Attached are the crunched numbers.

Here's how I did the analysis. I assumed that each DMM was measuring the same EMF. Because each DMM was acquiring data at the exact same time, the actual voltage of the EMF is unimportant. I'm simply looking for time-dependent *changes in differences* between voltmeters.

So to this end, I imagined that each set of voltage measurements created a vector in three-dimensional space (e.g. [DMM #1, DMM #2, DMM #3]). If each DMM was properly calibrated, the angles between each unit vector and the vector created by the collection of voltage measurements should be the same. But as these DMM's are not at all calibrated, the vector should be skewed off to one side a bit.

Now if the Vrefs of these units drift at the same rate, these angles should stay about the same. But one if drifts more or less than the others, the composite DMM vector will start to point towards or away from that DMM's axis.

Attached is a PDF of the spread sheet.