How do I calibrate my hygrometer?

To calibrate, use a saturated solution of plain table salt (NaCl) and water. This solution will maintain 75% RH in a sealed environment at room temperature. All you’ll need is a tablespoon of salt, a small bottle cap-sized container, and a Ziploc bag. Place a tablespoon of salt into the small container and add a few drops of water so you have a wet pile, not dissolved salt. If you add too much water, just add more salt. Place the wet salt solution and your hygrometer into the bag, making sure not to spill the solution on your hygrometer. Press most, but not all, of the air out of the bag and seal it (make sure that the bag is well sealed). Leave it for a minimum of five hours at room temperature before checking the reading. After five hours, read the hygrometer through the bag. It should read 75%. If not, note the reading and repeat the procedure. Take the average of the two readings for a good calibration figure. For instance, if you have readings of 70 and 72%, your calibration figure should be- 4%. When checking your humidor using that hygrometer, always add 4% to get the accurate reading.