Documentation; add `inline_resistor` to PT1000 section?

Developers
1

Octopus needs inline_resistor: 4740 but Configuration reference - Klipper documentation doesnt list it as an (pt1000) config option, easy to think its not valid (for pt1000) as it is listed for thermistor

2

I would not be aware that you can connect a PT1000 to the BTT Octopus board.
Are you talking about the BTT MAX31865 V1 or V2 board?

3

you can connect pt1000 to any board; some just work better in terms of accuracy and or precision. Octopus uses 0.1% pull up and series resistance to get both(ish).

But my post isnt Octopus specific - I just came across the docs only showing the inline_resistor in the regular thermistor section and not the pt1000 section while it is valid in both it seems.

4

Although you can connect a PT1000, it hardly makes any sense IMHO. The measured voltage drop across a PT1000 is only 1/10 of the expected value. In combination with the needed ADC the result probably is questionable.

Iā€™d guess this is the reason why there is no inline_resistor setting for the PT1000. Though in theory you are right, the same logic could be applied as for the regular thermistors.

5

One is NTC the other is PTC, so at higher temps that difference becomes smaller (1/5th at 300C); reducing the pull up resistance or adding a series resistance has pretty much the same effect on the ADC resolution in terms of degrees C pr bit.

And not to mention, the parameter actually does work for PT1000, and is probably needed on Duet boards as well as Octopus (and possibly others); but the way the parameter is shown in the current documentation it appears to not be valid for PT1000 which is unfortunate imho.

I didnt intend for a discussion on the merits of PT1000; got that bit covered.

1 Like
6

This would be a great change to submit as a GitHub pull request, perhaps including some of the caveats discussed here.