Hello MarvinTsoi,
Thank you for taking the FFT of your signal. You have confirmed that you are not introducting a 50 to 60 Hz power line frequency into your signal, as this would be reflected in the FFT graph. Considering the hardware being used, I would say that this data is appropriate. A fluctuation of 1 degree can be compensated for by averaging your data in software.
Reducing Thermocouple Noise to Improve Measurements
Another thing to consider is that you do not have CJC. Take a look at the following article which describes this, and why it is needed for improved thermocouple measurements.
What is CJC?
Again, the hardware you are using is not ideal for thermocouple measurements. For your reference, take a look at the following developer zone tutorial.
Taking Thermocouple Temperature Measurements
Take a look at the DAQ Systems for Thermocouple Measurements section for some different available solutions. For example, the NI-9211is a low-cost, plug and play solution which provides the signal conditioning necessary for proper thermocouple measurements.
Rod T.
