RFMicron moisture sensors are specifically designed to sense varying amounts of moisture and to provide a measurement value. This numerical value is called the Sensor Code.
Water and moisture affect the RF performance of the antenna. Moisture effectively moves the antenna tuning frequency. While it is possible to design an RFID sensor able to tolerate moisture, any such design suffers from severe limitations. A normal RFID tag could be designed to operate at the correct wireless frequency in the presence of moisture, but this same tag would probably not be readable when dry because the antenna tuning frequency would change without the moisture. This is impractical in the real world. The other issue with normal RFID tags is that they have no means of detecting moisture other than the frequency shift. Without a measurement circuit, the typical RFID tag can not report moisture levels.
RFMicron moisture sensors take a more sophisticated approach that uses the moisture/tuning frequency dynamic to measure the moisture levels. RFMicron incorporates a highly-patented tunable RF frontend called the Chameleon™. The Chameleon engine manages the tuning process so that RFMicron moisture sensors communicate over a range of moisture levels. For real world applications, the self-tuning frontend is essential for reliable sensing. The amount of moisture can be read from the Sensor Code, and the sensors continue to communicate with and without moisture.