## Diff between RMS & True RMS

## Diff between RMS & True RMS

(OP)

Hi,

What is the difference between RMS & True RMS? Is it simply the sample rate?

Thanks & best regards,

What is the difference between RMS & True RMS? Is it simply the sample rate?

Thanks & best regards,

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

Cheaper meters measure only average values which

are not accurate if the signal differs from a

SIN wave.

If a meter measures RMS it gives the effective power

of the signal accurately.

Beware, all RMS meters have limitations on their measurement ability. Bandwidth and Crest factor.

The phrase TRUE RMS is a marketing gimmick.

Rodar

## RE: Diff between RMS & True RMS

True RMS is a phrase dreamed up by instrument manufacturers to distinguish RMS-measuring instruments from average-measuring instruments with scales calibrated to show RMS values for a sinusoidal input.

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

TRMS at will perform within the limitations of crest factor and bandwidth.

TTFN

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

A "RMS" meter, control or whatever, likely means they get the average value and scale it. They assume a pure sine wave so the scale factor is according to a sine wave. If your waveform has any distortion the reading of an "averaging" (RMS) control will be erroneous. These types of devices tend to display a lower value than what is actually there. This is what makes the fluke 87 such a nice meter is it is a "true RMS" meter.

A review of the data sheet will tell you what you have. If they talk about a crest factor then you have a true RMS device and not an averaging (RMS) device. Technically, there is no differentiating between RMS and true RMS but the manufacturers and salesman use this verbage all the time so it is best to know the difference between them.

## RE: Diff between RMS & True RMS

Without going deep in details, cheap electronic meters measure not RMS but rectified amplitude value.

They are calibrated so that user reads RMS, provided that the signal is of pure sinusoidal shape.

TRMS meters must use more complicated processing to be really TRMS.

Probably, sometimes it's really only marketing gimmick.

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

when taking measurememts of values that have harmonics

present. There is a way, although I don't recall the exact

process,where the readings of a true RMS and average reading

meter are analyzed to obtain the harmonic distortion of the

measured value.

## RE: Diff between RMS & True RMS

Digital meters can either be "mean reading RMS calibrated" or the more expensive "RMS reading RMS calibrated" type, which it is easier to state as "true RMS". The "true RMS" title is therefore vitally important since there are so many mean reading RMS calibrated meters out there. As other respondents have said, crest factor is only found on the true RMS type of meters and is a limitation above which the instrument will read incorrectly, as indeed is bandwidth.

Note that analog true RMS meters do not "sample". In fact these big 6 ½ digit DMMs with true RMS are not necessarily digitally RMSing the value either. One technique uses transistors and log-antilog stages to do the averaging of the squared valued. This AC to DC converted signal is then measured as a DC voltage.

## RE: Diff between RMS & True RMS

Buzzp does indeed piont to the truth. But I dont know if the phrases gimmick marketing etc are all valid (I admit I have seen some s**t meters that qualify.)

Yes RMS is a defined standard, ("the power delivering " capability or "effective value" snitched from 2 handy books in my office)

A long time ago primitive techs only dreamt of DVMs like the flukes (I got one and thought my dreams had come true)

Moving coils give average and for RMS one did the calc and got a pen out and hey presto RMS meter. Use a CRO to verify

sinusiodal rms = .707 x peak

triangular rms = root (peak squared /3) [checked in book]

full wave rect = peak /root2

so, no-- rms meters have a place but true RMS (when they are) are heaps better

as an aside: youonlygetwatchyapayfor

Don

## RE: Diff between RMS & True RMS

A pretty clever fundamental method I thought.

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

rgds

Zeit.

## RE: Diff between RMS & True RMS

Thank you for your input.

Regards,

Lakey

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

I can tell you there are other ways to calculate the RMS value but they are software intensive and well beyond the ability of your small microcontrollers (cheaper meters or controls).

Now you can filter out whatever you do not want (harmonics). The question is what harmonics are inlcuded? Out to the seventh, fifth, what? To me, it should include the fifth at least, although the ones I have come across filter frequencies above the fundamental.

Yes calculating the real RMS value can be done and is done with software. But let us not forget the old op-amp configurations (square, square root, and differential) that will spit out the true RMS value. So there are plenty of devices capable of calculating (based on amplitude measurements) the RMS value. Your expensive utility type relays have this ability as well as some low/high end panel meters. So doing RMS measurements is possible and is being done by several companies.

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS

## RE: Diff between RMS & True RMS