Tweaked PID controller to stabilize target swings. Still needs further tweaking, maybe even adding Auto Tune as the oven behaves differently at different temperatures. Increased the PWM from 25% max to 100% max to do a full load test.
Wrapped most of the oven in foil, shiny side inward. Made a big difference to perceived heat emission and it completely blinded the laser temp sensor (was reading room temp all over the surface)
Ran up to 800C to check the thermal stability of everything hooked up. Some findings:
- Max external oven temperature measured was 350C and seems to mostly range between 77C and 123C.
- Between 1400W and 1600W needed to keep oven at 800C. Calculations for ideal oven suggest 817W.
- Controller box was cold to touch with the 80mm fan on. The SSR heat sink reading was max 29C at ambient 24C.
Overall the construction seems sound enough and electronics/electrics can handle the design well.
Think there is a massive loss to convection caused by the gaps between bricks. Also noticed the max power the coils are willing to draw peaked at about 1650W suggesting a 3rd coil might be needed to reduce the resistance. Trying to keep the max current consumption under 10A to avoid needing a special wall socket.
Some other random notes:
- Mains fluctuated between 245V and 226V during the day. Suppose as people start cooking in the complex towards the evening the grid takes a bit of a knock.
- Guesstimating it took about 3 hours effort to get to 800C from 24C room. Was tweaking the PID controller a lot during the day, pausing the temperature on its way to 800C for extended periods. According to my calculations it can take about 72 minutes min.
- Aluminum foil has a little dance on its surface when exposed to convection at ~200C. Suppose its a bit of contraction/expansion competition across the surface as the hot air flows under it trying to escape to room temperature.
- Power meter for the day suggested 6kWh spent on the experiment.
Next potential steps:
- Investigate a layer of Kao wool and brick interleaving to reduce convection and thermal conductivity.
- Reflect on the coil layout to see if a 3rd might fit.
- Start uploading some code to see if the community can identify any glaring improvements.