Ascertaining outdoor humidity levels is as easy as watching the news or reading your local paper. However, the moisture content in your home will not be the same as it is outside. To get an accurate measure of moisture content in your home, you must use a hygrometer. These tools are easy to find and come in mechanical and electronic varieties. Both will give you accurate measurements, but the mechanical versions are less expensive and easier to calibrate.
Calibrate a Hygrometer
Add the salt and water to the cup and stir it well. Place the cup and the hygrometer in the plastic bag and seal it. Place the bag where it will be out of direct sunlight and the temperature is consistent and leave it for 12 hours.
Check the reading. If it does not indicate 75 percent humidity, adjust the pointer so that it does. If you cannot adjust your hygrometer, take the humidity difference into account when taking the room's measurement.
Take your hygrometer out of the bag and place it in the room where your plants are housed. Do not place the hygrometer near a radiator, vent, window or any other place with fluctuating temperatures.
Read the hygrometer after it has been in place for at least three hours.