This is a simple and surefire method to accurately calibrate any digital or analog hygrometer. The kit uses a 75.5% relative humidity standard—widely used and accepted by chemists and lab professionals to determine the accuracy of any hygrometer (also known as the “salt test”). Humidipak’s One Step Calibration kit is used by some of the top museums in the world to calibrate their instruments, including the National Gallery of Art in Washington D.C. Shelf life: 2 years inside the plastic wrapping. Hygrometer in picture not included.