Analog (dial-type) hygrometers are often shipped from the factory out of spec. To calibrate, you need plain table salt, a few drops of water and a see-through Tupperware container with a tightly fitting lid.

Place about 1 teaspoon of salt in a small shallow open container, such as the cap to a jar. Add a few drops of water to the salt. You DO NOT want a solution or to dissolve the salt, only get it damp. Place the container with the salt solution in the Tupperware container. Next, place the hygrometer in the Tupperware container face down. This will allow you to view and check the reading of the hygrometer before you open the container. CAUTION - do not spill any salt or solution on your hygrometer! Place the lid on the Tupperware container making sure it fits tight. Allow to stabilize for at least 6 hours. Without opening the container, check your reading. It should read exactly 75%. If not, note the deviation and or adjust the hygrometer.

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