Bed Thermistor - readings are off, im at a loss.

Struggling to get the bed heater temperature to read correctly on the Ramps 1.4.. I bought brand new 100k NTC 3950 thermistors on Amazon (since I used old spare ones and couldn't figure out what they were). I have it programmed in Repetier 0.92.9 correctly as "100k NTC 3950". It still shows the temperatures way way off. Like this:
60c displayed on the LCD is = 105c measured by a laser thermometer
55c displayed on the LCD is = 82c measured by a laser thermometer
50c displayed on the LCD is =65c measured by a laser thermometer
45c displayed on the LCD is = 54c measured by a laser thermometer
40c displayed on the LCD is = 46c measured by a laser thermometer

The printer originally had a ferrite core (small black magnetic clamp) which was clamped on the heater wires and the thermistor wires.

When the NEW thermistor wire is in the ferrite core with the heater wires,, the readings are insane, like 900c. When I take the wire out of the ferrite core it settles down to about 20c which is room temperature in that room. So its causing some variations in the readings for sure.

Any ideas? I'm at a loss. Printer was originally a XYX Davinci 1.0a that now has the Ramps 1.4 board

Your touching the thermistor wires on the ferrite core... you get a high temp when resistance is lower, ie when reading the iron in ferrite core instead of the thermistor

Thanks for the reply! I noticed that when it was in the ferrite core. I thought it was strange it was originally in the core from the factory.

It's separated from the core now and the readings are still a bit off. The thermistor is sandwiched in-between a fiberglass insulation mat and the heating pad. Im curious if the thermistor is having a hard time reading it since its hot on the heating pad side but not the fiberglass side so its off. Im going to try to add thermal compound and some kapton tape on it to thermally insulate it directly to the heated bed. I'll report back

I forgot to report back!

Problem is solved.

Basically I had the thermistor sitting ontop of fiberglass insulation and the heated pad above it. The thermistor was reading the difference between the two and my readings were off.

My solution? I added thermal grease to the thermistor and taped it to the heting pad with thermally insulating Kapton tape.

Just like that, the numbers were spot on!

Edited 1 time(s). Last edit at 06/03/2020 02:13PM by Proxlamus.