A tool and a retractable assembly for measuring at least one measured variable of a medium in a containment, comprising: an essentially cylindrical housing; an immersion tube, which is movable axially in the housing between a service position run out from the medium and a process position run into the medium; and an engagement apparatus secured on the housing; wherein a movable piston is provided in the engagement apparatus and at least a first position and a second position of the piston are provided. The engagement apparatus holds the immersion tube in the service position when the piston is located in the first position, especially when no energy supply is present, and the engagement apparatus holds the immersion tube in an intermediate position between the service position and the process position and prevents further movement of the immersion tube in the direction of the medium when the piston is located in the second position.