Simply place your hygrometer inside the bag, close the zipper, and wait 24 hours. A perfectly-calibrated hygrometer will read 75% RH. If your hygrometer reads above or below 75%, adjust the calibration by the number of percentage points it is off from 75%. Completely maintenance free - no activation required. Easy to use. Lasts approximately one year, good for multiple calibrations. Shelf life before opening is a minimum of 2 years. Produces an environment of 75-percent humidity Exterior Bag Dimensions: 5.5" x 7" Interior Bag Dimensions: Approx 4" x 4.5"