Anti Flood Device by Magiplug®
The Magiplug is a bath plug which opens when the head of water over it reaches a certain level. It is intended to prevent bath overflow. The design has a significant advantage over conventional (built-in) overflows in that the emergency outflow takes place under a very much greater head and thus at a very much greater flowrate. The Magiplug will be able to "keep up" with continuing inflow to a greater degree than a conventional overflow.
The Magiplug is constructed from moulded plastic parts. The seal is maintained by a spring. Compression of this spring under increasing head of water over the plug eventually causes the plug to open, allowing water outflow. At some lower level, the spring will restore a closed position and should shut off outflow. The plastic forming the open / close sealing plate has been modified by the addition of a temperature-sensitive colouring. This aspect of design is to give an immediate visual indication of potentially over-hot bath water.
When the pressure plate turns green this can be taken to indicate water definitely not too hot for safe exposure levels (eg for a babys bath).
School of Engineering & Electronics, University of Edinburgh - April 2004