The accuracy of hygrometers drifts over time, so calibrating them with salt is an easy way to keep them delivering accurate readings. Salt calibration is a simple process anyone can do at home. All that is needed is a Ziploc bag, a small jar, table salt and water (see below for a chart that shows different kinds of salts that can be used and their corresponding relative humidity at certain temperatures).

Step 1: Take a small jar or mason jar lid and fill it with salt. Add water very slowly, a little at a time. Mix until it reaches the consistency of wet sand. Make sure there is no standing water. If it's too wet, add more salt. If it's too dry, add a tiny bit more water.

Step 2: Place the lid with salt mixture inside the Ziploc bag, along with your hygrometer(s), then seal the bag to create an airtight environment. If you are using wired hygrometers, use tape to seal the bag.

Step 3: Place the sealed bag in a location with constant temperature. Avoid: windows, exterior walls, or areas with temperature fluctuations. Leave undisturbed for several hours. Once the salt solution and air in the sealed bag reach equilibrium, the relative humidity will stabilize at the salt's characteristic value for that temperature. The humidity reading should remain constant (within the precision of the hygrometer) as long as the temperature stays stable.

Step 4: Check the table below to see the humidity value that corresponds to the temperature in the ziploc bag. If you used Sodium Chloride (Table Salt), the hygrometer should read 75.29 % at 25°C (77°F). If the reading is off, set an offset in the hygrometers app or settings (e.g. if it reads 76.7 % set an offset of -1.41).

Remember, temperature stability is crucial for accurate calibration.