I have added an item to my web store that I think is a great product. It is made by Boveda and is one of their humidipak items. It is called a "One-Step Calibrations" kit.
It comes with a ziplock bag with a double seal and a 75% humidipak. Just open the bag and place your hygrometer in the bag, one side of the bag is clear so you can see the display on your hygrometer. Leave it in there for 24-36 hours and then see how accurate your hygrometer is. If it is off note the difference and add or subtract that amount to the displayed level when you put it back in your humi.
It comes with a ziplock bag with a double seal and a 75% humidipak. Just open the bag and place your hygrometer in the bag, one side of the bag is clear so you can see the display on your hygrometer. Leave it in there for 24-36 hours and then see how accurate your hygrometer is. If it is off note the difference and add or subtract that amount to the displayed level when you put it back in your humi.