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I upgraded to an Mk9 dual extruder, and it came with thermocouples installed instead of the thermistors I had before.

No matter what I did with the thermocouples, the indicated temperature jumped around by as much as 30C or more. In short, after several weeks of fiddling I never got the thermocouples to work well, and replaced them with thermistors, which have been fine.

So my question is: what is required to get thermocouples to give reliable, consistent, accurate readings? Are they just incredibly touchy?

Some things I tried include:

  • Of course, one must add circuitry (typically a thermocouple amplifier board such as http://wiki.ultimaker.com/Thermocouple_Sensor_Board_v1), to convert the tiny voltage differences to larger differences usable with Arduino or similar analog inputs.

  • Place those boards close to the thermocouples, but far enough that they are at pretty stable temperature themselves.

  • Have absolutely no wire extensions of splices, changes of wire types (material), etc.

  • Avoid doing repeated measurements too fast.

  • I replaced a thermocouple board with 5V through a potentiometer to the analog input pin, to rule out problems in the Arduino, pin configurations, or software, and got stable readings.

  • I checked for shorts-to-ground from the heater block, both sides of the thermocouples, the heater itself, etc. None found.

  • The thermocouple wires are surrounded by a braided shield (not common or shorted to either thermocouple wire); I tried grounding that at either end and at both ends, to the heat block, the printer frame, the power-supply ground, and the RAMPS board ground. These had various effects (sometimes large), but I couldn't find any configuration that made the readings stable (much less accurate!).

Anything I'm missing?

Thanks!

Steve

1 Answer 1

It sound like you just have a defective thermocouple. But, I just did a google search for "Why are thermocouples inaccurate" and found this article on identifying bad thermocouples and preventative maintenance. I never thought of a lot of the tips in the article, but I haven't had such drastic temperature errors either.