A pressure responsive diaphragm has on its periphery axially spaced annular projections to define a peripheral annular groove therebetween which receives an annular projection at the coupling surface of a coupling flange. The diaphragm is thus releasably retained on the coupling flange and encloses a recess formed in the flange which recess communicates with the instrument. A compressible sealing ring for the recess within the flange is positioned on the peripheral surface of an annular projection of the diaphragm. The diaphragm may also be provided with an axially extending annular projection which is received in a corresponding axially opening groove formed in the coupling flange. A compressible sealing O-ring may be positioned within this axially opening groove radially outwardly of the diaphragm projection or may be positioned on the peripheral edge of the second annular projection of the diaphragm and arranged so that the O-ring contacts two or three metal walls of the coupling flange.