PT100 is out by a few degrees

  • My PT100 sensor is giving a value slightly higher than the other 2 sensors in my enclosure. Actually the other 2 are exactly the same reading:

    PT100 shows at 22.7

    Chamber shows 20.8

    Bed temp shows 20.8

    My PT100 config line is set at: M305 P2 R4700 T100000 B3950

    What do I need to edit to get the PT100 value to be the same as the other 2?

  • That M305 line looks like it's for a 100K thermistor, not a PT100 sensor. Do you have a PT100 amp board installed? If so your line should look more similar to M305 P2 X200

  • I have myself confused now, here is the entire heater section for all 3 Sensors

    ; Heaters
    M307 H0 B0 S1.00 ; disable bang-bang mode for the bed heater and set PWM limit
    M305 P0 T100000 B4138 R4700 ; set thermistor + ADC parameters for heater 0
    M143 H0 S120 ; set temperature limit for heater 0 to 120C
    M305 P1 X200 ; PT100 on RTD1
    M143 H1 S280 ; set temperature limit for heater 1 to 280C
    M141 H2 ; Set H2 as Heater Chamber - No Heater yet
    M143 H2 S90 ; Set Max Chamber Temp at 100 Degrees
    M305 P2 R4700 T100000 B3950 ; heater 2 is monitored by a 100K thermistor with B=3950 and a 4.7K series resistor

  • Just in case you need to know my setup, I have the RTD PT100 Daughter board, with a 2 wire PT100 connected to RDT1 pin 2 and 3 on the Hotend. On the bed is the standard 100k Thermistor and the chamber is also a 100k Thermistor. I hope this helps.

  • @MichaelMD1978 said in PT100 is out by a few degrees:

    My PT100 config line is set at: M305 P2 R4700 T100000 B3950

    It is confusing as you said. Becuse this is not your PT100 config line, this is your chamber thermistor config line. Your PT100 config line is M305 P1 X200

  • @MichaelMD1978 It's more likely that the PT100 is correct and the two thermistors are different. Are you using the same type of thermistor for the bed and chamber, or are they different (you have different B values for each one - are they both correct)? Take a look at the merits of various sensor types here

  • Couple things to consider.

    1. PT100 sensors are more predictable and accurate than most thermistors. I would generally trust the PT sensor reading is more accurate. A caveat below:

    2. PT100 sensors are more sensitive. Particularly with a 2-wire setup, there is no way to compensate for the resistance of the wires and connectors. This won't make a huge difference unless the wires are tiny or long... for example a meter of 26awg wire would be a~0.15 ohms, which will push the temp reading up by ~0.5deg. If you have longer / smaller wires or inline connectors with a bit more resistance this will throw the PT reading out further. Electrical noise will also affect the PT100s to a greater degree. This can be compensated with a 3 or 4-wire pt100.

    3. Therms are not terribly accurate. The most common are +- 3% which comes out to around 0.8-1 degree at room temp generally. Your readings are only off by 1.9 degrees, so 1 degree low from therm variance, plus a half degree from pt100 extra resistance and there's most of your difference.

    A more likely scenario is that the thermistor values are not tuned 100% in. Therms have non-linear response so it's a bit tricky to get the temp readings to accurately track over the entire temp range from 0-300C (or whatever). If you mostly care about the readings matching, you could tweak the thermistor control values. A good approach here is to use a calibrated thermocouple or a known good sensor and build your own temp/resistance chart. There are a few online calculators which will generate SH values from temp/resistance values.

  • As mentioned above, a PT100 sensor is very sensitive to wiring resistance. To really get accuracy out of them, they are generally used with 4-wire connections. A 2 degree error on a PT100 is less than an ohm, which would be easy to find in wiring. Is the daughter board capable of 3-wire or 4-wire connections? If so, the PT100 would become your thermometer of reference, as it is very good over a very wide range.

    I put a PT1000 on my DuetWiFi for the hot end. I now trust my high-temperature readbacks to a degree or two. That is limited by the accuracy of the bias resistor in the readback bridge. With a PT1000, of course, wiring resistance is much less significant.

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