Hygrometers are very easy to check for accuracy:
Hygrometer Testing
To test a hygrometer you will need:
1/2 cup table salt
approximately 1/4 cup water
coffee cup
hygrometer
large re-sealable freezer bag
1. Place 1/2 cup of salt in the coffee cup, and add the water. Stir for a bit to totally saturate the salt (the salt won't dissolve, it will be more like really wet sand).
2. Place the salt/water mix in a re-sealable plastic bag, along with the hygrometer, and seal the bag. Note: make sure none of the salt/water mix comes in direct contact with the hygrometer.
3. Let this bag aside at room temperature for 8-12 hours, in a location where the temperature is fairly constant.
4. After 8-12 hours, check the reading of the hygrometer. It is best to read it while still in the bag.
The relative humidity in the sealed bag with the salt/water mix should be 75 percent