Abstract: The apparatus includes first and second support elements of fused quartz and an intermediately positioned thin diaphragm of the same material. Each support element has a recess in one surface which defines a support surface. The support elements are oriented so that the support surfaces contact and support opposite surfaces of the diaphragm. There are openings in each support element which extend between the recess and the exterior of the element, through which openings a reference pressure and the pressure to be measured are directed. The support surfaces of the support elements and the diaphragm are bonded together so as to form a dimensionally stable, unitized apparatus. Thin film capacitor plates are provided on both surfaces of the diaphragm and in the recesses of both support elements to define two capacitors. Electrically conductive leads extend from the capacitor plates to the exterior of the apparatus, so that the capacitance of the capacitors can be measured.