MEASUREMENT OF 0-1ML 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. Investigations focused on the desi gn and operation analysis of the oscillator differential structure in which the oscillation frequencies of the two oscillators are very clos e, and on the application analysis of capacitive-dependent crystals. I n addition, the excitation of the entire sensor with stochastic test s ignals has been analyzed by the correlation deconvolution method which is also called the direct digital method (DDM). The compensation of t emperature and voltage influences, as well as disturbing noise signals , are included. The area of operation and the uncertainty of the senso r with and without the test signal are given as well. When designing t he capacitance sensor, the problems regarding the source of stable osc illation, compensation of temperature, the influence of supply voltage , noise, and A/D and D/A conversion occur in the operation range under 1pF. The pulse width module, which forms pulse-width modulated high-f requency 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 increa ses linearly if it is charging by a constant current. As the charging is affected only by the current pulses which require an adequate curre nt, the disturbing noise signals do not affect the capacitor charging. The correlation determination of the measuring value is primarily imp ortant for the determination of end values. Several experiments have b een carried out to investigate the method's possible applications. The method is linear in the range of work and ensures the uncertainty in the. range below 0.01%. The experimental results of 0-1 ml volume meas urements are shown. The volume measurement uncertainty (0-1 ml) is les s than 0.05% (T = 15 to 25-degrees-C).