Hi, I've finally had a chance to look at the SVR-T measurements that I made last week. First off, I'm an organic chemist, not an ee - I still have a lot to learn about this metrology stuff! My good meter is an old Keithley 196 6.5 digit. The only thing I know about its calibration history is that I haven't done it! It is in nice shape and seems very stable to me. The 196 has GPIB, however I don't have an automated measurement system working yet - it's in the planning stage for now.
I powered the SVR-T at 15.00 v from a very stable and quiet Power Designs 4010 precision power source. I let everything stabilize overnight before I took any data. For the cable connection between the SVR-T and the 196 I tried the various solutions "in the box" from cellularmitosis as well as my own teflon insulated twisted pair from plenum grade Cat5e and standard RG-59 coax with Pomona BNC to banana plug adapters. I was unable to discern any difference between any of them. In the end I used the coax cable for the measurements.
What I did was each morning for 3 days I took manual readings of the K196 every 30 seconds for 15 min. I checked the lab temperature on my Fluke thermometer at the beginning and end of each run. For the third run I decided to record the temperature every 30 sec also.
Looking at the 3rd day plot with the temperature, it appears that the slight voltage drift may be correlated with the temperature fluctuation. Like Conrad, my lab is in my basement. Its temp and humidity is not well controlled and I took no measures to prevent drafts on the SVR-T. It looks like that might be a good idea.
I also took 10 min of data on day 3 with the Keithley 196 averaging filter ON. It did seem to smooth out the data somewhat, but not entirely. I'm still working on understanding how this filter works, so I didn't use it for the main part of the experiment.
Overall, the SVR-T appears to be very stable and quiet. In the time I had it up and running it's value changed less than 10 PPM.
Graphs of the data are below for your enjoyment.