M305 Steinhart-Hart thermistor C parameter help?

robm

I have a 250mm square aluminium bed with a 24V silicone heater stuck under it.
I fasten a K thermocouple lead to the top centre of the aluminium bed with blue painters tape, and set a steel plate (old bed) on top of that.
I get the temperature up over 100C, turn off the DuetWifi (fw 1.21), pull the thermistor lead and monitor the resistance as the temperature drops. As my thermocouple only reports in integer degrees C, I average the two R values from when the reading transitions in and out of a given value.

I usually set the bed to 65C for PETG but occasionally print ABS with the bed at 100C, so I collect the following values:

T : R
25 : 92800
100 : 5010
65 : 19220

I plug these into this calculator (subsequently verified with this one) in the order shown above (because the beta part of the calculator uses the first two and not the first and last).

The beta and S-H model curves are similar, and I can plug other R measurements in and get T values in the Model Calculator window reasonably close to my readings.

I generate this M305 line:

M305 P0 S"bed" T92800 B4330 C5.247675313E-7 R4700

and finish (M303/307) tuning.

Now when I set the temperature though, I find the DWC reported temperatures are 20-30 degrees higher than the thermocouple on the bed reports (the bed is not as hot as the Duet reports). If I remove the C parameter, the DWC readings jump to the correct value.

I've tried e.g. C5.24E-7 and C0.000000525 with no effect. I can also get the correct DWC readings changing to E-8 (e.g. C5.24E-8) but I don't know if this is just because it is closer to C0.

What am I missing?

dc42

You don't normally need to use the C parameter for the bed thermistor because the temperature range is small enough that the B model works well enough.
If you change from using the beta model to the Steinhart-Hart model (or vice versa), you must change the B parameter. Adding the C parameter makes a substantial difference to the correct B parameter.
If the bed is glass then expect a thermocouple reading to be much lower than the thermistor reading. There will be a temperature drop of typically 5 to 10C across the glass. The contact area between the thermocouple and the glass will be small and the thermocouple wires will conduct heat away from the thermocouple junction better than the glass can supply heat.

robm

@dc42 said in M305 Steinhart-Hart thermistor C parameter help?:

You don't normally need to use the C parameter for the bed thermistor because the temperature range is small enough that the B model works well enough.

Cool.

If you change from using the beta model to the Steinhart-Hart model (or vise versa), you must change the B parameter. Adding the C parameter makes a substantial difference to the correct B parameter.

Ah! That was it. I was misreading the intent of the M305 'B' documentation; have made a small tweak in case others read it as I did.

If the bed is glass

Interesting point, but inevitably bang my hotend tip on bed clips when I use that method of attachment - and can't think of any other way to do glass.

Please, someone in the know look at M305 docs parameter T"c" : "The thermistor type letter, default K" - is that supposed to be "thermocouple type"?

Thanks David!

dc42

@robm said in M305 Steinhart-Hart thermistor C parameter help?:

Please, someone in the know look at M305 docs parameter T"c" : "The thermistor type letter, default K" - is that supposed to be "thermocouple type"?

Thanks, I've corrected that.