In view of the character that humidity measurement for steam with high temperature and pressure is difficult and inaccurate

a coaxial cylindrical capacitance sensor is developed by capacitance method based on the fact that water and steam have great different permittivities.By means of this sensor

on-line measurement of the steam humidity can be realized.The measuring circuit made with timer NE555 can transform the capacitance to frequency output

and then reflect the steam humidity with the frequency.The sensor can measure not only gas humidity but also the water content of solid materials after serious calibration.