Does the MAX31855 thermocouple converter output accurate temperature values?

Question

I'm trying to figure out why my MAX31855 thermocouple converter appears to be outputting inaccurate temperature data. I'm curious if this is due to non-linearity in the converter. Link to data sheet: https://datasheets.maximintegrated.com/en/ds/MAX31855.pdf

The accuracy of the thermocouple seems to be okay around room temperature, however, it it becomes very inaccurate at higher temperatures. Around the 400-500F range, it can be off by +80F.

I'm somewhat familiar with the physics of thermocouples, but I'm definitely not an expert.

So my question: does the MAX31855 actually output accurate temperatures? The data sheet states that it outputs in units of 0.25 deg C. From this, I assumed that it would output accurate temperature data since it's using deg C as units.

However, I'm curious if this is a bad assumption. The FAQs page of the MAX31855 states that it does not linearize the results (link to FAQ page: https://www.maximintegrated.com/en/support/faqs/max31855-faq.html). This confuses me, why would it the datasheet say it outputs in units of 0.25 deg C if it's not actually linearized/valid?

Am I required to post-process the data by myself to get accurate temperature values?

Answer 1

Thermocouples are approximately linear (a few percent for many popular types) so it makes sense to specify the resolution in units of temperature.

Yes, you have to apply a lookup table, polynomial or other linearizing function if you want accurate temperature readings over a wide range. If you only care about some particular temperature a linear scaling may suffice.

Also make sure you have the correct type of thermocouple attached. The difference between J (Iron-Constantan) and K (Chromel-Alumel), for example, is about 20% of temperature difference from cold-junction. The cold junction compensation of this chip is a crude linear approximation of the temperature output of the thermocouple near room temperature, so it also won't work properly in general if you attach the wrong thermocouple type (though some are very close to each other such as T and K).

You don't specify the type of T/C but I suspect you may have a J T/C connected to a K chip. J thermocouples of North American origin generally have black color code.