Background
We recently purchased a couple of used kilns for firing ceramics – one 5 KW kiln and a larger 10 kW kiln. These kilns are equipped with what is known as infinite control switches, basically knobs that allow you to adjust the percentage of time the heaters are on.
They also incorporate a device called a kiln sitter that is designed to shut off the kiln when it reaches a specified temperature. It does this by mechanical means where you insert a cone that is designed to melt at a specific temperature. The cone is inserted under a wire, when the cone begins to bend the wire drops, releasing a weighted lever arm that falls and shuts off the switch. On the smaller kiln there is also a safety timer that will shut off the kiln if a set time is exceeded.
In order to fire ceramics in these kilns it is necessary to adjust the settings on the infinite controls as the kiln heats up. It would not be good just to turn the heaters all the way up from the start, as the temperature would shoot up too fast and the ceramic pieces would either explode from trapped air or water, or crack from the rapid temperature change. You must start at a low setting and turn it up as the temperature increases in order to maintain the correct temperature profile for good results. Then when the cone bends enough to shut off the kiln, it simply cools at whatever rate is determined by the mass in the kiln and the amount of heat escaping from the kiln.
More modern kilns often include an electronic temperature controller that is capable of running temperature profiles on its own, and can also regulate the cool-down rate if desired. Obviously this is a big improvement over the need to manually adjust the controls during the firing. This project is an attempt to improve on those controllers using off-the-shelf computer and networking hardware to build a controller with a web interface that allows remote operation of the kiln.
The Design
The diagram above shows the physical devices and interconnections in the system. The kilns are located in a shed that is separate from the house. Communications between the shed and the house happen over a wireless link using a reprogrammed Linksys router in the shed.