Imagine you have a periodic signal F which is nothing but a sinewave (S) plus some DC offset (D).
Here is an illustration of such signal:
Imagine I want to measure the RMS value of this signal F.
The rms value of this signal mathematically can be written as:
rms_of_F = sqrt[ (rms_of_S)^2 + D^2 ]
So far so good.
First of all, if I use the DC settings of a voltmeter I would obviously not read the rms value.
Then I may use AC settings.. But recently I came to a question: What value does ammeter or voltmeter measures (RMS, Average or Peak )? where the answer is chosen as follows:
Measuring RMS values is a bit more expensive than measuring average values, so most multimeters avoid the former. Instead they presume your signal is a sine and measure the average value for the rectified sine or the peak value, after which they apply a conversion factor to find the presumed RMS value.
Does that mean the rms value of my signal F can never be mesured accurately by using a voltmeter?
I'm asking because in case the voltmeter in AC settings doesn't measure the rms value: if one calibrates an instrument and relate the rms value of such signal output to the device reading(any parameter like temperature, flow ect.); and if another person would use a multimeter/voltmeter to use the calibration expression, this could lead him a wrong conclusion because he will not read the true rms voltage but something else. Is that right?