Capacitive blood pressure transducer

A capacitive transducer was developed to measure blood pressure using a catheter interfaced to the body. The transducer was made of fused quartz in order to achieve a low temperature coefficient and it consists of a circular body and a 1.25 mm thick sensing diaphragm that mates concentrically with the body. The body of the transducer has a 10 micron deep circular cavity, plasma etched into it. One of the electrodes of the sensing capacitor is vacuum deposited in the etched cavity and the other, which is an oversized electrode, is similarly deposited on the underside of the diaphragm. To obtain a stable reference capacitor, a ring electrode is also deposited in the cavity that forms a capacitor with the oversized electrode. The diaphragm and body are brazed together using a tin-gold alloy. A typical transducer has a full scale of 300 mm of Hg with an overpressure capacity of 3000 mm; a TC sensitivity of 0.015% of FS/degrees C; a thermal zero drift of 0.01% of FS/degrees C; linearity and hysteresis of 0.5% of FS; a volume displacement of 0.02 mm3/100 mm Hg. The pressure module is 29.3 mm in diameter and 7.6 mm high.