Oven temperature logger up and running

The MAX31856 thermocouple breakout board (shield / hat / cape??) thing arrived to day. Or more accurately, a MAX31855 board arrived, as the tossers behind that particular Amazon storefront can't describe their product correctly. NVM, the difference isn't an issue in this situation:

The thermocouple is a K type, so that's OK. and I'm measuring the temperature of a domestic oven, so the reduced resolution really isn't a problem. And the 50/60Hz filtering isn't going to be an issue I don't expect. So apart from requiring a slightly different firmware in the ESP32, it's not much of a change. You can get Claude to reflash the ESP32 over wifi if you need to change the firmware.

Let's connect up the MAX with the ESP32. Here's the wiring table:

Here are the 2 boards. I'm just going to use a breadboard, so positioning the MAX board upside down with the pins downwards orientates it in a similar way to how it will be in the implementation. 

That worked, as did the Home Assistant dashboard. With the original 1 minute updates and the broad limits for the Y axis, the resolution is bit crude. There's clearly some cycling going on. That's perhaps not a surprise, as the temperature controllers in these things are hysteretic with a fairly low switching frequency. It has little impact on the average temperature and saves the relay contacts.

But the MAX can acquire and report at 10Hz, so I can find a middle ground between 1 minute and 100ms, like once per 10 seconds. I switched in the middle of this record and now we can see the oscillatory nature of the temperature "control" as a result of the on-off control scheme. 

The cycle time is just shy of 2 minutes at this condition. Obvs I had to ask Claude (now living within the Home Assistant annals of the Raspberry Pi) to capture some values and report. This is with the oven set to 180C and the thermocouple near the front, next to the door. Obvs if I placed it right next to the fan heating element, it would read a higher temp. The cycle time seems to be around 110 seconds, which will be dependent on the rate of heat loss (ie kW lost through the insulation) and will increase as the set temp is raised.

And here's what happens when the temperature is turned down from 180C to 130C:

And sure enough, the cycle time increases to something like 3-4 minutes. There's a lot of residual heat stored in the oven and the casserole. I expect it will be a bit shorter once the system has found a longer term equilibrium.

Now we just have to keep an eye on things over the next few days and weeks. This uncertainty over the temperature control needs to be resolved before we go near it with an expensive turkey in 2 months' time....