Fluctuation in indicated temp - thermistor failure ?



  • For some time now I am seeing rapid fluctuations in indicated bed temperature. The fluctuations are too rapid to reflect actual temperature changes. Originally I thought this might be related to a loose connection someplace but it only occursScreenshot from 2019-10-25 20-13-11.png during heat up and cool down periods. The temperature graph is totally rock solid during printing.
    The blue trace is the bed temperature.
    Am I seeing the beginning of a thermistor failure or are there other things that may be causing this kind of behaviour ?



  • I’d wager the thermistor is lose if it isn’t directly touching the bed. Also keep in mind that electrical noise can cause similar fluctuations. Check the wire path and make sure the thermistor is tightly adhered to the bed and not loose.

    But then again, thermistors are literally a dime a dozen in some cases, so replacing it anyway is probably a good move.



  • @TLAS , the thermistor is integral to the bed heater and can't come loose.
    The thermistor wire runs in the same wire loom as the 120V AC bed power and noise was one of my thoughts but in the graph I included you can see that the problem also occurs when power is shut off (during cool-down). I would also think that the issue, if noise related, would be present once the bed is at operating temperature and being maintained by the PWM 'd AC power.
    I am currently waiting for some high temp tape to tape a replacement thermistor onto the heater but would really like to understand why this is happening.



  • @jens55

    Also do you get any noise prior to cooldown? Could be a fan that is blowing directly across the thermistor cooling it faster intermittently or where the thermistor solder might have cracked and you have the intermittent connection as thermal expansion and contraction occur.

    Also, if it’s a surface mounted one, there could also be a chance that you have broken or semi-broken trace in the board.

    What testing equipment do you have? You could always use a heat gun or soldering iron over specific areas to do a bit more testing.



  • @TLAS said in Fluctuation in indicated temp - thermistor failure ?:

    @jens55

    Also do you get any noise prior to cooldown? Could be a fan that is blowing directly across the thermistor cooling it faster intermittently or where the thermistor solder might have cracked and you have the intermittent connection as thermal expansion and contraction occur.

    The noise happens only during heatup and cooldown. There is no fan involved as we are talking bed heater. I have never seen solder used on a thermistor (not suitable for hot service) but I will double check.

    Also, if it’s a surface mounted one, there could also be a chance that you have broken or semi-broken trace in the board.

    I believe - but can't be sure, that it is just a bead inserted into the bed heater and covered with a silicone patch.

    What testing equipment do you have? You could always use a heat gun or soldering iron over specific areas to do a bit more testing.

    I have access to pretty much anything in terms of test equipment. The heat gun is a great idea ! I will give that a try and see what I get!



  • FYI thermistors come in essentially two types surface mounted resistors and glass bead. Glass bead is essentially still a surface mounted resistor just one that has two small wires already attached and is covered in glass.

    If you use a glass bead, there will always be imperfect contact with the bed as a) only the edge of the round glass has direct contact and 2) the glass bead is usually adhered with tape or adhesive. If the adhesive comes loose or creates an air path, it can create contact problems.

    Generally a surface mounted thermistor is much more reliable because it is directly soldered to the board.

    I suspect in your case that the thermistor is loose or if it’s a glass bead type, that one of the wires is having problems contacting when a thermal gradient exists (heating / cooling).



  • Ok, I think my next plan of action is to attach a different thermistor once my high temp adhesive tape arrives.
    Thanks for your help !


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