The invention relates to a retractable assembly for immersion-, flow- and annex-measuring systems in analytical process technology for measuring at least one measured variable of a medium in a process containment, comprising an essentially cylindrical housing having a housing interior; an immersion tube, which is axially movable between a retracted service position in the housing and a process position extended from the housing. In the service position, the immersion tube is positioned in the housing interior; a closure element on an end region of the immersion tube facing the medium for sealing off the housing interior from the process containment when the immersion tube is located in the service position, and a proximity detector in or on the end region of the housing facing the medium for detecting the closure element in the service position.