Hi. I have two reference PT100's. What would be an acceptable difference between the two probes in the same bath at the same temperature. Mine seem to be about 2mK apart all the time. Is this good or bad?
It looks like you are getting very good readings there!
Your results will be based on multiple factors such as the temperature stability of the baths (5 mK is a fairly typical specification for a good bath), the accuracy of the probes (good probes have an accuracy around 15 mK and annual stability around 10 mK), the meter accuracy (5 mK for a good meter), factors such as immersion depth, etc.
All in all if I had two PT100 probes that are consistently 2 mK apart I would be very satisfied!
I would never be satisfied with 2mK ... but I would understand that it's pretty darn good... .
DW is right, many variables in stirred liquid bath, including temperature gradients in 3 dimensions. You might get better if you used the same display channel, probe in same location, each probe's Ro checked in a properly realized ice bath. But why bother-- unless you're getting paid to do it!