Should this cold-junction compensated thermocouple-to-digital converter (MAX31855) output an accurate temperature?
I worked with a guy who had 8 degree C error on his type-K thermocouple PCB.
His tempsensor was in a corner, well away from the type-K connector; a HOT MCU sat in between, but quite near the tempSensor. He did have the ADC near the type-K. We thought and drew diagrams of heat flow for a few days; we moved the Tempsensor to be between the two (wide thick copper) pins of type-K; we installed a 2cm thermal_gap between MCU and type-K/TempSensor; we used lots of vias to thermally short the 3 GRND foil layers under the type-K to create an isothermal regions(about 50 vias, one every centimeter); he also changed the MCU to a low-power version. Result? Next pass of PCB was less than 1degree C error. We had developed the understanding of thermal shorts and thermal opens.







simulate this circuit &ndash; Schematic created using CircuitLab

Your MAXIM IC needs to measure the temperature of the K_connector.
Copper foil has thermal resistance of 70 degree Centigrade per watt, per square of foil (any size square). A heat flow of 0.1 watt will cause 7 degree C error.

Simply putting your face near the PCB or junction or header or MAX will cause the "temperature" to change. Our heads dump 100 watts into the environment; at 0.2meter by 0.2meter area, or 20 by 20cm (400cm^2), our face is 1/4 watt per square centimeter. Your face, or you hands, or a soldering iron, causes errors.

What else is around, needing to dump heat?