How to Check/Calibrate Your Hygrometer?

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Hygrometers available on the market are initially calibrated, but this setting may evolve with time, meaning that the displayed values are not always reliable. The most sophisticated hygrometers can be recalibrated, but the cheapest ones usually do not include this option. For calibrating orchecking your hygrometer, it would seem necessary to make small enclosures, inspired from a standard (ISO 483:2005) and the literature (Greenspan1977).It is based on the fact that the relative humidity (RH) above a salted solution is regulated by the nature of the salt and its concentration. Hence the easiest way to achieve a reproducible concentration is to consider saturated solutions. If the water evaporates,the salt will precipitate yet its concentration in solution will remain the same. Moreover, for some specificsalts, the RH above the saturated solution is practically independent from the temperature. Table 1 lists someof the most suitable salts. It is advisable to calibrate/ check your hygrometer over a range of RH that is comparableto the application (for instance, a hygrometer could work perfectly between 40 and 60%RH while displaying biased values over 80% RH). The difference between the displayed value and the physical RH value of the enclosure should remain below the accuracy stated by the supplier. If not, the hygrometer should be re-calibrated.