OK so we now know that there is at least a positive aspect of the DC coupling, so if Brymen could manage to adress the "overrange problem", it could be safer on the mV scale that some other meters.
I was trying to characterize the meter to know above which level I can not trust it anymore on the AC mV scale. In the worst case scenario when the width of the squarewave is just above the declared minimum of 350us this safety limit can be as low as 400-430mV. Above which the meter starts to show BS values. So it is not necessarily has to reach the 650mv displayed value to have something way higher real value. Even with 50% duty cycle It starts to show invalid values above 500mV.
Unfortunately we still don't know what happens, if we just raise the voltage to lets say 50V. Maybe it will overrange. What is this lowest safety marging with the 869?
The other question is, what could be the lowest safety margin with some kind of completely different waveform? Could it go even further down? Would be really nice to know.
Anyone has a newer firmware than the 08?
So can I can get the BM789 to read 0 mVAC with a signal applied as I suggested. You bet. Do I care, other than for a small bit of entertainment, not at all. I could run others but will wait and see what bdunham7 comes up with.
What was the exact waveform here?
And regarding the AC coupled mode:
Similar to the ipact wrench, or cordless drill tests where they are getting attached to eachother to see which is the stronger, we might see multimeter fights as well in the future!
It is really interesting that this issue surprises even experienced engineers. Might there are tonns of equipment which are getting damaged all the time because of this regulary, just people are not avare of the cause. Wonder how long they keep the charge.