Is there a way to accurately measure the true rms value of the mains? I have a generator test box with a moving-iron volt meter which I want to calibrate. The frequency meter is easy - just use the mains but I can't think of an obvious way to calibrate the volt meter. Thanks.

 

Are you asking about making a ballistic average meter read True Root Mean Square, which requires integration and other computation, or are you just asking how to put a scaling resistor and/or trimpot in series with the meter?

 

I simply want to know how to adjust the meter via the black screw on the front so it gives an accurate reading of the mains rms value. I could compare it to a DMM but how accurate are they?

 

The black screw on the front of the meter is for adjusting the zero setting.
With no power to the meter, adjust to show zero on the meter.
After that the meter should read correctly if it has been properly calibrated at the factory.

 

Thanks - I've adjusted it to zero then compared it to the reading of two DMMs and it seems spot on.

 

Nice to hear.