Voltage reference:
Not precise UT61B, but I like it:
2.499:
5.003:
7.495:
9.999:
More precise multimeter in Owon handheld oscilloscope:
2.499:
5.003:
7.495:
9.999:
Owon B41T+ Just received. It has the same chips as in UT61E (and same precision) and also bluetooth module. It has a lack of protections for high voltage usage, but I’m not measuring voltage more than in the socket directly 
:
2.499:
5.003:
7.495:
Not so accurate at 9.999 maybe not the best probes:
Some other stuff for Owon:
Bluetooth app:
Chart:
Also just received Agilent oscilloscope probe 10073C:
