Incorrect readings of the PT100 sensor

  • Everything was in order, but one day the PT100 sensor (Heater 1) of the 3D printer began to show the wrong temperature:

    0_1558475576732_Screenshot from 2019-05-21 22-51-00.png

    Can you please tell me how to fix this problem?

  • Maybe this sounds a little silly, but you're sure its not correct? A fault somewhere could lead to excessive current flowing through the sensor turning into a heater.

    If it is wrong then config and wiring needs a checkup. Try moving it to another input. Measure the resistance of the sensor if you have a multimeter.

  • @bearer It's weird, but now the temperature has dropped ...
    (I just turned off the printer, and turned it on after an hour.)

    0_1558481925532_Screenshot from 2019-05-22 00-34-35.png

  • administrators

    If PT100 channels show the temperature higher than you expect, but not a spike to 2000C, then this usually means you have a bad connection in the PT100 wiring. In the 2-wire configuration, each additional ohm of wiring resistance increases the reading by around 2.5C.

  • @dc42 Does this mean that you recommend changing the wires to thicker ones?

  • No need to change the wires unless they are unusually thin or long. Just measure the resistance if in doubt, or calculate from wire thickness and length if you don't have a meter. If they cause an offset it can be compensated in the M305 config, but for common setups it should be negligible.

    The thickness of the wires are fixed and as such not responsible for the temporary increase in temperature.

  • @bearer In that case, what do you think, why did my sensor "double" the temperature? And what can I do to avoid this in the future?

  • Pointless speculation beyond the suggestions offered initially to try and isolate the cause. Intermittent faults are inherently difficult to track down. Faulty wiring somewhere is the best bet.

  • @vapvap said in Incorrect readings of the PT100 sensor:

    @dc42 Does this mean that you recommend changing the wires to thicker ones?

    I think IF you would need to change something on the wires AFTER you have measured and posted here the resistance of your 2-wire-setup... I think 4-wire-setup would/could help you more than thicker wires...

    since the 4-wire-measurement-setup seems to have been invented (on that island with those strange brexit-people... just kidding) to cancel out resistance of wires, in a 4-wire-setup the thickness is for short distances and for this application neglectable -> thicker wires could help of course in a 2-wire setup if you are too lazy to have your measured resistance calced out of the way by the duet2... that written here I am one of those 4-wire guys (not from the beginning)
    All of this is of course only true if your wires are not infinte thin with infinite resistance... 🙂

  • administrators

    @vapvap said in Incorrect readings of the PT100 sensor:

    @bearer In that case, what do you think, why did my sensor "double" the temperature?

    Probably because there is a bad connection in one of the connectors in your PT100 wiring. Most likely a bad crimp.

  • @dc42
    I changed the wires to those that were bundled with the PT100 sensor. But the result was even worse (temperature difference ~ 12°C).
    Tell me please, what should be the correct resistance in the wires?

  • administrators

    The PT100 should have a resistance of 100 ohms at 0C and about 108 ohms at a typical room temperature. if you use a 2-wire connection then each 1 ohm of resistance in the wires or connectors will cause the reading to increase by about 2.5C.

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