Auto Bed Leveling troubles

[printbus]

Agreed. It is kind of like when I was younger, my mother would always set the oven 5 or 10 degrees hotter because that was what was necessary to get things to bake right. You just knew that was the case and adjusted things.

Exactly. BTW, if that was an oven with the old mechanical type knob, there would have likely been a screw on the back of the knob that allowed the knob dial to be adjusted for calibration. I offered to set my mom's dial once, but she refused since it would have thrown off everything she knew about baking in that oven.

Is there a defined testing methodology described somewhere? Hell, I've got a fluke multimeter with a thermocouple somewhere . . . I'm not saying for everyone to calibrate their own, but i would be happy to publish my Epcos clone thermistor and makerfarm 8" bed results. control a few variables like ambient temp and top surface of bed/type of glass and it should be close enough.

There probably is, but it's likely buried in blogs or the reprap wiki. And you'd have to start with the fundamental question about whether the thermistortables file is meant to capture theoretical temperature measurement data for just the thermistor by itself, or is meant to include compensation for real-world losses affecting the net temperature gain from the heaters involved.

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I couldn't resist the temptation and spent some time digging into the data, starting with truth data direct from TDK/EPCOS. This was quite interesting. The amount of raw data is a bit overwhelming. If I think of a way to compile the info, I'll start another thread to share it. Here, however, is a summary.

The type 1 data in the thermistortables file closely follows the resistance-vs-temperature data provided by EPCOS, after converting for the simple voltage-divider approach feeding a 10-bit analog to digital conversion in the Arduino Mega processor. This suggests that the thermistortables data is intended to reflect data for just the thermistor by itself, not factoring for any real-world effects on the measurement. I'm not saying this is particularly good or bad - it's just the approach being used.

As Roxy pointed out, the type 6 data IN THE MARLIN BASELINE differs considerably for the higher end of the temperature curve. My conjecture is that Type 6 was originally someone's attempt at coming up with a set of data that did reflect real-world effects on the measurement. This is supported by the comment in configuration.h for Type 6 where it says "created using a fluke thermocouple".

And then there's the type 6 thermistortable data provided by MakerFarm. It turns out that this data is very close to being the same curve data as for Type 1. Here I conjecture that MakerFarm (or their firmware source) started using the Marlin type 6 data and found it undesirable. For whatever reason, the type 6 thermistortable data was altered instead of just changing configuration.h to specify use of the type1 data. Comments in the MakerFarm configuration.h file that imply the type 6 thermistor data is not as accurate as type 1 are wrong, since the comments refer to thermistortable data that has been changed in the MakerFarm baseline.

I doubt anyone would notice the difference between using Type 1, or using Type 6 as long as the MakerFarm thermistortable alterations go along with it. I would NOT, however, pull in the Marlin thermistortables.h file and stick with the Type 6 setting in configuration.h. That'll give you some wonky temperature results, as Pronus indicated earlier. You'd still be able to make it work - your "oven knob" would just be off more than it should be and it might take you a while to get used to adjusting the temps correctly. I can't help but wonder how many people have unknowingly struggled with that.

FOLLOWUP COMMENT: The compilation of data is now available here - http://3dprintboard.com/showthread.p...rtables-h-data