True RMS ?

[SG-1]

Why do DMM manufactures claim "True RMS" on the AC settings when they strip out the DC component ?

In order to get close to the correct reading two measurements must be made & summed or if so equipped the meter must be in AC+DC mode.

It seems to be an industry standard that the DC is NOT included & some assembly required.

[helius]

When you put a meter on a AC setting, you would be greatly surprised if it was including DC in the measurement. TRMS is simply a description of the measurement technology, which involves measuring the heat dissipated by a resistor. This permits correct readings for non-sinusoidal waveforms, up to a specified crest factor which will be in the DMM's spec sheet. Meters without TRMS converters perform Average measurement of AC signals, and then apply a correction factor that assumes the signal is sinusoidal, \$ \pi \over {2 \sqrt 2} \$ times the average.

[JohnnyMalaria]

From a useful Wikipedia article:

Analog electronic converters

Analog electronic circuits may use:

an analog multiplier in a specific configuration which multiplies the input signal by itself (squares it), averages the result with a capacitor, and then calculates the square root of the value (via a multiplier/squarer circuit in the feedback loop of an operational amplifier), or

a full-wave precision rectifier circuit to create the absolute value of the input signal, which is fed into an operational amplifier arranged to give an exponential transfer function, then doubled in voltage and fed to a log amplifier as a means of deriving the square-law transfer function, before time-averaging and calculating the square root of the voltage, similar to above, or

a field-effect transistor may be used to directly create the square-law transfer function, before time-averaging.

[Tomorokoshi]

This application note has some useful information:

http://www.analog.com/media/en/technical-documentation/application-notes/an106f.pdf

[SG-1]

My main point is that when someone is measuring a signal with the meter in ac mode they my or my not be reading the RMS value of the signal. The manuals do not mention this.  The only way to know is to test the meter.  I do see an advantage with it being done this way, in that you can see the contribution of the AC & DC separately. It would be nice if this were part of the instructions.

Can the true rms technology actually measure a voltage signal rich in harmonics correctly ?  (Provided they are within the stated frequency range of the meter.)

I have a functional RMS meter built in the late 50s in a wood case. It does not strip out the DC component. As a matter of fact it does not have separate AC & DC voltage settings.

[JohnnyMalaria]

I think most True RMS meters say so on the outside since it's such a strong selling point. The two I have do (a Fluke and an Extech).

[bugi]

Also, it seems to be common to advertise it if the meter has any DC+AC -modes (there can be multiple such), that is, modes which show both the DC and AC components with the dual display.
As an example, brymen's 860-series datasheet
https://brymen.eu/wp-content/uploads/biall/102092/102092.KARTA_EN..2015-07-09.1.pdf
It touts DC+AC true rms quite a few times.
Manual, on the other hand, is less clear about whether it has true RMS or not and when or how it might be in use. But on the other hand, it includes quite good general explanations on page 4:
https://brymen.eu/wp-content/uploads/biall/102092/102092.INSTRUKCJA_EN..2014-11-04.1.pdf

[LazyJack]

Every reputable meter says in the instruction manual, how it measures RMS. Fluke even gives examples of how to calculate the combined values.
It also specifies the frequency limits for the harmonic content where the measurement is within specs.

In most of the usual use cases it is more useful to have the separate DC and AC RMS values. You can still easily add them in the few cases where it is needed.