Measurement of 0-1 ml volumes using the procedure of capacitive dependent crystals

In the paper the use of a differential oscillator sensor structure in a capacitance sensor is presented. Investigation was focused on the design and operation analysis of the oscillator differential structure in which the oscillation frequencies of the two oscillators are very close, and on the application analysis of capacitive dependent crystals. 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 in the operation range under 1 pF. 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. In this way we benefit from the fact that the capacitor's voltage increases linearly if it is charging by a constant current. 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 correlation determination of the measuring value is primarily important for the determination of end values. The experimental results of 0-1 ml volume measurements are shown. The volume measurement uncertainty (0-1 ml) is less than 0.05% (T=15 to 25/spl deg/C).<<ETX>>