Heat cost allocators are devices attached to individual radiators in buildings that measure the total heat output of the individual radiator. Heat cost allocators can be either electronic, where one or two electronic thermosensors and a microcontroller are used to calculate the heat consumption of radiator by the temperatures of radiator and air in room, or evaporative, where a special, calibrated liquid in a capillary tube records the total heat output of the radiator.
References
Heat cost allocator Wikipedia(Text) CC BY-SA