People on here are always saying that many stick on hygrometers are wrong. This is probably true, but there is a way to correct this. To calibrate a hygrometer you need:
1-2 teaspoons of IODIZED(table) salt- cannot be sea salt
A freezer bag
A bottle cap, about the size of a vitamin water cap
Water
And the hygrometer
First, put the salt in the cap and add water so that it is moist, NOT waterlogged. This will probably be about 1/4-1/2 teaspoon.
Second, put the cap and the hygrometer in the freezer bag. They should not be touching and no salty water should leave the cap. Seal up the bag very well. There should be absolutely no airflow.
Third, put this basically anywhere at room temperature for about 12 hours. When you come back, it should say 75 percent humidity. If it does not, adjust the small screw on the back so that it does say 75 percent. These gauges are always off linearly, so once corrected,they will stay that way. Not all hygrometers have an adjustable screw, so check to see if it does before you start. It should be on the back.
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