It is often assumed that high precision instruments are better not switched off but left on running 24/7, just because it is to tedious to wait for the specified “warm-up” period. Mostly, this seems to be more of a habit than a real requirement, at least for multimeters and their build-in references. Let put it to a test.
With all good references back in Germany, we first need to fabricate a little substitute, a +5 V reference based on a MAX6350. This has roughly 0 ppm tempco around ambient temperatures, and +-0.02% initial accuracy.
Some caps were added to keep noise down.
With this reference fully ‘warmed up’, two instruments were used to monitor the output, a 34401A, with current calibration, and the recently repaired 3455A, with about 4 years old calibration.
The 3455A provides 10 µV resolution, in the 10 V range. The 34401A – the displays shows 6.5 digits, same as for the 3455A, but the build in resolution, when read through GPIB, is about 1.3 µV.
As you can see, there is really no warm up. It is stable to ppm level, after a few seconds.
The offset of the two meters:
About 4 ppm difference – not too bad; certainly, well within spec of these two instruments.