Printed flexible capacitive humidity sensors for field application

A new type of printed capacitive humidity sensors with stacked parallel-plate electrodes is presented in this work. The high capacitance and high sensitivity associated with this type of sensors allow the use of low-cost electronic circuits for detection. The use of a polymer sensing material with extremely low hysteresis and a grid top electrode design ensures the sensor’s stable performance and rapid response to humidity changes. Our study shows that the sensor performance can be significantly improved through printing processes and the performance is affected by environment temperature through its influence on water sorption in the sensing material. The temperature effect on the sensors was investigated in detail and the sorption heat of water molecules in the sensing material was estimated. The printed sensor was attached to a printed circuit board (PCB) to become a standalone and functional sensing unit. The PCB board is equipped with light energy harvesting, capacitance measurement, and wireless data transmission, as well as the calibration capability for converting measured capacitance to relative humidity. The obtained unit met the needs for application in high performance building management.