Diff between RMS & True RMS 8

[Lakey]

Hi,

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

Thanks & best regards,

 

[cbarn24050]

Hi, TRMS takes into account the waveshape of your signal, conventional meters are only accurate for sinewave signals.

 

[rodar]

To me there is no difference.
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

 

[ScottyUK]

I agree with Rodar - RMS has a clear mathematical definition.

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.

 

[cbarn24050]

I don't think it is fair to call it a "marketing gimmick", or a "phrase dreamed up". That would imply that it has no real value.

 

[IRstuff]

If anything, it's the "rms" meters that have created a marketing gimmick by using a cheesy approach that only works for sine waves.

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

TTFN

 

[ScottyUK]

True RMS and TRMS are both totally meaningless terms: a quantity is either an RMS value according to the definition of RMS, or it is not. The idea of a meter reading 'true' RMS is nonsense - it either reads RMS, or it doesn't. Black or white, no shades of grey in between.

 

[buzzp]

Well gimmick or not, careful attention needs to be given to data sheets and marketing material. RMS does not necessarily mean it is reading the real RMS value. If you see literature identifying true RMS then you have the real deal. Most equipment will use this verbage.
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.

 

[xyzz]

Hi everybody.
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.

 

[60hzhmmm]

buzzp hits the nail on the head. You can certainly have an RMS meter that is exact only if the measured signal is a sinewave. If you aren't measuring sinewaves you better have a "true" rms meter. That's the industry standard nomenclature.

 

[electromike]

The function of "True RMS" reading meters is not just marketing. What they do is give a more "true" RMS reading
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.

 

[logbook]

There are a lot of the correct answers stated above. Perhaps some historic perspective is useful here. AC quantities such as voltage and current are always RMS values unless otherwise stated. The mains voltage at 230V is 230V RMS and so on. Most analog meters have been described in more technical books as "mean reading RMS calibrated" devices. Whilst it is possible for an analog meter to be a true RMS reading device, it is not usual. Such meters are "electro-dynamic" or some other square law technique, such that they read the mean of the squared value, scaled to read as RMS values.

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.

 

[don01]

hi all
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

 

[60hzhmmm]

logbook: For a while the "BEST" rms meters were actually just a resistor that converted anything that came by, (crest-factor be damned), to heat. The resistor's temperature was monitored acurately with great detail and calibrated to "true" rms.
A pretty clever fundamental method I thought.

 

[cbarn24050]

Hi, yes there were alot of thermister based trms meters in the 50's and 60's, it was the only way to do it at the time.

 

[zeitghost]

And thermocouple RF ammeters before that!

rgds
Zeit.

 

[Lakey]

To all,

Thank you for your input.

Regards,
Lakey

 

[buzzp]

With the age of electronics as old as it is, seems most cheap meters sample a period, find the average, and scale it to get the rms value.

 

[Heydave]

The sampling is not quite so easy. There must be a large enough memory to store a lot of samples so that samples at the beginning of a sample window are deleted in some way. This can occure by recomputing the entire array for each measurement or by subtracting the old value from the total. To keep the bandwidth high yet still deal with mostly unwanted power line frequency, the sampling array needs to be large. To maintain stability near the nyquist frequency, an anti-alias filter is needed. To get the voltage, a square root calculation is needed and this also takes some processing. Now try to compete with the power draw of dual slope meters. The advantages of the digital approach are vast if you have use for them. Any investors out there as this instrument designer is ready with patents in hand.

 

[buzzp]

Let me tell you all how one manufacturer does their RMS measurements. First, at a zero cross, sampling (magnitude) occurs for a half cycle. At the end of the half cycle (next zero cross) the sampling stops. The average is found and scaled appropriately to come up with the RMS value. So only half cycles are sampled. One waveform is only sampled every 40 cycles. Four calculations are made (total of 160 cycles) and compared to the trip level or displayed.

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.