A measuring probe has a housing for receiving a sensor element. Hence, even if a processing fluid adhering to a substrate or an evaporant of the processing fluid moves toward the main transportation path while the high-pressure processing unit transports the substrate wet with the processing fluid, there are the processing units located which the processing fluid or its evaporant must arrive at before reaching the main transportation path. A sterilization element of silver or of another metal having an oligodynamic action may be arranged in the intermediate space between the projection of the valve seat and the elastic seal.