How Does a Hair Hygrometer Work?

What is a Hygrometer?

A hygrometer is a device that measures humidity. The most basic kind of hygrometer uses two thermometers: The first one is dry and the second one is wet, and measures what is known as the wet-bulb temperature. Evaporation from the wet-bulb will cause the measured temperature to be lower. This difference in measurements will allow for the calculation of the current humidity. Another method of measuring humidity is to use a human hair, which can be used to make a hair hygrometer.

Hair Hygrometer Theory

The first step in making a hair hygrometer is to understand the basic structure of the human hair. Every strand of hair has a similar makeup. The outer part of the strand has a layer of flattened cuticle cells. These cells have a layer of lipids, which are a part of oils. These oils repel water. The cuticle cells surround the fibrous cortical cells. In the very center of the hair are the medulla cells. Under a microscope, the cuticle cells have a scaly appearance, and exhibit a multilayered covering on the surface of the hair cell. The cortical cell is visible because its pigment contains granules, which look like small, dark packets under a microscope.

How to Make a Hair Hygrometer

The basic starting point of a hair hygrometer is a piece of human hair. This can be obtained from anyone. To make a hair hygrometer, it is best to have multiple hair strands--at least three. Clean them with a paper towel soaked with 1/5 rubbing alcohol and 4/5 water, to help remove any residual oils. This will allow water to get into the hair. Then, affix the hairs to a piece of plastic. Finally, create reference points for 0 and 100 percent by drying the hairs and then saturating them in moisture. Because the hairs will expand and lengthen in response to absorbed moisture, the length of the hair can be used to measure the humidity of the air.

Using a Hair Hygrometer

The most basic step in using a hair hygrometer is to mark off the reference points from the 0 and 100 percent humidity marks. Since the hair should be fixed to a piece of plastic, these marks can be put on the plastic using a permanent marker or pen. The space between these measurements can then be subdivided to provide approximations for various other degrees of humidity. For example, four marks between the 0 and 100 percent measurements could then be used as approximations of 20, 40, 60, and 80 percent. More accurate measurements can be made by using another kind of hygrometer to get basic readouts, then making corresponding markings on the hair hygrometer.