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.
Best Answer
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 :)