Incorrect temp values
JL last edited by
I'm building a CoreXY printer and for the hotend and bed I use two different K-Type thermocouples connected to a MAX31856 thermocouple dautgher board connected to a duet3 6HC main board, latest firmware on everything. So far the printer is up an running, but I noticed that on both sensors the temp is not correct. E.g. I set the bed temp to 60°C and it heats up and hold the temp correctly, and the displayed value in DWC is 60°C, but the actual temp in my bed is 50°C measured with an external thermocouple connected to a multimeter. Same for the hotend, Temp in DWC is 220°C, measured is 190°C.
After checking and changing the thermocouples without finding a problem I measured the temp readings with a multimeter directly at the connection to the MAX31856 PCB. 50°C on bed measured with external sensor, 50°C measured with bed sensor and mulitmeter, but still DWC shows me 60°C when the bed sensor is reconnected to the PCB. I also exchanged the thermocouple daughter board, but still the same error.
I noticed, that the error is larger at higher temperatures, seems linear to me. So I tried to change the sensor type from K to N in the config, but oddly that didn't change the temp readings in DWC at all, no matter what type of sensor I configured.
Right know the config for the sensors is
M308 S0 P"spi.cs0" Y"thermocouple-max31856" A"Bed" ; define temperature sensor number 1 as a Thermocouple Type K on the first port of a temperature daughter board
M308 S1 P"spi.cs1" Y"thermocouple-max31856" A"Tool 0" ; configure sensor 1 (S1) as thermocouple via CS pin spi.cs1
So, any ideas on how to solve that mystery
@JL while type K is the default, you can specify type with the K parameter in M308. Otherwise, it generally comes down to wiring. See the troubleshooting section here, which also includes a check without wiring: https://duet3d.dozuki.com/Wiki/Connecting_thermocouples#Section_Troubleshooting