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 haven't found any online reference to other people installing a custom Repetier build on a Wanhao I3.
- I have no idea what values to use for the temp conversion table.
- I can't even find anyone else reporting my issue.
- I'm fairly new to this; I rather not brick my printer, and I haven't found any good guides to installing firmware on the device either.
- I'm not convinced this is the right option. Should I be looking at replacing the thermistor instead? And if so, what's the correct part?
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.