Optimization study of a microwave differential technique for humidity measurement in gases

We present a simulation study of precision humidity sensing in gases by a microwave differential technique. We investigate the conditions for the optimization of the system capabilities. The small quantity at the parts per million (ppm) level of water vapor in gases is determined by measuring a shift of the resonant frequency of a measuring resonator relative to a reference one. Because of some unavoidable differences in the physical dimensions and electrical characteristics of the setup components, we studied the variation of the circuit parameters that could significantly have an impact on the sensitivity of the method. We also considered possible asymmetries between each branch of the circuit. It is concluded that our circuit design can tolerate imperfect symmetry of the components and still provide good sensitivity, as shown by our experimental results.