I'm trying to interface to a type-K thermocouple with an ATXMEGA128A4U and have been experiencing some difficulty and I'm hoping someone can confirm the process I am using.
Details.. 1. TC is connected to standard A/D input via an AD8223 instrumentation AMP with a gain of 106 and an offset of half my A/D reference voltage of 1.25V offset. My A/D reference voltage is 2.5V 2. I am using a standard 10K NTC for the cold junction reference. It's soldered directly to the TC socket and I have calibrated it in an incubator and am confident that my cold junction temperatures are being converted and processed correctly by the processor.
Basically, I am not getting the output that I would expect across a fairly narrow temperature range of 0-80C. I'm not 100% sure that I'm processing properly. If we ignore the calculations I am using for the conversions for now, am I correct to think that I can take the TC output in mV and use the NIST lookup table to convert that mV to a temperature and then subtract my cold junction temperature (which will always be close to Ambient 20C) to get my tip temperature or is there more to it?
To illustrate my problem, I just placed the ThermoCouple in a mug of 76.4C water (confirmed with Fluke 51II ThermoCouple Meter) and Im getting an output of 5.151mV with my NTC reading the Cold Junction at 18.4C. According to the NIST table, 5.151mV equates to 125.5C. When I subtract the 18.4C cold junction I'm left with 107.1C. Clearly not correct. I'm actually using rational polynomials for the conversions in code but I thought I would refer to the look-up chart analogy to keep things simple for the sake of this post. I'm hoping that someone might be able to confirm my logic before I go any deeper.
Perhaps I'm thinking about this the wrong way. Feedback is greatly appreciated.