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I'm using a b 31855k breakout board (a chinese one at that) with the olimex A20 and a K-type thermocouple (which works OK on another device with the 31855k).

The problem is, that the thermocouple temp reading fluctuates wildly (up to +- 10deg C) and slowly drifts into negative values. Communication with the IC via SPI is ok, because the cold junction temp is correct.

So, for example, once I start the reading after the board has been off for a while, everything seems fine. The thermocouple temp is the same as cold junction around 25 deg C(+- 2 deg C, but that is expected). It then slowly but surely drifts downwards and after 5- 10 minutes it reads max 20 deg C - basically fluctuating around 17 deg C. This is then an "unstable" situation. The temp reading remains at those values as long as the thermocouple is not heated or cooled. Once that happens (I've used a hairdryer and my freezer) the thing goes haywire and starts drifting into negative temperatures at room temperature in a room where there are no drafts or air currents. It also starts reading -10 deg C and +10 deg C within the span of half a second, etc ...

Any ideas what could be the issue? The breakout board has a capacitor for the max31855 to stabilize VCC. I'm betting on the 31855k being a cheap Chinese knock off, and if it is only that, it's good.

If it's not however, I have no clue what to do. Any tips would be appreciated.

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    \$\begingroup\$ Please link a data sheet of the (ahem) chinese breakout board or read what it says about performance then check to see if the original manufacturer has a recognized quality policy and while you are at it, can you find other reports from disgruntled customers who also bought this er (ahem) device? \$\endgroup\$
    – Andy aka
    Commented Oct 25, 2020 at 20:45
  • \$\begingroup\$ Did you place 10nF ceramic across T+,T- and 0.1uF across IC? \$\endgroup\$
    – D.A.S.
    Commented Oct 25, 2020 at 21:05
  • \$\begingroup\$ @Andyaka Well, the breakout board is just the reference design for the max38155 (datasheets.maximintegrated.com/en/ds/MAX31855.pdf) - so a 0.1uF between VCC and GND and connectors for all the IC pins. I've bought the boards from ebay - with thousands of feedback for "everything". It is either that the chip itself is defective, or that it is a bad bootleg. I have a couple more to test - at work, but I am currently in quarantine so working with what I have at hand. \$\endgroup\$
    – drkrek
    Commented Oct 26, 2020 at 12:48
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 There is a 0.1uF accross the IC, but the 10nF ceramic is not present. I will try that, thanks! \$\endgroup\$
    – drkrek
    Commented Oct 26, 2020 at 12:50
  • \$\begingroup\$ That will make a big improveent on noisy readings and it’s in the data sheet! How did you miss that? \$\endgroup\$
    – D.A.S.
    Commented Oct 26, 2020 at 23:50

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Turns out it was indeed the lack of the 10nF capacitor across the T+ and T- pins. With the capacitor added, the temperature measurement is "rock solid".

Reading the datasheet, I mixed up the 0.1uF and 10nF capacitors and assumed (yeah, mother of all f...) it is the same thing.

Hopefully others do not make this mistake :)

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