Without proper calibration, a hygrometer's readings are merely guesswork. And guessing bout RH puts moisture-sensitive products at risk. Chemists and lab professionals determine the accuracy of a hygrometer with a "salt" test. The Boveda One-Step Calibration Kit is the ultimate salt test. Boveda's Calibration Kit ensures a consistently accurate reading by adding and removing pure water vapor to achieve and maintain 75% RH in the controlled environment of the Calibration Bag.