Nice score, 4.5 digit meter
The manual is hilarious
"4.1 Zero Adjust
Everybody can adjust to "0" at initial screen to get a exact reading value."
From the nice picture you posted, do you see the label "ZeroAdj" and the hole next to it?
My guess is that this is for adjusting the zero, with a pot trim tool (small flat blade screw driver if you do not have one).
Yes, wait till the meter warms up and stablizes. This can take upto an hour or two.
Then adjust the zero.
Would I be expected to check and adjust zero voltage every time I want a precise measurement?
Yes but in practice you probably only need to adjust it once a month or once a quarter.
The important part is to let the meter warm up and stablize.
You only need to worry about it if you want the Precise measurement. Most of the time you will be OK with the ballpark figure ie you do not need to worry about the warm up time ie do you really need to know it is 3.289V or is 3.3V good enough?
Or calibrate once when it's fully stable, but have to wait an hour for a full warm up?
Yes, just what I said above.
Is this a faulty unit or just the way it works?
My guess is that is the way it works.
I have a Thurlby 1905a (5.5 digit) and a Solartron 7060 (6.5digit) - yes I have to wait for 1 hour for warm up before I can take accurate readings.
FYI, it is worth getting yourself a voltage reference such as the AD584 - it is a single chip precision voltage reference that provide 2.5V, 5V and 10V +/- 30mV worst case. Plenty good enough for a 4.5 digit meter. Just buy the chip, put it on a circuit board (strip board will work) and provide it with 12V or more and it will just output those voltages.
If you get a chance to check the AD584 against a calibrated meter - you then have some form of traceability.