Passive wireless irreversible humidity threshold sensor exploiting the deliquescence behavior of salts

Passive RFID transponder applications, requiring integrated sensors, can greatly benefit from unconventional sensing strategies. Especially in situations where there is the need to continuously monitor environmental properties, but without having access to an energy source or an omnipresent reader station in communication range. Since in many cases the violation of a threshold value is of interest, alternative sensing strategies, exploiting irreversible phenomena not considered or even avoided before, have the potential of finding a successful use in low cost (e.g. chip-based sensor RFIDs) or lowest cost (e.g. chipless sensor RFIDs) wireless sensor applications. In this work, a stand-alone, simple, passive, wireless humidity threshold sensor is presented, which exploits the deliquescence behavior of salts. Based on a double planar coil arrangement, an oscillating circuit is formed. Its resonance frequency irreversibly changes, if a threshold relative humidity is exceeded for a certain exposition time.