How to modify MAX31865 temperature readings?

[0scar]

Basic Information:

I’m using a Mosquito Magnum+® - 2.85 mm with 2 PT1000 sensors on an Octopus Pro v1.1 controller board.

One RTD is connected to the internal MAX31865, the other to an external MAX31865 board (Adafruit clone).

Issue:

The sensor connected to the external board shows a consistent averaged 5.4 % higher temperature than the one connected to the internal MAX31865:

If I could multiply the readings with a fixed value, the readings would be exactly the same.

How can I adjust the temperature readings from the second RTD by multiplying with a constant to match the output of the first RTD?

[EddyMI3D]

Hello @0scar !

Are the sensors at the exact same spot?

[0scar]

No, but the hotend is symmetrical and 2 identical heaters are used.

Nevertheless, this isn’t a problem for the question, the question is how can I edit the values of one RTD to match the other. My measurements already conclude there is a very good relation between both.

I’m not seeking an answer why they differ, I’m seeking an answer to make them the same.

[EddyMI3D]

@0scar

I’m not seeking an answer why they differ, I’m seeking an answer to make them the same.

And how do you know what one is the correct one?

[0scar]

I have a separate PT100 to measure it, but again, this is not relevant to the question.

Temperatures are all irrelevant to be honest, it is what you schedule in the slicer what is your temperature, that doesn’t necessarily need to be the actual temperature, filaments have temperature ranges, it is you who tunes the hot end with a temperature tower to find the “temperature” the printer prints well with.

Please explain me how I can adjust the second one rather than delving into the difference or the correct sensor.

[EddyMI3D]

@0scar

Temperatures are all irrelevant

Are that sure?

The readings are not for observation, they are for setting the hotend to the proper temperature for the used material. A few degrees can decide if there is stringing or clogging. The reading should be quite precise. Not be ironed down to a proper view.

@0scar

Please explain me how I can adjust the second one rather than delving into the difference or the correct sensor.

Scaling temperature readings

I measured the offset between bed thermistor and print surface at various temperatures so I can define a linear or parabolic formula to scale the readings to obtain the “real” temperature. Clearly the goal is to have the set temperature to also be the real one, without having to remember the offset, even more so when multiple printers are in use and each one has a different offset. However, it’s not clear to me, after reading the docs about thermistors, how to do it, if it’s even possible. Ca…

[Sineos]

@0scar

Please explain me how I can adjust the second one rather than delving into the difference or the correct sensor.

Simple: you cannot.
But as you said:

@0scar

Temperatures are all irrelevant to be honest.

Here I agree, given you do not rely on any pre-defined values but dial in your filament yourself.

And on this hot-end, it is even more irrelevant, since the second RTD is intended as a redundant fail-safe, and this is something Klipper does not support anyway.