Calibration with MAX31850K connected

[georgeharker]

First of all, awesome project. I’ve done the MAX31850K tc board mod. And flashed the firmware etc. it’s behaving much better but still under the right temp (tho it tracks better).

In pursuit of understanding I taped some external tcs to the boards and managed to see the relevant discrepancies. Which can be modeled as gain and offsets. This theoretically gets them to match the board temps. (Ie negate the issues of having boards not at where the built in tcs are / them not being where the board is). Only issue is it didn’t do anything when I entered calibration data.

Is my reading of the code correct, in that the calibration does not apply if the MAX31850K is connected? Looks like it only applies in the non External TC codepath.

If that’s correct it might be nice to add it in.

Can have a shot at a PR but wanted to discuss first and will want to have a shot at making that mod and flashing the firmware before I’m comfortable recommending it.

George

[georgeharker]

Update: I was seeing a roughly 18deg C difference between the pcb temp and the registered values from the inbuilt fixed thermocouples using the thermocouple interface board. That's not super surprising as the thermocouples are closer to the heaters and also higher up.

Left and right tcs differed by approx 3-5deg and the graph of on pcb values was at a gain of roughly 0.87 compared to the built in averages / left right values.

I'm using a lead free paste which has a melt at about 240deg and was totally failing to get hot enough to reflow despite allowing longer soaks, longer reflow times and peaking the reflow temp a fair bit higher than rated in the data sheet for the profile I made.

I modified the firmware to allow gain and offset to be applied to values read by the external tc board. After measuring values on pcb and comparing to the built in values I was able to find a decent set of gain and offset values and immediately got a nice reflow.

So... I'm happy to make a PR to allow calibration to be applied to the external tc board values. That seems valuable. It would be used by measuring on pcb temps and developing adjustment values. It's still pretty manual.

It feels like with the two extra tc converters it should be possible to add a mode that would auto generate / calibrate values for offset / gain. (I'm sure this is pcb height and thermal capacity dependent, so would be for tuning a consistent process run rather than totally one-shot).

Or alternatively (and possibly more hazardously) to switch to on-pcb thermocouples as feedback tcs for pwm of the heaters.

Those latter changes are a bit more work. But would be awesome. I might (time allowing) have a shot at those.

At any rate having firmware with source code is a great leg up in getting the oven to a usable state. Thanks.