Thanks for the comments...
Darn thing just reset itself! - sitting right here on the bench..... (good strong wi-fi signal)
- dodgy psu?
I've seen the same behaviour with two separate psu's - both rated 2.5A
- Faulty Pi?
It's a possibility. I've 2 x 3b's and one PiZero here.
I seem to recall that I swapped out the previous 3b for the current one - but I could swap it back, just to see....
Out of curiousity, I set a led flashing in the program's idle loop (it takes a temperature reading then sleeps for 60 secs before taking the next one).
Seemed to flash perfectly happily until, suddenly, the Thonny window closed itself and terminated the program.
I don't think the Pi was 'out' long enough to have rebooted, but it dropped communications with the VNC Viewer on the PC.
One or two of the previous times it 'reset' itself, I had htop running - but, when I came back to the machine the htop window wasn't active... which, as you say, suggests that the Pi had reset itself...
Just noticed that I'd soldered (badly!) a reset pushbutton to the reset pads - soldering clearly not good enough as one of the leads to the board just parted company with the pad on the board.. Wonder if it's been waving around and causing a (seemingly random) reset?
Reset wires now removed - we'll watch & wait.....
If that proves _not_ to be the problem then I'll happily post the code. It's a scruffily-hacked version of the Adafruit MAX31855 test program - so nothing particularly exotic. I think, without a 31855 attached it'd likely sit there until hell freezes over, waiting to read the device....
Anyway - I'll run this thing for a bit and see what happens...
During this 'debug' journey, I did find a line in the original code that would cause the program to terminate if the MAX31855 returned a 'NAN' (not a number) result, which could happen if noise found its way into the thermocouple circuit.
The kiln it's monitoring has a 7.5kw / 240v heating element, switched via heavy-duty contactors, so it's not the cleanets of electrical environments!
Thanks again for your suggestions - I'll report back!