Second thermistor for bed



  • I am trying to set up a secondary temperature reading for my heated bed. At this point I am only interested in seeing a display of the second bed temperature in the temperature graph in the web display.
    I have connected the thermistor to E2 on the Duex5 board

    I have inserted this line in config.g:
    M305 P103 T100000 B3950 C0 R4700 S"TEMP2" X3

    This was derived from another posting where DC42 was trying to assist somebody else doing a similar thing.

    As far as I understand, I named this sensor "Temp2", the X3 says it's on the Duex board.

    I have configured the web display to give me a graph of this second sensor but it is about 10 degrees below the primary sensor. Both sensors use the same B, T and R values. Do I just tweak the B value until the temperatures match ?
    I must confess that I have some basic lack of understanding here which likely doesn't help. I would appreciate someone explaining to me what exactly a 'virtual' heater or sensor is.
    I would also like a bit of background understanding on why I am using P103 and X3. Which part says that I am using the E2 connector on the Duex board ? Why do I need both P103 and X3?

    Thanks for any and all help offered!



  • "Virtual" Heaters are a side-effect of the fact that Duet/RepRap V2 firmware cannot be configured to have a temperature sensor separate from a heater.

    This was a design decision, or probably more fair to say somewhere between an unintended behavior and a blind spot, when the firmware was originally designed.

    RepRap firmware V3 allows many kinds of sensors, including temperature, to be defined. Independent of anything else. V3 is still in Beta, fairly early Beta, and I wouldn't go to it for just this reason (yet).



  • RE: 10 degrees different. Two very likely causes:

    First, most temp sensors are on the bottom of the heater, which is in turn, on the bottom of the "heat spreader" (thick alu plate) of the bed. Spots on the bottom of the heater can be a few degrees off from each other. The "spreader" evens this out, but only as the heat works its way upward; the bottom remains uneven.

    Second, Thermistors are CHEAP and are only accurate to +/- a few degrees.

    What to do about it? Take a look at the bed with an IR thermometer and make sure it is fairly even. Then, either don't worry about it, or play around with several thermistors and micro-placement. And corrections in the config, if you wish.



  • Thanks for posting. Yes, I was aware that V3 requires more configuration with the benefit of more flexibility. I will eventually upgrade to V3 but not until it comes out of beta.
    Let me see if I have this right ..... since I can't configure a sensor without a heater, I use a 'virtual' heater. The sensor is not a 'virtual' sensor, just the heater.
    I think that makes sense .... thanks !



  • The original/primary sensor is built into the heat bed and the secondary sensor is placed on the underside of the bed so the temperature difference makes sense to some degree (no pun intended). The part that bothers me is that the temperature difference stays well after the heater has stabilized.
    The secondary sensor is maybe 3 inches away from the primary sensor so yes, it makes sense that there is some difference. It could also be that the primary sensor is closed to a heater wire than the secondary so it makes sense again.
    If I play with the beta value of a thermistor, what does that effect - does it affect high end temperature, low end temperature or all temperatures? Does it affect anything else ?
    I wouldn't mind tweaking both beta's to get them closer together but only if it isn't going to cause all kinds of weird side affects.



  • BTW, in the inactive state at room temperature, both sensors report the same temperature (well very close anyway)


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