Humidity measurement using the procedure of capacitive-dependent crystals

A differential oscillator sensor structure in a capacitance sensor and the experimental results of humidity measurements are shown. In addition, the excitation of the entire sensor with stochastic test signals has been analysed by the correlation deconvolution method which is also called the direct digital method (DDM). When designing the capacitance sensor the problems regarding the source of stable oscillation, compensation of temperature, the influence of supply voltage, noise, and A/D and D/A conversion occur. The pulse width module, which forms pulse-width modulated high-frequency current pulses, is the proposed solution. With these pulses the capacitor in the integration element is charged or discharged. As the charging is affected only by the current pulses which require an adequate current the disturbing noise signals do not affect the capacitor charging. The uncertainty of humidity measurement is less than 0.5% (T=15 to 25°C: and humidity=10 to 98%)