Experimental Research on Steam Wetness Measurement by Capacitance Sensor

It is well-known that, according to the dielectric theory, the static dielectric constants are quite distinct for the saturated water and its vapor at the same pressure and temperature. Therefore, based on it, the feasibility of using the capacitance method to measure the flowing steam wetness was explored. As air and water vapor have similar static dielectric constants, the wet steam was substituted by spraying water into air, whose wetness was measured by a novelly designed coaxial cylindrical capacitance sensor. The output signal of the measuring circuit is the voltage which is in direct proportion to the capacitance. The experimental results shows that as the wetness of the wet air flow increases gradually, the output voltage and capacitance get increased accordingly. And for wetness below 12%, there appears an obvious nonlinear correlation between the change rate of the capacitance and the wetness. However, for wetness in the range of 3%~8%, this correlation shows a perfect linearity. The analysis of experimental results indicates the feasibility of capacitance method for the steam wetness measurement.