In order to begin learning how to calibrate a hygrometer, you will need a small container which will hold one teaspoon of iodized salt. A completely flushed out laundry detergent bottle cap might be just the thing, or you could find something smaller still. To the level teaspoon of salt, and on a flat surface, add just enough water to produce a wet paste. You will want to be careful not to flood or over moisten the salt. If your hygrometer is digital, be sure to turn it on before proceeding. Analog and digital models should be clean and at the ready.
Your next step is to find an airtight storage container or sealable plastic storage bag that will easily fit both the cap of salt solution and the hygrometer that you mean to calibrate. Because this salt and water solution will always produce a humidity reading of 75%, this a fail proof way of testing and setting your hygrometer. Remember that the digital type hygrometers will most likely lack a way of resetting or calibrating the readings that they take, so will need to adapt to this using other means.
Once the bag or container has been sealed in an airtight manner with both the slat solution and the hygrometer, you will let it sit unopened for no less than 6 hours, with 7 hours being ideal. At this point, you will open the container and check the reading on the device. If your hygrometer is calibrated correctly, it should read within 2 degrees either way of 75%. If it does not read within this allotted range, your analog hygrometer will need to be adjusted with the use of a flat head screwdriver. For a digital misread, you will need to calculate how many degrees it is reading off by and add or subtract them each time your test your device.
