How to use true rms function on multimeter

[sairfan1]

Hi, I want to know what is True RMS function on multimeters, in what scenario we really need it and how do we check it.

Thanks.

[shobo]

 true rms applies when measuring ac currents,and it's not a function that you select manually.it gives you a true meaning of voltage and/or current.

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[schmitt trigger]

The only place you DON'T need an RMS meter is for a sinewave with little or no distortion.

For everything else, an average responding meter will provide false results. If I recall correctly, a square wave will read 11% higher with average mode than true RMS mode.

[glarsson]

The only place you DON'T need an RMS meter is for a sinewave with little or no distortion.

You also do not want it when following old repair manuals with reference readings taken with an old averaging (non true RMS) meter.

[wraper]

It's a passive function. It's either always enabled or is not there at all. What it does allows accurately measure effective voltage/current on non sinusoidal waveform.

[sairfan1]

Thanks, so what i understand, its not something that i turn on/off that is something multimeter using to measure, and its not something that i use in particular scenario, its preset all time, wow, i was thinking quite differently.

Just want to clear one thing, as @shobo said, does it applies to only AC current?

[MosherIV]

Just want to clear one thing, as @shobo said, does it applies to only AC current?

No, any AC measurment.
That is it applies to Volts AC and Amps AC

[David Hess]

Some multimeters allow average and RMS AC measurement mode to be selected.

Average measurements settle faster which may be important when making adjustments or trying to follow changes.  If I am trying to adjust a precision square wave for calibration purposes, then I use average mode with the 11% correction factor Schmitt Trigger mentioned because it is faster.

I think the only places where I have absolutely needed RMS measurement capability in a multimeter were for noise measurements and phase control measurements.

Just want to clear one thing, as @shobo said, does it applies to only AC current?

No, any AC measurment.
That is it applies to Volts AC and Amps AC

And AC+DC measurements which are the root of the sum of squares of the DC (1) and RMS AC values.  If you have an AC waveform with a DC offset, then this is what you use but you can also make the DC and RMS AC measurements separately and do the math.  The normal RMS AC measurement will not work because it is AC coupled.

[Mr. Scram]

Thanks, so what i understand, its not something that i turn on/off that is something multimeter using to measure, and its not something that i use in particular scenario, its preset all time, wow, i was thinking quite differently.

Just want to clear one thing, as @shobo said, does it applies to only AC current?

Simply put RMS looks at the actual waveform to determine the equivalent voltage or current, while non RMS multimeters assume a waveform and guesstimate. Obviously, the latter can be off quite a bit if the waveform isn't perfectly predictable. RMS is more accurate and what you'd always want to use if it's available.

You need this equivalent voltage or current, as it allows you to use your value in various formulas. You can use the formulas as if it's DC.

[MosherIV]

Do not get too obsessed with RMS.

As David Hess, it is only useful for certain applications where you need to know the true average Voltage of a non Sinusoid waveform.

Most of the time, AC volts is just used to check mains voltage is there and is it the right number of volts.
Or maybe checking the secondary side of transformers.
I have never had any reason to need the rms feature of any of my dmms.
(Maybe I just do not do enough electronics or the wrong type of electronics )

In any case, the most useful tool for looking at waveforms is the Oscilloscope.

[Mr. Scram]

Do not get too obsessed with RMS.

As David Hess, it is only useful for certain applications where you need to know the true average Voltage of a non Sinusoid waveform.

Most of the time, AC volts is just used to check mains voltage is there and is it the right number of volts.
Or maybe checking the secondary side of transformers.
I have never had any reason to need the rms feature of any of my dmms.
(Maybe I just do not do enough electronics or the wrong type of electronics )

In any case, the most useful tool for looking at waveforms is the Oscilloscope.

Mains waveforms can be fairly messed up nowadays.

[ogden]

while non RMS multimeters assume a waveform and guesstimate.

Some (many?) cheap multimeters just rectify AC and scale it. If using such you measure 12 volt battery in AC mode, it will happily show 16.92 VAC. This can lead to many misunderstandings and not only. Trap for young players (c) Dave

[Brumby]

As David Hess, it is only useful for certain applications where you need to know the true average Voltage of a non Sinusoid waveform.

Poor use of the word "average", I'm afraid.  The mathematic purists would be apoplectic.

RMS is a mathematical function.  Applied to a voltage, it takes any given waveform and calculates the equivalent DC voltage that would produce the same power dissipation in a resistive load.  For a sine wave, this calculation is very easy - the RMS voltage is 0.707 times the peak voltage.  As a mathematical function, it can be applied to any quantity measured against another.  Voltage (instantaneous) against time is the classic, but instantaneous current against time is just as valid.

Because sine waves are common enough, the cheap non true RMS meters have this calculation effectively built in.  Measure a sine wave with them and you'll get the right answer.  However, measure a non-sine wave and you will be shown a lie.  The less the waveform looks like a sine wave, the bigger the lie.

A true RMS meter pays attention to the actual waveform - not just the peak - and will give you an accurate measurement for a wide variety of waveforms other than sinusoidal. Note: There are limitations that affect even true RMS meters.  Check out "crest factor" if you want to get a full understanding.

Most of the time, AC volts is just used to check mains voltage is there and is it the right number of volts.
Or maybe checking the secondary side of transformers.
I have never had any reason to need the rms feature of any of my dmms.
(Maybe I just do not do enough electronics or the wrong type of electronics )

If any of your meters have true RMS, you've almost certainly been using it.

[David Hess]

Mains waveforms can be fairly messed up nowadays.

The power line AC signal is not normally distorted enough for this to matter unless something like phase control dimming is being used.  The error produced by typical waveform distortion on the power line is not any greater than the AC accuracy of the meter.  With power factor corrected switching power supplies becoming more common, I have noticed that the typical power line distortion has been getting better over the past decades.

I tested this on my DMM916 which supports both average and RMS AC measurements and the difference between modes with a low distortion signal was greater than the error caused by measuring a real life distorted AC line in averaging mode.

[notlegato]

Specifically, the math of RMS is not too complex, though requires a little bit of calculus.

you take the square root of the mean of the square of the function, ie.

sinewave: sin^2(x) is the function squared.
integral of sin^2(x) from 0 to 2pi times 1/(2pi) gives you the average value of the function squared (area divided by width).
square root of that is then the RMS value => sqrt(1/(2pi) * int. sin^2(x) from 0 to 2pi)

true-RMS means the meter is able to do this sort of thing for arbitrary waveforms, not just sine, where the multiplier would be 1/sqrt(2), or ~0.707

[IanMacdonald]

The only time RMS voltages have any significance is when powering purely resistive heating loads. In electronic circuits you are generally more concerned with peak, peak to peak, and rectified/smoothed voltages, none of which are indicated by a True RMS meter any better then by an averaging meter. In fact, the AC ranges of a DMM are seldom used in electronics work. A 'scope is what you need here. Even in radio/audio work the limited frequency response of the AC ranges generally makes them a poor choice for measurement of signal levels, unless you just want an approximation.

Another common justification for spending a lot of money on a True RMS model is the mistaken assumption that it will allow you to measure power in reactive AC circuits.  NO IT WILL NOT!! Separate current and voltage measurements will not give you the power consumption, true RMS or no. For that you need a bona-fide wattmeter. A few high-end DMMs have AC wattmeter capability, but most do not.

[mzacharias]

The only place you DON'T need an RMS meter is for a sinewave with little or no distortion.

You also do not want it when following old repair manuals with reference readings taken with an old averaging (non true RMS) meter.

Non-RMS models are slightly more accurate with low level, low frequency (like 50 or 60Hz) readings because the RMS converters add a bit of noise.

Any difference would normally be inconsequential in practice.

[Vtile]

This might be answered already up above ^^^^.. But just to give another opinion about quote below .. no (t)rms is not purely a mathematical function, it is purely physical function as a direct derivative of Joule heating applied with mathematics. https://en.wikipedia.org/wiki/Joule_heating

Average tension/current is purely mathematical function.

As David Hess, it is only useful for certain applications where you need to know the true average Voltage of a non Sinusoid waveform.

Poor use of the word "average", I'm afraid.  The mathematic purists would be apoplectic.

RMS is a mathematical function.  Applied to a voltage, it takes any given waveform and calculates the equivalent DC voltage that would produce the same power dissipation in a resistive load.....

[harrimansat]

With the fluke 867 you can select RMS or Average

[harrimansat]

true rms applies when measuring ac currents,and it's not a function that you select manually.it gives you a true meaning of voltage and/or current.

Sent from my SHV-E250L using Tapatalk

With the fluke 867 you can select RMS or Average