Ghetto Hygrometers

Seeds of the common weed "StorksBill" or "clocks" (Erodium circutarium, Erodium malcoides) work as an organic hygrometer.
They curl up in low humidity and uncurl in high humidity.
The wisp at the end of the seed acts as an indicating dial if you mount the seed.
The plant grows throughout most of North America.

paper soaked in Cobalt Chloride turns pink in high humidity and blue in low humidity:

A Foiled Gum Wrapper, a Nail and a Board can be used to make a hygrometer:



BUILDING A HYGROMETER
Hygrometer
Materials
Long human hair (Run a brush through hair and collect the strands that fall out. The longer the hair, the greater the difference in length due to changes in humidity.)
Rubbing alcohol
Large paper clips, thumb tack, glue gun
Paper scale (a little longer than the hair)
Wall thumb tack will stick in
Hot water or source of steam and hair dryer or source of heat

Clean hair, thumb tack and one paper clip with rubbing alcohol and let dry. Carefully place one end of the hair on the paper clip and use a bead of hot glue to hold them together. Let dry. Secure other end of hair to the thumb tack. Let dry.

Secure the tack to a vertical surface--the paper clip should hang freely from the hair. Carefully add paper clips as necessary to make the hair hang straight and taut. Tape the paper scale behind the hair and determine how to record the change in hair length over time.

Think about the following statements. Discuss them in your group and determine how to calibrate your hygrometer. Prepare to explain to the class how the hygrometer works.

A hygrometer measures the amount of moisture (humidity) in the air. What happens to the hair when the humidity is high? How could you test this? (What happens when you run very hot tap water, or have water boiling on the stove?) Check with your teacher or other responsible adult before conducting your test.

Do all types of hair react the same way to humidity? Does the length of hair make a difference in how much the hair stretches or shrinks due to changes in humidity? Set up a system to test your hypothesis. Would this hygrometer work with hair from another animal--like a horse's mane? How would you find out?



MAKING A HAIR HYGROMETER

Equipment:

  • Empty milk carton
  • Large sewing needle
  • Broom straw, 2" long
  • Scotch or masking tape
  • Penny
  • 9" human hair, wiped clean of oil
  • 4 thumbtacks
  • Paper clip
  • Dishpan

Cut the carton so as to make a small horizontal slit near the top; insert the paper clip. (Fig.1) Cut a vertical slit near the bottom. Then cut horizontal slits perpendicular to this cut at its end points - like an H on its side. (Fig.1)
Pry out the flaps thus made and bend them to an upright position. Insert the needle through these flaps. (Fig.2)
Tie the hair to the paper clip, wind it around the needle, tape the penny to the other end of the hair, and let the penny hang over the end of the box, which should be lying on its side.
Put a card with a scale on the side of the carton under the straw which has been pushed through the eye of the needle. (Fig.3)
Place the hygrometer on a wet towel in a dishpan and cover with a damp cloth. After 15 minutes remove it from the cloth and set the straw at numeral 10 on the scale. Watch to see whether the straw moves.
Since humid air causes the hair to stretch and dry air causes it to shrink, the straw should move toward the dry end of the scale as the hair dries.




DESIGN OF A HYGROMETER TO DETECT CHANGES IN RELATIVE HUMIDITY

Materials

A hair, at least 10 inches long (39 cm); a broom straw; a couple of thumbtacks and a cardboard box as a chassis. One can glue a small piece of paper to the box under the sweep of the 'needle' (tip of the straw) on which to mark gradations from calibration in a very dry environment and then, a very damp environment (like the shower).

Using a long blonde hair attached to a straw on a pivot, I built a hygrometer which was qualitatively calibrated.
I am sure any color of hair will do but a straight hair fiber might be easier to work with than a curly one.

Either way, the hair generally shortens if the air is laden with moisture and lenghtens when the air is dry.


Someone built a hair hygrometer using human hair, a mirror and a laser pointer:

Humidity-Detector Science Fair Projects

Dial hygrometers household versions this kind of hygrometer can be found at thrift shops.

Radio Shack makes a very useful digital thermometer/hygrometer combo
(this isn't a radio shack unit, but most of them look like this: