A connecting device for measuring instruments includes a body member attached to a member which forms a boundary surface of a process. The body member has an aperture therein for communication with the process. The process is measured through the aperture by a measuring instrument affixed to the connecting device. The measuring instrument is affixed to a selector member mounted movably with respect to the body member. The selector member has a hole communicating with the aperture of the body in the measuring position of the connecting device. The selector member has a closing surface closing the aperture of the body in the closed position of the connecting device.