A radio frequency identification (RFID) tag can include a die attached to an inductive-capacitive (LC) circuit including a capacitive element coupled to an inductive element. The LC circuit can have a resonant frequency that varies according to properties of fluid proximate the RFID tag. The RFID tag can further include a coating material disposed around the die to form an outer surface of the RFID tag. The coating material may have a thickness over a portion of the LC circuit, to permit a conductivity property of a fluid proximate the outer surface to affect the resonant frequency of the LC circuit such that the resonant frequency shifts to a second resonant frequency. Additional apparatus, systems, and methods are disclosed.
Methods and Apparatus for Evaluating Downhole Conditions with RFID Mems Sensors
Goodwin, S., Roberson, M., Bartee, C., McGuire, K., Roddy, C., Rothrock, G., & Ravi, K. (2016). Methods and Apparatus for Evaluating Downhole Conditions with RFID Mems Sensors. (U.S. Patent No. 9,494,032).