the cylinder bore 15 provided by the controlling clearance 22 is sealed by the lower valve stem seal 23 the interior of the spring housing and stuffing box shell 4 is connected to the atmosphere through the relief bores 18a so that it will be prevented in any case that misrouted cleaning or disinfecting agent will be present under pressure in the interior of the spring housing and stuffing box shell 4 even in case of a damaged seat surface 6a it is ensured by the sealing through the lower valve stem seal 23 in the area of the controlling clearance 22 that no cleaning or disinfecting agent can get into the product space or into the sample the relative lift displacements a b c indicated in fig2 are defined as follows the relative lift displacement a is the distance the upper edge of the lower valve stem seal 23 must travel in order just to close the controlling clearance 22 the relative lift displacement b is the maximum distance the lifting and closing member 6a must travel from the closed position of the sampling valve to get into abutment on the closing element 5 the relative lift displacement c is the distance the spring housing and stuffing box shell 4 and consequently also the valve stem 6 must travel from the closed position of the sampling valve in order that the upper piston valve seal 19 within the piston valve member 18 will just establish a connection between the interior of the spring housing and stuffing box shell 4 and the atmosphere by dimensioning the relative lift displacements to be b gt a gt c it is ensured on the one hand that the supply of cleaning or disinfecting agent through.