The Dometic DTM01C holding tank monitor system includes a full-tank sensor in a threaded cap. It is a reliable and well proven design that is easy to install and is used by the world’s leading boat builders. The Dometic DTM01C offers unparalleled reliability at a single glance as a bright red warning light indicates when holding tank is full and that the toilet should not be flushed. This is a simple solution to prevent any overfilling of the tank. The single float switch assembly comes with a cap that fits on to a 3" diameter threaded tank opening.