hair strands can be used as the basis for a hygrometer, a device which measures the humidity level in the air. Will a hygrometer help you to predict bad hair days(!) or can you use it to help predict the weather?

Objective

The goal of this project is to make and use a hygrometer, an instrument for measuring the level of humidity in the air.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

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Last edit date: 2013-01-10

Introduction

Changes in humidity are an important part of daily weather. In this project you will build an instrument to measure the humidity level in the air. The instrument is called a hygrometer, and it's sensing element is strands of human hair.

In order to understand how a hygrometer made with strands of hair works, it is important to understand the structure of a shaft of human hair. Figure 1, below (Tobin, 2006), illustrates the microscopic structure of a human hair. The left-hand panel of the illustration (Figure 1a), is a cartoon of a human hair shaft with a cut-away view to show the inner structure. Each strand of hair has an outer layer of flattened cuticle cells (Cu), which surround the fibrous cortical cells (Co). The medulla (Md) is a central core of cells in the hair shaft. Also shown is a microfibril (MF) within a cortical cell. The middle panel (Figure 1b), shows an actual hair shaft under the microscope. You can see how the flattened cuticle cells (Cu) have a scale-like appearance when magnified. The dark central medulla (Md) is also visible. The right-hand panel (Figure 1c), shows a cross-section of a fine human hair. Here you can see that the cuticle cells (Cu) are highly flattened, and wrap around the cortical cells (Co) in many layers. The cortical cells contain the dark pigment granules that give each hair strand its natural color.

microscopic structure of human hair shaft
Figure 1. Microscopic structure of a human hair shaft. Part (a) shows a cutaway cartoon of a single hair shaft. The labels show cuticle cells (Cu), cortical cells (Co), the medulla (Md), and a microfibril (MF) within a cortical cell. Part (b) shows a transmitted light micrograph of a single hair strand. The scale-like layer of cuticle cells (Cu) is clearly visible, as is the central medulla (Md). Part (c) shows a cross-section of a fine hair strand. The flattened cuticle cells (Cu) wrap tightly around the cortical cells (Co), which contain many dark pigment granules (Tobin, 2006).

The cuticle cells have a coating of specialized molecules that repel water. These molecules are called lipids. By repelling water, the lipid molecules help to protect the hair strand.

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 contain a large number of a particular amino acid called cysteine. Each cysteine in the keratin molecule is a potential attachment point, where the keratin molecule can be tightly connected to another cysteine, forming a chemical bond called a cross-link. The keratins in hair have many such cross-links, making a hair strand strong and flexible. There are also many weaker bonds, called hydrogen bonds between the keratin molecules. Hydrogen bonds can be disrupted by water, including water vapor in the air. In fact, this is what causes hair strands to expand in moist air and to contract in dry air.

If you are interested in finding out about how hair grows, you should do research on hair follicles, the specialized structure in the skin that produces each individual hair strand.

"A single lock will stretch about two and a half percent as the humidity goes from 0 to 100 percent. While that’s only a smidgen of length, the rate of this change is very dependable, so it’s possible to obtain highly accurate humidity readings by measuring these tiny shifts. Hair hygrometers are best suited for keeping track of humidity in closed settings such as office buildings, where the variations are small and the goal is to keep humidity constant." (Weather Notebook, 2005)

You can build a hair hygrometer and use it to measure humidity changes inside your home, or outside to help you predict the weather.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

  • human hair strand, structure, composition, how it grows:
    • cuticle,
    • cortex,
    • pigments: eumelanin, phaeomelanin.
    • keratin,
    • hair follicle.
  • relative humidity.

Questions

  • How does the structure of the hair strand change as the relative humidity of the surrounding air varies?

Bibliography

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • a scrap piece of wood or flat styrofoam (about 25 cm long and 10 cm wide),
  • a flat piece of plastic (about 8 cm long and 8 cm wide) thin enough that you can cut,
  • 2 small nails,
  • 3 long strands of human hair (about 20 cm long),
  • a dime,
  • glue,
  • tape,
  • hammer,
  • scissors (strong enough to cut plastic),
  • rubbing alcohol,
  • cotton swab.

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Experimental Procedure

  1. For each hygrometer you will need three strands of hair.
  2. Wipe down the strands with alcohol.
    1. Make a solution of 20% rubbing alcohol (chemical name: isopropyl alcohol) and 80% water.
    2. Use a dab of this 20% alcohol on a cotton swab to wipe down the hair strands.
    3. This will remove any residual oils and allow water to permeate the hair strands more easily.

    human hair hygrometer
    Figure 2. A simple hygrometer made with strands of human hair. The hair cells expand when the humidity is higher, and the weight of the dime moves the pointer lower. The hair cells contract when the humidity is lower, and the contracting hair pulls the pointer higher. (Image courtesy of The Franklin Institute Resources for Science Learning, www.fi.edu/learn.)

  3. Figure 2 (above) shows what the completed hygrometer looks like. Here's how to make it:
    1. Use the template below to cut the piece of plastic into a triangular shape to make the pointer. Make sure each of your pointers is the exact same size.

      pointer template, human hair hygrometer
      Template for making the pointer. (Image courtesy of The Franklin Institute Resources for Science Learning, www.fi.edu/learn.)

    2. Tape a dime onto the pointer, as shown in the illustration below. Again, make sure that you tape the dime in the exact same place on each pointer.

      detail image (pointer), human hair hygrometer
      A close-up of the pointer, with dime attached. (Image courtesy of The Franklin Institute Resources for Science Learning, www.fi.edu/learn.)

    3. Poke one of the nails through the plastic pointer, near the base of the triangle. Again, it is important to use the exact same position on each pointer. Wiggle the nail until the pointer moves freely and loosely around the nail.
    4. Glue the hair strands to the pointer, between the dime and the nail hole (refer to the illustrations above). As usual, use the same position for each of the pointers.
    5. Position the pointer on the wood or styrofoam base about three quarters of the way down the side. Attach the nail to the base. The pointer must be able to turn easily around the nail. As usual, use the same position for each of the hygrometers.
    6. Attach the other nail to the base about one inch from the top of the base, in line with the pointer. As usual, use the same position for each of the hygrometers.
    7. Gently pull the hair strands taut, so that the pointer points parallel to the ground. That is, make sure the point of the pointer is perpendicular to the hair. The hair should hang perfectly vertical and the pointer should point perfectly horizontal.
    8. Glue the free ends of the hair strands to the top nail. If the hair is too long, trim the ends.
  4. Calibrate the two extremes on your hygrometer scale, 0% and 100% humidity.
    1. For 0% humidity, dry the strands gently with a hair dryer until the strands no longer decrease in length (pointer no longer moving). Mark this location as 0%.
    2. For 100% humidity, close up the hygrometer in a plastic box with a wet sponge or paper towel. Wait at least ten minutes. Mark the pointer location lightly, re-seal the box and wait another ten minutes. Keep doing this until the pointer location is no longer changing. Mark the final pointer location as 100%.
    3. For intermediate values, you could just divide the space between 0% and 100% into five equal increments and mark the locations for 20, 40, 60, and 80%, then divide each of these in two to mark locations for 10, 30, 50, 70, and 90%. However, the best way to calibrate your hair hygrometer would be to compare it to another instrument. You could use a psychrometer (see the Science Buddies project Make Your Own Psychrometer), or you could buy an electronic hygrometer (e.g., Radio Shack part #63-1032 or 63-1089).
  5. Test how the hygrometer responds to changes in humidity.
    1. Put the hygrometer outdoors in an area that is sheltered from precipitation.
    2. Monitor the hygrometer regularly throughout the day over the course of several days.
    3. How does the humidity change during the course of a "normal" day?
    4. How does the humidity change when a storm system moves through your area?

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Variations

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