Temperature offset feature.

[rsilvers129]

My thermistor is under the heated platform, and currently a thermistor reading of 115C is an IR reading of 96C (even though they agree when everything is at room temp).

Can we have a way to have a correction factor?

[foosel]

Something like that would only influence the temperature display though, it wouldn't in any way change how your printer firmware handles temperature control. I wonder if that really is worth the hassle. Once you figure out the correct temperature to use firmware-side for your printing material, you wouldn't really care that the surface temperature reads like 20°C below that, as long as the print succeeds, or would you?

[mrongen]

On reprap based systems you can tune the thermistor values set in the firware via the M301-M304 commands (at least for the Marlin firmware). (Dont forget to save the settings via M500.)

[rsilvers129]

That is very useful. Thank you.

When I set the temp to 110C, I get 105C on the top of masking tape via IR.

What exact command will correct this, and what command will save it to firmware?

[mrongen]

A couple of thoughts on this:

1. The IR assumes perfect black body radiation. For non metallic materials the true temperature will usually be a couple of percent higher (due to the reduced emissivity). So your thermistor might actually work perfectly well.
2. Especially for the print-bed the absolute temperature is not all that important. Just find a value that works for you and go with it.

If you still want to "correct" your firmware, you will need the thermistor resistivity at 25°, the value of the series resistor and the beta value (see Steinhart-Hart-equation, this can usually be found in the datasheet). You can then adjust the firmware via M304 Hh Bb Rr Tt, H0=Bed, H1..n=nozzle, b=thermistor beta value, r=series resistor, t=thermistor resistance as 25C and store the settings to the firmware via M500 (commands may differ for your firmare, this is Marlin).

If you cant find the values anywhere, you can read the currently stored values via the M503 command and modify beta slightly until you are satisfied.

[rsilvers129]

I put on some masking tape to avoid the emissivity problems of the aluminum.

M503 sends back to me:

Send: M503
Recv: echo:Steps per unit:
Recv: echo: M92 X63.36 Y63.36 Z2272.72 E590.00
Recv: echo:Maximum feedrates (mm/s):
Recv: echo: M203 X60.00 Y60.00 Z2.00 E14.00
Recv: echo:Maximum Acceleration (mm/s2):
Recv: echo: M201 X2000 Y2000 Z30 E10000
Recv: echo:Acceleration: S=acceleration, T=retract acceleration
Recv: echo: M204 S3000.00 T3000.00
Recv: echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum xY jerk (mm/s), Z=maximum Z jerk (mm/s)
Recv: echo: M205 S0.00 T0.00 B20000 X20.00 Z0.40 E5.00
Recv: echo:PID settings:
Recv: echo: M301 P22.20 I1.08 D114.00
Recv: ok
Send: M105
Recv: ok T:32.14 B:109.18 @:0

[mrongen]

That list seems a little short. What machine, firmware are you using?

[rsilvers129]

Printrbot Simple with a Printrboard. Firmware is whatever Marlin they use. Nothing unusual.

[imitation]

I've got my thermistor on the bottom as well, with a 0.5mm aluminum sheet and 3mm glass on top of the bed. When i measure the top layer of Kapton tape, i get pretty much the temperature my IR thermometer tells me.
What i'd do to eliminate the offset would be modifying the thermistors.h directly.
On a related note, can this be closed since it's something the firmware should do and not Octoprint?