If anyone still reads here, in a final attempt to measure anything useful I bought the handheld flicker meter Fauser LiFli ($300), and maybe the most important thing I found out so far is that flicker-free monitors do flicker, although very little, and even at 100% brightness. My two BenQ EW2740L flicker at 2% and the totally unusable Dell U2515H flickers at 4%. Still doesn't explain why one of the BenQs is not perfectly usable. Maybe very small differences in percentage make the difference. But 2% is the least you can measure with this device. My cheap Hantek 6022BE oscilloscope which I connected to the LiFli seems to suck too much to pick up such small differences. Currently I'm not willing to spend any more money for a better oscilloscope. I have an LED bulb that flickers at 4% when you turn it on and then it slowly continues down to 0% over some minutes. During the first seconds I get PWM symptoms. Later this LED is much better. So it seems the safe percentage for me might be somewhere between 0% and 2%. Which is insane since we're talking of frequencies between 20 kHz and 400 kHz. However, even this 0% LED is still buzzing slightly, made audible by the LiFli device's speaker. So there still is some ripple. It seems true flicker-free is not possible with wall-plugged devices. My small LED battery-driven flash light is at 0% without any buzzing.
I didn't measure flicker percentage (a-b)/(a+b) but ripple percentage (a-b)/a, which makes more sense to me.
Where a is max and b is min voltage.
https://www.fauser.biz/li/lifli_e.htm
Edit: With my new oscilloscope setup (a few posts below), it turned out that the low percentages (like 0-2 %) the device measured was probably the display's 60 Hz refresh rate flicker. I did not even know that LCD matrixes would pulsate at each refresh when displaying a constant image, but apparently they do.
