A Solid-Dielectric Capacitive Pressure Transducer

Abstract

We are attempting to develop a direct-reading pressure trans-ducer that will be as precise and stable as the piston gauge so that it may be used as a pressure transfer Standard with these gauges. The development has centered on a capacitive transducer in which the change in capacitance of a solid-dielectric capacitor is measured. The capacitors consist of discs of dielectric material with metallic electrodes and guard rings deposited directly on their surfaces, forming, in effect, parallel-plate capacitors. When subjected to hydrostatic pressure, the capacitance changes as a result of the combined effects of the change in size of the capacitor and the change in the dielectric constant of the material. Andeen, Fontanella, and Schuele [1] were the first to demonstrate the feasi-bility of such a device. They used single-crystal CaF£ as the dielectric medium with which a pressure change of 100 MPa produces a capacitance change of only 0.37%. Although this change is small, by using the three-terminal capacitor method, modern ac bridge techniques, and accurate ratio transformers, capacitance measurements can be made with accuracies in excess of 1:10⁷ and with resolution approaching 1: 10⁹. It should, therefore, be possible to measure pressures with an uncertainty of less than 1 kPa, which is about the uncertainty of piston gauges intended for the 100 MPa ränge. The drawback with using CaF₂ capacitors is their large temperature dependence. To realize the pressure resolution of 1 kPa with this material requires that it be thermostated to within 0.1 mK which is clearly impractical. To overcome this pronounced temperature dependence, we conducted an extensive search for materials having a more favorable ratio of temperature-to-pressure dependence. Several materials were found which were an order-of-magnitude better than CaF₂, but this was not considered a sufficient improvement.