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You mean you measure at the 2 zero crossings of the 4kHz sample frequency right?
Because at the zero crossings of the signal would be strange.
So you actually measure with 8kS/s then?

…60Hz waveforms, what the majority…
Americans… :P Most of the world uses 220V-240V @ 50Hz :P
But that shouldn’t make a big difference in the measurements.

About my measurement methode: (Interesting to know for most people)
I connect the Fluke 5522A calibrator directly with the mooshimeter through a shielded test cable.
Only for current I use standard twisted banana cable because shielded cables are not made for higher currents.
For lower resistance (<10k) I use compensated 2 wire (4 wire to the calibrator) which is generated with 1mOhm accuracy

For high resistances with tight tolerances (the high end meters) we use special measurement banana cables which are better insulated.
That is where leakage current comes into play. We keep the + cable apart from the – cable of course.
Our accredited lab also has to

The calibrator generates the resistances, DC/AC voltages and currents with very nice specs.
For example I generate the 240mV with 0.00242%, the 9A with 0.0556% and 1MOhm with 0.0034%

Besides that is the 5522 every year measured in our accredited lab.
The measured deviations are logged to ini files and used in the shown uncertainty on the report.
The interesting part: it doesnt matter if it actually generates 1.1V when we want 1V,
as long as it only drifts 13ppm per year, we still have an accuracy of 0.0013%

I can take a few more measurements at 8kHz with 256 samples,
even better would be 8kHz/512 samples since that would be the same time period as at 4kHz.

about the 1kHz:
The spec website states:
Frequency: Better than 1% accuracy up to 1kHz
Sampling : 4kHz analog bandwidth for most measurements

Your measurement methode:
I dont know what the amplitude tolerance of your frequency generator is,
but I guess about 2% assuming it is recently callibrated.
The PM5136 has for example 2% AC voltage accuracy

Fluke 115:
45 – 500Hz : 1% + 3 digits
500Hz – 1kHz: 2% + 3 digits
Since the bandwith of the meter is only 1kHz you have to cross reference it to know what is does at 1kHz.
So I think you measured with 3% tolerance…

This is the reason why cheap DMM’s often dont meet their specs:
they didn’t care or they didnt have the equipment to verify the specs.

Of course I dont blame you for not having such costly equipment :P
We can buy a small house with that money… (its 70k with the 1.1GHz scope option)

So to be continued…