I have a Monoprice Maker Select, which is a rebranded Wanhao Duplicator I3 V2 (Prusa clone). I've found that the heated bed temperature values on the LCD are incorrect. The heater works, and the controller maintains the bed temp just fine, but the temperature reported isn't the true temperature. When using the PLA preset, with the bed set for 60°C, once the temperature stabilizes at 60°C on the display, I can measure it with a laser thermometer and get a consistent 54°C across the buildplate. With the ABS preset of 90°C, I read 80°C. And with the bed set for the maximum value of 120°C, I'm only reading 102-104°C.
I've checked these values with the bare aluminum build plate, and I've allowed the temps to stabilize for at least 10 minutes to ensure that I have consistent readings. I believe the the firmware is using the wrong temperature curve for the thermistor in my device.
I had hoped that there was a simple scaling constant that I could adjust, but that doesn't seem to be the case. I've researched enough to learn that my device is running Repetier 0.91 firmware, and this page on temperature control on the Repetier wiki says that each type of thermistor needs a custom voltage->temp lookup table, and talks about building a custom table in "configuration.h". I've tried to follow this line of research but I feel like I'm going around in circles. It sounds like I could build a custom version of the firmware to install on my printer, and in doing so I could potentially specify a better conversion table. This gives me pause for several reasons:
I've also looked into the Melzi board inside the printer hoping for a variable resistor to tweak the thermistor voltage divider, but no such luck, at least in the schematics.
I could just live with it; I've been using a 67°C setting to achieve a true 60°C bed temp for PLA, but I'm starting to work with ABS and I'd like the option to get above 100°C bed temp. Am I on the right track? Suggestions? I'm still pretty new to the device and 3D printing in general, so I may have overlooked something obvious.
Update: additional question in light of Tom's answer: what is the expected max actual bed temp achievable on a Wanhao/Monoprice I3? I'm measuring 100°C with an LCD reading of 120°C, but I'd like to get to 110°C if possible.
It is completely normal for the surface of the bed to be cooler than the indicated temperature. The thermistor goes on the underside of the bed, near the heating traces. The top of the bed (which is further away from the heating traces) will naturally be cooler.
It would be possible, though a lot of work, to build a custom thermistor table that more accurately reflects the surface of the bed, though that means your thermistor table will no longer reflect the characteristics of your thermistor, but instead reflect the particular circumstances under which you build your custom table (which would be affected by ambient temperature, any drafts, ...).
The accuracy of an infrared laser thermometer depends on the characteristics of the surface you're measuring for, so unless you adjusted the thermometer specifically to measure the aluminium surface of the bed, it's possible your thermometer is off as well.
The exact temperature of your bed doesn't matter anyways (you just need it approximately in the right ballpark) so I would suggest to just live with it. The 60C/90C figures are not supposed to be for the surface of the bed. They're meant to reflect the temperature reported by the printer. That said, 90C is a bit low for printing ABS (but the bed/power supply on your printer might not be powerful enough to get any higher).
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