A Passive Wireless Humidity Threshold Monitoring Sensor Principle Based on Deliquescent Salts and a Diffusion Based Irreversible State Change

Passive RFID transponder applications that require integrated sensors can greatly benefit from unconventional sensing strategies especially in situations where there is a need to continuously monitor environmental properties without having access to an integrated source of electric energy or an omnipresent reader station within communication range. Since, in many cases, information about the violation of a threshold value is of interest, alternative sensing strategies exploiting irreversible phenomena not considered or even avoided before, have the potential to find successful use in low-cost (e.g., chip-based sensor RFIDs) or lowest cost (e.g., chipless sensor RFIDs-either with or without transistors) wireless sensor applications. In this paper, a stand-alone, simple, passive, wireless humidity threshold sensor concept, and its realization are presented. This exploits the deliquescence phenomenon of salts. Based on a double planar coil arrangement, for which an electric model is given and a transfer function has been deduced, an oscillating circuit is formed. Its resonance frequency changes irreversibly, if a threshold relative humidity is exceeded for a certain exposition time. The sensor principle is demonstrated by the example of sodium chloride. Various measurements demonstrate the feasibility of the presented sensor approach. Sensor solutions based on a threshold activated irreversible state change might be a promising approach in order to monitor environmental parameters without a permanent supply of electric energy.