Animal Care/Hygrometer calibration

Relative humidity gauges are called hygrometers and their delicate mechanisms can be jarred out of adjustment. As a result they must be calibrated before the readings can be trusted. To do the calibration you need an airtight container (mason jar, ziplock bag, plastic tub, etc.) a small dish, bottle cap, or shot glass, 1 tsp salt, water, and if the hygrometer is adjustable, a very small, screwdriver. Put the salt in the dish and add just enough water to completely wet (not dissolve) the salt. Seal the dish and the hygrometer in the container. (Optionally, put everything in a second, larger, airtight container to be confident that things are airtight.) After waiting at least 6 hours, note the reading on the hygrometer. If that test reading is not 75%, the screw on the back of an adjustable hygrometer can be used to gently correct the reading. If the hydrometer is not adjustable, you will have to mentally adjust the reading every time you use the gauge [actual reading + (75 - test reading)].