Thermisters not heating equally

Yeah, I’m working on a heated bed for my DIY 3D printer. Not using a hot pad because my print dimensions are 12" X 12".

So I’m going with the resistor technique.

9 resistors in series (3 ohm each) running off of household current. (About 4 amps 450W)

My Arduino will control the heating by reading thermistors and using a SSR.

So I did a test run today and found the following sad news. It appears that the resistors are not heating equally. I had thermistors on R1, R3, R5 and R9.

Take a look at the graph. Anyone have any ideas?

Power was applied at 00:00:36
Power was removed at 00:05:29

thermistor circuit.jpg

Unless you have perfectly matched power resistors, 10k resistors and thermistors, you will get differences.

What tolerance resistors are you using?

If you use 10% tolerance 10k resistors, you may have 9.5k on one and 10.5 on another. Then you have the difference in values for the thermistors.

If accuracy is required, you need accurate components in your measuring system.

You could start with 1% tolerance 10k resistors, put all thermistors on the same heat source, take the readings and work out a correction value for each.

But as you are using only one on/off power source, you cannot control each heater individually so why not simply take the average of all readings?