- 1). Place a teaspoon of salt in the bottle cap and add water one drop at a time to make the salt damp but not wet. Place the hygrometer in the zipper bag along the bottle cap of salt.
- 2). Allow some air into the bag and seal it. Allow the bag to sit undisturbed for at least six hours. Read the hygrometer without opening the bag. A correctly calibrated hygrometer should have a reading of 75 percent.
- 3). Record the correction factor of your hygrometer by subtracting the hygrometer reading from 75 percent. Your next step will depend on whether or not your hygrometer is adjustable.
- 4). Add the correction factor to your hygrometer's reading if your hygrometer is not adjustable. Be sure to apply the correct sign of your correction factor.
- 5). Adjust the hygrometer by the appropriate correction factor if your hygrometer is adjustable. These types of hygrometers will typically have a dial in the back which will change the reading.
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