The humidity indicator card must be used with a desiccant sachet, in a sealed package. The principle is simple: the pastilles present on the card turn pink as soon as a certain relative humidity level is reached. For example, for a 4 spot card, the thresholds are 10, 20, 30 and 40% RH.
The humidity indicator light is metallic. It is designed to be fixed on watertight boxes or containers. Its fixing is easy thanks to its lock nut and sealing is ensured by a seal.