With the stock thermistor and my added thermocouple epoxied to the M2’s nozzle, I stepped the temperature upward, let it settle, and recorded the temperature from the Pronterface status display and my Fluke thermocouple meter:
Because the firmware servos the temperature through the stock thermistor, that line is dead straight at the exact setpoint values: the reference never disagrees with itself. The thermocouple, however, reads low by about 12%: according to it, the nozzle runs much cooler than the thermistor value.
Huh?
Several explanations come to mind:
- The firmware is using a lookup table that doesn’t match the thermistor
- The Fluke thermocouple meter reports the wrong value
- The thermocouple junction is defective
- Despite the epoxy blob, the two sensors aren’t at the same temperature
- Something else is kaflooie
This obviously calls for more data…
The Smell of Molten Projects in the Morning 
Comments
5 responses to “Makergear M2: Thermistor vs. Thermocouple”
Or the excitation voltage is off, being usually specced at +/- 10%. Or the divider/scaling resistors likewise, or or or or…
Don’t you have a noncontact IR thermometer for a third measurement? :)
The fundamental problem is that the thermistor table in the firmware has absolutely no identification of the thermistor and circuit; it’s just a bunch of ADC vs. temperature pairs. It’s “Table 1”, which suggests it’s been around for a while and certainly predates the RAMBo board’s circuitry, so I’m not convinced it’s appropriate for this thermistor.
Which I trust as far as I can throw it for anything that’s not a big, flat sheet of painted wood. It assumes 0.95 emissivity, which doesn’t match metallic surfaces, Kapton tape, or glass and the spot size at the muzzle is bigger than the M2’s entire hot end. It’d give me another number, that’s the only sure thing! [grin]
Not sure how to pull it off, but the boiling water calibration test sounds like a good one… Assuming a aluminum foil “pot” for the nozzle, (a close to conformal fit, I’d think) you should be able to get fairly close to your boiling water temp.[Note to self: look up water boiling point for 4300 feet.]
Make sure it is distilled water as well. You might as well do the zero point (freeze distilled water ice cubes and then put them in a bucket of cold distilled water and wait a bit for it to equalize). It’s relatively accurate and gives you two relatively accurate points for a graph function. Should at minimum give you more accuracy than the +/-30° you are seeing.
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