Buffalo catering coffee percolator problem
(I'm going to answer my own question as the easiest way of documenting what I've learned, for the sake of anyone searching on this device or this fault, but feel free to comment.)
This Buffalo 15L 100 cup coffee percolator was faulty from new. A replacement percolator was sent by the supplier but they weren't interested in taking back the faulty one. I was therefore asked to see what I could do with it. When I tested it, the boil-dry thermostat cut out prematurely while it was still brewing.
Principal of operation
Electric coffee percolators typically contain two heater elements: the main one, in this case 1500W, and a keep-warm heater of 125W. Unlike a kettle, the main heater only heats a small portion of the water in a well at the centre of the base. This boils a small amount of the water, sending a mixture of steam and boiling water up a central tube and onto a spreader, which allows it to drip through to the coffee tray. The base of the tray is perforated, allowing the water with percolated coffee to drip through to re-join the main bulk of the water.
The bulk of the water, therefore, is not heated directly, but rather by the hot coffee dripping back into it. Only when this approaches boiling point does a thermostat cut out the main heater, leaving the keep-warm heater on.
Here is the circuit diagram:
While the ready thermal switch remains closed, the keep warm heater and green indicator are bypassed and the full mains voltage appears across the main heater and the red indicator. Once the ready thermal switch opens the two heaters are in series. Since the keep warm heater has around 10 times the resistance of the main heater, 90% of the mains voltage appears across it, sufficient to light the green indicator. At the same time, only 10% of the mains voltage now appears across the main heater, insufficient to light the red indicator and much reducing the heat the main heater produces.
(Stand by for the fix.)