Bed temp reading too low if using long cables

paboman

I need to use long cables for the bed termistor (NTC100k) and the readings on the duet are 10°C lower compared to what I get when measuring with a thermocouple or IR thermometer.

Ho can I compensate for higher resistance due to longer cables ?

;Bed
M301 H0 S1.00 P10 I0.1 D200 T0.4 W180 B30                               ; Use PID on bed heater (may require further tuning)
M305 P0 T100000 B3950 C0 R4700                                		  ; ADC parameters for heater 0 T= 100000 is ohms at 25 celsius B= Beta(was 4725) C= Steinhart-Hart C coefficient (was 7.060000e-8), default 0 R= Series resistor is 4700 for Duet
M143 H0 S120                                                            ; Set temperature limit for heater 0 to 120C

thanks!

dc42

It's unlikely that the cable length is affecting the thermistor reading, the cables would have to be several miles long to do that. You probably have either an inaccurate thermistor or some leakage between the bed heater circuit and the thermistor circuit.

paboman

ok, so will play with the beta values to find something that work well for me in the 50/80 range
What is the purpose of the C value in M305 command ?

dc42

The C value allows RRF to model the variation of resistance with temperature more accurately, for better temperature reading accuracy. Thermistor manufacturers rarely quote the C value, but some publish resistance vs. temperature tables, from which it can be computed.