Okay, i thought i had a pretty good grasp on how thermistors work and behave, until today...
I've assembled my arcol.hu v4 hotend and went on to test the heater and thermistor.
Setting the thermistor in Marlin to one of the 100k defines (thought it doesn't matter much which for a first test, just wanted to see if connections are okay and thermistor is working), i got a reading of 68-70 deg C at ambient temperature of 30 deg C.
Okay, i thought, maybe the 2printbeta/arcol.hu guys screwed something up and packed a 10k thermistor...
So i reconfigure Marlin (instead of thinking it through - haven't had my coffee yet) and - lo and behold - i get a reading of 15 deg C.
Ooooh-kay... So it's somewhere between 10k and 100k?
I check the Arcol.hu wiki (http://wiki.arcol.hu/thermistor-100k
) and it rather clearly says it's a 100k 104GT-2 part.
Thinking maybe it's just faulty, i turn the heater on and set it to 100 deg C and temperature starts rising rather quickly, hitting the pain threshold at about 85-90 deg C. That's 45-50 deg C, which is more or less the expected value, meaning the reading is just linearly shifted up by about 40 deg C in that range. Or is it...
Okay, let's check the thermistor resistance... Here's where things start getting really weird for me.
I hook up a multimeter, set it to 20k range and the readout starts steadily climbing, from 0 kOhm to 15 kOhm, at which point i disconnected it, set it to the 200k range and reconnected it. And it shows - i shit you not - MINUS 30k rising towards 0k. So i think, perhaps it's an artifact of different voltages the multimeter is applying to the test leads at different ranges, or something (i don't know, don't look at me like that, i still haven't had my coffee). I reconnect the multimeter and set it to volts and measure a steady -20mV on the thermistor. Now the weird bit. I swap the leads, get +20mV, switch to ohms and get a normal reading.
As if that isn't making my morning bizarre enough, the reading i get is around 67 kOhms. Indicating i'm quite probably stuck with a 50k thermistor under 2cm of fire cement that i currently have no will nor intention of getting off the hotend.
I'm guessing it's probably a Semitec 503GT-2.
Discounting all the bizareness with measuring the resistance (polarized thermistors? first time i've heard of that) and assuming it is a 50k one, do you guys think that will have enough resolution at ABS temps? Furthermore, is there a convenient pre-compiled table for 50k thermistors out there or should i just generate my own with createTemperatureLookup.py?