I have a old thermocouple reader which, when the filter caps were replaced, reads the temperature with a fluctuation of +- 0.2 degrees Celsius, meaning it will jump between 2.8 and 3.2 for instance. It still jumps when I put it inside of Styrofoam box. I have tested the atmosphere in my room prior with a RTD and found the temperature is much more stable then that.
It has some tantalum capacitors for filtering the input (which I checked the ESR of, they seemed fine), and some filter capacitors for the rectifier (which I replaced with excess).
I used this method for checking the capacitors:
http://fullnet.com/~tomg/esrscope.htmReplacing the supply filter cap reduced the fluctuations significantly. They tested bad. The tantalums on the thermocouple input tested out fine. Is this type of fluctuation normal for a type T thermocouple? I also tried wrapping the thermocouple in ferrite rings but that did not really do anything. I had like 4 rings with 30 windings installed near the screw terminals on the unit, out of frustration, but that did not really do anything.
Maybe since this is an old unit it was just not built for the amount of noise that switchmode supplies throw out now?
I was thinking about maybe making some sort of matched low pass filter but that seems rather difficult for such low voltage measurements.
The wire is also twisted.
DUe to a lack of probe at this time I have just twisted together the extension wire to form my thermometer but I don't think this has any effect.