Wireless Salinity Assessment in Liquid Using Auto-Tuned RFID Tags

A wireless salinity sensor based on RFID technology is presented. It uses an RFID tag as a transducer and an auto-tunable chip to detect salinity in liquids. Water samples with varying NaCl concentrations are brought into contact with the tag, where changes in salinity alter the sample’s conductivity (σ) and the tag’s capacitance, affecting impedance and power transfer to the chip. The auto-tuned chip compensates for mismatch by switching internal shunt capacitors, whose binary states are backscattered to the reader as a hexadecimal sensor code, indirectly reflecting salinity levels. Circuit modeling and full-wave simulations show that increased salinity results in a lower sensor code. Experimental validation was conducted up to a distance of 1 meter using custom PDMS-based sample holders and saline-soaked paper, confirming effective salinity sensing.