Dry & Dry [15 Packs] Premium Cobalt Free Humidity Indicator Cards(Reusable 6 Spots) Card Dimension : 1.5"(3.8cm) x 4.5"(11.5cm). Humidity indicator cards are sensitive to temperature, humidity, atmospheric pressure, and wind. Therefore, they do not work well in open air. They work well and display accurate relative humidity level in enclosed space. Cobalt Chloride Free Premium Quality - REUSABLE Eco Friendly Humidity Indicator Card When You Initially Open the Card, All Dots Sometimes React and Change ColorBecause They are Exposed to Sudden Humidity. If You Wait for About 30 minutes, the Card will Calibrate and You will be able to Read the Humidity Level. - 6 Spots Humidity Indicating(10%/20%/30%/40%/50%/60% RH - Clear Color Change) Indicator Changes Green to Purple(Violet) - Green(Dry) to Purple(Humid) CONTINUOUSLY & ACCURATELY Displays Humidity Level - If the humidity level becomes lower, the strip will change back to Green (the original color). Just because the color has changed to the next level does not mean it cannot go back to the previous color. The colors will change depending on the humidity level. Colors may fade with long term use of more than 20 times.