the holding part 49 which leads in turn to the measuring opening 30 of the screwed crown 11 the screwed crown 11 is represented in the closed position in fig7 and in the open position in fig8 inserted tightly in a radially penetrating case opening 50 is a guide shaft 51 which projects radially outward and is provided with the axially penetrating measuring opening 30 in this arrangement the outer lateral surface which runs around radially the guide shaft 51 forms a sliding surface on which a sealing ring 52 surrounding the guide shaft 51 is seated such that it can be displaced both axially and in the circumferential direction the sealing ring 52 is seated with its radially external region in an annular groove 53 of a crown head 54 which reaches over the guide shaft 51 in the fashion of a pot the crown head 54 has a center pin 55 which projects into the measuring opening 30 an annular gap 56 is formed in the free end region of the pin 55 between the pin 55 and the wall of the measuring opening 30 while at the free end region of the guide shaft 51 the measuring opening.