In the search for a reality check I picked up an LM1031 precision voltage reference IC. That's designed to provide 10.00 volts +- 0.05% accuracy, i.e. 9.995 to 10.005 volts when fed a slightly higher input voltage.
Checked the B&K meter, which read 9.97V, and the Centek, which thought 9.34 was more correct. (Trimmed both to 10.00 afterwards anyhow.)
Of course that's a crude measurement randomly shot at one temperature and I've no idea how they'll drift, but it confirms the quality of raw Centek is slightly better than a test lamp, while an inexpensive "half percent" meter can sometimes perform better than rated. I'm thinking of wiring
up a couple of the LM1031 into Altoid tins, with two 9v battery
clips, a power button and a couple of output posts. Who'd have thought field calibration could be so simple and convenient?
Or maybe some lithium cells, epoxy
pot it, and sell a "marine grade" version.<G>
0.05% ain't a bad way to start if you don't have a calibration lab handy.