High Frequency and Ultrahigh Frequency Radio Frequency Identification Passive Sensor Transponders for Humidity and Temperature Measurement Within Building Structures

This paper presents the development of passive high frequency (HF) and ultrahigh frequency (UHF) radio frequency identification (RFID) sensor transponders for humidity and temperature monitoring. The transponders use RFID integrated circuits with a digital interface for communicating with a ultralow-power microprocessor. The microprocessor handles the measurement, either by charging sensor and reference capacitors, or by commanding a digital sensor. The read ranges of the sensor nodes are 5 cm and up to 40 cm at HF and UHF, respectively. The sensor nodes are demonstrated in concrete casts, where the HF node provides better than 5%RH-repeatability of the moisture measurements from sensor node to another. At UHF the repeatability is about 15%RH.

I. Introduction

Wireless sensor networks have recently been widely studied, especially for environmental sensing [1]. There are a multitude of implementations using commercial protocols, such as Zigbee [2], as well as proprietary protocols [3]. Active systems attain a life time of about 5 years with standard AA sized batteries, which is acceptable in indoor environmental monitoring, where the sensor nodes are easily available for battery replacement. However, when monitoring moisture inside building structures, the battery replacement is not possible and application life time should be in tens of years.