There are 2 elements per ring in a Skutt kiln. Each element loops twice. On low the switch sends power through both elements in series, creating 1/4 the available power. On medium it send power through one element, so 1/2 power. On high it turns on both elements in parallel for full power. The controller regulates the temperature by cycling the relays/elements on and off. However you can't just swap the Sitter with a controller and put some relays in the box. The existing box is not made to house a digital system. It's not vented well enough and there's no heat baffle. If you want to convert it to a digital system you'll need to build a separate box and mount it to the wall. The kiln plugs into the box and the box plugs into the outlet. Look at the Skutt KM-1 wall mount controller as an option. That type of system uses one big relay to run everything rather than individual relays for each section. Or you can buy a complete new digital box from Skutt, with their controller. For the money I'd replace the whole box so you don't have to deal with having the manual system and the digital system.
