strange PT100 RTD MAX31685 freezing

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ToniSniff
 
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Joined: Mon Jul 15, 2019 5:24 am

strange PT100 RTD MAX31685 freezing

Post by ToniSniff »

Hi everyone,

this is a strange error I experience with several of your PT100 RTD MAX31865 boards

The Problem:
From time to time the temperature (and resistance) readings freezes and returns identical values up tp the last digit. After some time, the reading comes back to life and jumps the where it should have been - see the plateaus of the blue curve in below plot in an otherwise monotonous cooling trend - with no apparent mismatch when extrapolating the previous behaviour.

From what I can tell, freezing and recovery takes place with no apparent external trigger by itself. However, un- and re-plugging the board typically removes the freezing.

ada.png
ada.png (86.26 KiB) Viewed 108 times
The setup:
There is a total of 9 RTD boads hooked up the the same Raspberry Pi 4. All share the same 3.3V source and GND. Sensing is done via SPI in star topology. PT100 are connected in 4-point geomety. Mounting holes are enlarged to about the outer diameter of the metallic ring. All boards are screwed to an anodized Al rail with a stripe of sticky tape on the bottom side to prevent unwanted shortcuts and crosstalk across the rail.

The core python code is basically identical with the Adafruit repo. All sensors are read in series once a minute (but sampling rates don´t seem to have any influence)

As far as I can tell, the freezing problem appears with all of the boards. Next to the blue curve I´ve plotted the temperature reading from another RTD board. Also here freezing appears, less prominently though, but there´s no apparent correlation to the blue freezing.

I´ve been using this kind of board for quite some time and I´ve never experienced (or recognized) a problem like this before. So I guess it originates somewhere in the comparably large number of boards in parallel, but otherwise I am out of ideas

I am desperate, any help is highly appreciated!!

Toni

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