Black or white under glass for ir probe



  • Help
    I am getting erratic readings from DC42s ir sensor - the trigger height is stable for a short while and then changes by about 0.2-0.4mm and remains stable for a period it changes After a reboot - I change the G31 instruction in the config file after calibrating the trigger height and reboot and find the trigger height has changed but remains consistent but incorrect for printing until I reboot and calibration gives another trigger height which remains the same until another reboot.
    I have black cartridge paper under glass
    The instructions for the ir sensor say to use black backing under the glass.
    I had a modified Hephestos 2 running a duet and ir sensor and this worked well and the bed was glass with white underneath - this printed well but I wanted a larger bed and height and so I am setting up a CR-10s 400 with the same duet and sensor. I used black paper under glass as recommended and then my problem with variable trigger height started. This makes it impossible to print as the hot end either starts too low or high even when I set Z0 accurately with G92 Z0 before printing.
    Can anyone explain why I have this variation and should I use white backing under the bed? Also does the ambient light affect the sensor ie sunlight versus fluorescent light
    I have carefully cleaned the glass and use no coatings. The ir sensor is also firmly mounted with no possibility of it moving.

    Rob H


  • administrators

    My recommendation is to use black paper under the glass, to reduced the effect of any coating you put on the glass, But for quite a long time I had an unpainted aluminium plate under the glass on one of my printers, and it worked well (without a coating on the glass).

    If you don't use any coatings on the glass, then a white surface below the glass may also work well, but the trigger height will probably be lower. By all means try it.

    Direct bright sunlight (no clouds in front of the sun) reflecting off the bed into the sensor affect it, and when this happens either the probe reading will jump to 1000 or the LED will blink rapidly (depending on software version) to let you know there is a problem. Other light sources don't affect it.



  • Hi
    thanks for reply
    I now realise that the black paper is rough and sits on a 240 volt heating pad which is not flat - it also is thin and I think moulds to the heated pad leaving small voids in places. I will try thicker card and also a thin aluminium sheet under the paper.
    I cannot put the heated pad under the aluminium bed due to the mounting screws#

    Rob H


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