Hygrometers measure the amount of moisture or humidity in the air. A simple hygrometer can be made from fairly common household items without a great deal of difficulty. This meter will not give the type of precise reading that can be obtained from a weather report, but can be useful in determining indoor and outdoor humidity. Making a hygrometer can also be a fun project for a science class.
Gather materials
Making a very basic hygrometer requires a few materials. Get a base for the meter, a piece of wood or flat Styrofoam about ten inches in length by six inches in width. The wood or Styrofoam should be thick enough to stand upright by itself, but can also be hung on a wall. A piece of plastic three to four inches square will be needed to create the pointer. You will also need one or two nails, three strands of human hair about eight inches in length and one dime. The tools needed are glue, tape, a hammer, and scissors or an Exacto blade.
Make the pointer
Draw an Isosceles type of triangle on the plastic. This triangle should have two sides equal in length and one, the base, shorter in length. It will look like an arrow. Carefully cut the triangle out of the plastic, keeping it flat without creases.
Next glue or tape the dime to the top, or apex, of the triangle, the pointed end. Use a nail to create a hole near the bottom edge, or base, of the triangle. The hole should be large enough for the triangle to move around on the nail but not be loose. Glue one end of each piece of hair to the middle of the triangle.
Attach the pointer to the base
Place the triangle on the wood or Styrofoam base about three inches from the bottom. Tack it into place using the nail hole in the bottom of the pointer. The pointer should move freely. With the base piece laying flat, position the pointer so that the arrow is parallel with the bottom of the meter base.
Extend the three pieces of hair upwards towards the top of the meter base. Cut them so that they are even in length and glue them in place. The hair should be tight from the triangle to this location. The hair should be perpendicular to the pointer end and bottom of the base. As an alternative, you can put a second nail in the base where the hair ends reach and glue the hair to the nail, again keeping them tight.
This basic hygrometer works because human hair will lengthen when it becomes moist and shorten as it becomes drier. The overall change in hair length will be about 2.5 percent from a zero to 100 percent humidity level. As the air gains moisture the hair will lengthen, and the arrow will point downwards. As the humidity lessens, the hair shortens and the arrow will point upwards.
To roughly calibrate the hygrometer, place it in a bathroom with the door shut and run a hot shower for fifteen minutes or more until the air is very steamy. Make a mark on the base where the arrow point is and refer to this as 100 percent humidity. Next take the meter out of the bathroom and gently dry the hair with a hair dryer but do not burn or melt anything. Once the hair is fully dry, mark the location of the arrow on the pointer and call this location zero percent humidity.
References:
http://www.fi.edu/weather/todo/hygrometer.html
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Weather_p010.shtml
