Choosing values for a low pass RC filter?
The rolloff frequency of a RC low pass filter is:

&nbsp;&nbsp;f = 1 / (2 &Pi; R C)

When R is in Ohms and C in Farads, then f is in Hz.  This comes up so often in electrical engineering that I permanently keep 1/2&Pi; in a register in my calculator.  I divide it by any two of Hz, Ohms, and Farads to get the third.

You say your highest frequency of interest is 10 Hz, but that sounds high unless you have a exceptionally tiny thermocouple.  Even so, we'll use that as the rolloff frequency.

With 100 &Omega; in series (the 31 m&Omega; of the thermocouple can be ignored relative to that) and 10 Hz rolloff, the capacitor needs to be 160 &micro;F.  While that is doable, I would want to avoid electrolytic capacitors in this situation.  With 1 k&Omega; in series, you need 16 &micro;F to ground.  That's more manageable.

If you really have such high common mode noise, you should treat the thermocouple lines as balanced.  A instrumentation amp would be useful here.  There are some explicitly intended for thermocouple use.