Auto Bed Leveling troubles

[printbus]

OK, maybe more rant. Perhaps someone finds it amusing.

I think the lesson here is to put more faith in your relative temperature results than in the absolute values. Pay more attention to the results you are getting and adjust accordingly than what others are using for temperature settings and what filament manufacturers offer as recommended temperatures.

Some day, I may revamp what I use in the thermistor tables based on data from a manufacturer for a thermistor I've purchased from a trusted component supplier. These are all typically 1% parts, and IMO 1% manufacturer specs are going to potentially be far more accurate than what some guy (like me) measured in his basement or home oven with some cheap IR thermometer or other questionable calibration scheme. Frankly, I'd rather want to see and be working with true measurement data (and likely throwing out everything I had previously learned about temperatures on my printer) than temperatures that have been biased in an attempt to compensate for heating and/or measurement errors, whatever the source. But even this gets challenging. I've noticed most reprap suppliers indicate they are providing EPCOS part number B57560G104F thermistors, since that's the reprap "standard". As best I can tell, TDK/EPCOS made that an obsolete part number years ago. So, are these suppliers "helping" by translating to the current part number behind the scenes for us? Or are they really obtaining their thermistors from some low cost "EPCOS compatible" source and the parts are really of unknown specs? And of course, the code comments only say the tables are for an EPCOS 100K NTC thermistor. Jeez - I think they have about a dozen different ones.

Finally, having done my own temperature-control designs, I had to sort of chuckle when I first saw the 1K or 4.7K pull-up voltage-divider approach used on the reprap boards. This simple approach sets up the possibility for measurement errors due to self-heating in the thermistor (current flowing through a resistance dissipates heat). It might work "OK" for the higher temperatures we're using in the printers, but can be unbelievably inaccurate at measuring lower free-air temperatures. Feel free to search on "thermistor self heating" for more info.