The device has a ring-shaped clamping body (10) with a valve seat formed as a part of a downpipe set, and a ring-shaped adaptor (30) surrounding the body at a radial distance. The adaptor has a tube socket (31) and a measuring device that is attached to the socket by a flexible tube or a tube line. The measuring device detects a hydrostatic water level in a sanitary basin or a measured variable proportional to the water level. The body has an opening (42) above the seat to define a water connection between the basin and the socket.