Highly sensitive humidity sensor for dielectric measurement (water absorption) in glass-fiber resins

An humidity sensor using a sensitive capacitive-dependent crystal is described. The authors describe the probe sensitivity (i.e. the dependence of df on dCx). In addition, a new idea of excitation of the entire humidity sensor with stochastic test signals is described, and the humidity surface-measuring method is given. This includes the influence of test signals on the weighting function uncertainty and on the A/D to D/A conversion. The results of air-humidity measurement and water absorption measurement in glass-fiber resins are shown experimentally. The uncertainty of the surface humidity measurement is less than 0.2% (T=15-25 /spl deg/C and air-humidity=50 to 98%).