Did anyone find the missing 2ppm?
At least we think we did, but I have to fully confirm this measuring the other references one after another.
Reason was, that K3458A was in parallel to K2002 influencing each other. I did the test yesterday: readings on K3458A can change, if you connect another meter in parallel to it. On the other hand, now that this issue was found I repeated measurements on first reference with K3458A and they now agree with the readings of HP3458A.
So this is the lesson learned for me at this summit, be careful paralleling meters and watch for changes in the readings.
Edit: Attached is the current result. In the area marked red are the findings with the meters paralleled, in the area marked green the readings with each meter individually connected to reference LTZ1000-1. The left point in the area marked green is the reading on HP3458A, the other points are K3458A, Prema 6048 as is (no adjustment) and both adjusted meters R6581D and S7081. They now agree within about +/-0.5ppm, with Prema 6048 readings somewhat higher.
-branadic-