Temperature fault at -4ºC / sensor for negative temperatures?

[pixelpieper]

For a university test setup I am trying to control a peltier element with a duet 2 maestro board which seems to work quite well with enabling the inversion in M307.

Unfortunately, when I reach about -4ºC the thermistor stops working and the printed temperature jumps to -273.1ºC, triggering a heater fault event. Is this a firmware limit or can I work around by using a PT1000 / PT100 / K-Type Thermocouple instead?

Relevant config.g snippet:

M308 S0 P"bedtemp" Y"thermistor" T100000 B3950 ; configure sensor 0 as thermistor on pin bedtemp
M950 H0 C"bedheat" T0                          ; create nozzle heater output on bedheat and map it to sensor 0
M307 H0 B0 I1 S1.00                            ; disable bang-bang mode for heater  and set PWM limit

[pixelpieper]

In case anyone ever runs into this: I replaced the 2k2 series resistance on one of the thermistor inputs with a 10k resistor which shifted the range down to about -19ºC. Make sure to configure the 10k resistance in your config.g using the R parameter of M308. This will likely limit the measurement range on the upper end (which isn't a concern for my application).

M308 S0 P"ctemp" Y"thermistor" T100000 B3950 R10000 ; configure sensor 0 as thermistor on pin bedtemp

[dc42]

@pixelpieper if you encounter this problem again, it can likely be resolved by calibrating the M308 H parameter. See https://docs.duet3d.com/en/User_manual/Connecting_hardware/Temperature_connecting_thermistors_PT1000#temperature-calibration-and-adc-tuning. However, for operating below room temperature I suggest using a 10K thermistor instead of a 100K one.

[pixelpieper]

@dc42 Its a bit of a tradeoff, I want to cover a range from -20ºC to 120ºC, have to compare whether a 10k thermistor would still work well on the upper end - thanks for the suggestion!