Development of an Accurate Double-Diaphragm Sapphire Cryogenic Capacitive Pressure Transducer

Abstract

Mechanical creep of the diaphragm is well recognized as a major problem for obtaining a reproducible cryogenic pressure transducer and it is mainly caused by the stress at the diaphragm edge, where this is rigidly constrained to the transducer body. The double-diaphragm design alleviates this problem. In this design, the rigid constraint of the diaphragm edge is replaced by a (non-ideal) hinge between two identical diaphragms, which allows for a considerable improvement of the reproducibility on thermal cycling. Sapphire has normally been used for this design because of its excellent elastic properties. However, the quality of the results critically depends on technological details of the fabrication. The paper describes the design criteria used for the actual implementation of the double-diaphragm technique, making use of monocrystal sapphire disks of limited dimensions (Ø 25 mm). Results on the stability for Ap = 0 of a capacitive prototype (0. 1 bar f.s., sensitivity 1.35.10-3 pF Pa-1), designed mainly for work in the range 3-30 K with a gas thermometer in the differential mode, showed a nearly zero temperature coefficient in that range, a zero shift decaying logarithmically with thermal cycling between 77 K and 300 K and limited to 0.025%, and a creep limited to 50 ppm in 8 days at 80-95 K.