A hygrometer is an instrument that measures the humidity in the atmosphere. It is a useful tool to have in your humidor as long as it is accurate. Analog hygrometers should always be calibrated upon installation as it is a mechanical device and can get out of whack during shipment. From time to time you may need to check the calibration of your hygrometer especially if you have problems maintaining the proper humidity in your humidor. The best method to check the calibration of your hygrometer is a very simple method that uses plain salt. Salt will maintain an exact humidity of 75% in a sealed environment.
Digital hygrometers are more precise than analog hygrometers but still most are accurate + or - 5 points. They are calibrated in the factory. We recommend that you check the calibration of your digital hygrometer using the salt method so that you know exactly the margin of error of your instrument.