I'm working on a reflow hot plate project, and am using a TM-902C K-type thermocouple with digital readout to monitor the temperature. Picture 1 shows the setup, with a circular saw blade 3/8 inch above the bare-element hot plate. I adjust the power with a controller driving an SSR that switches the power on and off per an adjustable duty cycle every 2 seconds, with duty cycle granularity of 20ms - i.e. about one AC cycle.
The problem I'm having is that even after running at the same duty cycle for a long time, so the temperature should be completely stable, the reading moves back and forth by as much as 10°C over a period of 30-60 seconds. The movements are always gradual, so it never jumps directly by 10°C, but that's just too big a variance to be useful. None of this happens at room temperature.
The second picture shows the exact same setup, but with a teaspoon of sand dumped on the tip and board. Doing that almost completely eliminates the variations, so that they never exceed one degree in total, and usually stay right on the money for extended periods - plenty good enough for this purpose.
So I think this means my thermocouple is functioning properly, and the temperature of the blade is in fact constant, but something is making the reading vary when the sand isn't there. Since this is out on the kitchen counter, and the air temperature is much cooler than the hotplate, the variations must be caused by air cooling off the wires near the tip, thereby cooling the tip itself by varying amounts.
The third picture is the thermocouple tip. I don't know whether the wires should touch ONLY at the tip, or if they are ok the way they are.
What I'm really curious about is all the toaster oven projects that use a thermocouple for feedback. From what I've read, you have to place the tip of the thermocouple right on the board, preferably in a plated-through hole. But even so, based on my experience you might easily get that 10°C variance, and that could substantially affect the controller's actions. Why would they not experience the variations that I see? Or do they, but just not know it? My understanding is that with quartz heating elements, heating is primarily radiant, so the air doesn't get all that hot, but I guess there would be less differential than with my hot plate setup.
Is my explanation of the cause of the problem correct, or is it something else? Since I can't be dumping sand onto the solder paste, is there some other way to solve this problem?
By the way, while I haven't given up yet, so far it seems that a hot plate just has too much thermal inertia to permit following the reflow protocol. You can't go from soak to reflow quickly enough without way overshooting. This makes me wonder why so many people prefer hot plates. Anyway, I may well end up with a toaster oven, and would need the thermocouple to work properly there. So I need to figure this out.
Any ideas would be appreciated.