Homemade Tools: How To Make Your Own Hygrometer

A hygrometer, which measures relative humidity, can be made with a few simple materials and three human hairs. It is a great science project.

A hygrometer measures the relative humidity of the air. Relative humidity is the ratio of the amount of water the air is holding compared to how much water the air can hold at the same temperature. A hygrometer is an important tool in meteorology. In fact, there was a hygrometer aboard the first manned balloon to cross the English Channel in 1785. Designs have evolved over the centuries into three different ways to measure relative humidity, in other words, three different instruments that each measure relative humidity by a different method.

The simplest, and the one that can be built as a science project, is the hair hygrometer. It works on the principle that organic fiber like hair absorbs water from the atmosphere. The more water a hair absorbs, the longer it gets. The drier the air, the shorter the hair. If the hair is affixed at one end and attached to a pointer at the other end, the hair will make the pointer move when the relative humidity changes.

To make one you will need three eight-inch human hairs. The fun part is deciding from whom to pluck them! You will need a base to which you can attach the hair at one end and the pointer at the other. It should be about nine by four inches and can be made of corrugated cardboard, Styrofoam or wood. If cardboard is used, there will need to be a way to stand it on end or hang it on the wall, as it will not be thick enough to stand by itself. To make the pointer cut a three by three inch square from the flat side of a plastic milk jug. Then, make it into a triangular shape by cutting from one corner to the midpoint of the opposite side. Cut from the other corner to that same midpoint and you have a pointer. Tape or glue a dime onto the pointer at the wide end of the triangle, not the pointer end. Glue one end of each of the three hairs in the middle of the triangle. Attach the pointer low on the front of the base (wood/plastic/Styrofoam) with a thumbtack. Move the pointer back & forth on the thumbtack, rotating it until the pointer is loose and can swing freely from the tack. The instrument, when finished, will stand on one four-inch end. Hold the pointer so that it will be parallel with the ground when sitting in that finished position. Extend the hairs straight up the front of the base and glue the free ends down so that they are taught when the pointer is pointing parallel with the ground.

The hairs will lengthen when the air is humid and the pointer will point downward. When the air is dry, the hairs will shorten and the pointer will point upward. You can calibrate it, if you would like. Take it in a very steamy bathroom, perhaps with a hot shower running. Wait a half hour, and then make a mark on the base where the pointer is pointing. That will represent 100% relative humidity. Remove the hygrometer from the steamy area and dry the hairs with a hair dryer. Again, mark the base where the pointer is pointing. That spot will represent 0% relative humidity.

This is certainly not a precision instrument and hair hygrometers need frequent recalibration. It is a fun project to build and a great way to "see" changes in the relative humidity. It is an instrument any budding meteorologist will enjoy. Best, a homemade hygrometer is sure to spark interest in the world around us.

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