The Boveda One-Step Calibration Kit is the simple, surefire method to accurately calibrate any digital or analog hygometer. Using the 75.5% RH standard, the One-Step Calibration Kit is also used by some of the top museums to calibrate their instruments, including the National Gallery of Art.
One of these came with a new (2nd) humidor I purchased and it worked great and easy to calibrate both my old and new hygrometers separately or together.
I have 3 hydrometers and each read differently. This item sure confirmed the actual humidity.