How to use true rms function on multimeter - Page 1
Forum section: electronics / beginners

author: sairfan1
author_rank: Frequent Contributor
timestamp: April 20, 2018, 07:25:46 pm
content: 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.

author: shobo
author_rank: Regular Contributor
timestamp: April 20, 2018, 07:28:53 pm
content: 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

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 20, 2018, 07:49:56 pm
content: 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.

author: glarsson
author_rank: Frequent Contributor
timestamp: April 20, 2018, 08:37:05 pm
content: Quote from: schmitt trigger on April 20, 2018, 07:49:56 pmYou also do not want it when following old repair manuals with reference readings taken with an old averaging (non true RMS) meter.

author: wraper
author_rank: Unknown
timestamp: April 20, 2018, 08:41:48 pm
content: 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.

author: sairfan1
author_rank: Frequent Contributor
timestamp: April 20, 2018, 09:06:01 pm
content: 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?

author: MosherIV
author_rank: Super Contributor
timestamp: April 20, 2018, 09:34:40 pm
content: QuoteNo, any AC measurment.That is it applies to Volts AC and Amps AC

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2018, 09:53:07 pm
content: 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.Quote from: MosherIV on April 20, 2018, 09:34:40 pmAnd 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.

author: Mr. Scram
author_rank: Super Contributor
timestamp: April 20, 2018, 10:08:51 pm
content: Quote from: sairfan1 on April 20, 2018, 09:06:01 pmSimply 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.

author: MosherIV
author_rank: Super Contributor
timestamp: April 20, 2018, 10:29:00 pm
content: 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.

author: Mr. Scram
author_rank: Super Contributor
timestamp: April 20, 2018, 10:59:56 pm
content: Quote from: MosherIV on April 20, 2018, 10:29:00 pmMains waveforms can be fairly messed up nowadays.

author: ogden
author_rank: Super Contributor
timestamp: April 20, 2018, 11:19:53 pm
content: Quote from: Mr. Scram on April 20, 2018, 10:08:51 pmSome (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

author: Brumby
author_rank: Unknown
timestamp: April 20, 2018, 11:36:17 pm
content: Quote from: MosherIV on April 20, 2018, 10:29:00 pmPoor 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.QuoteIf any of your meters have true RMS, you've almost certainly been using it.

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2018, 02:25:51 am
content: Quote from: Mr. Scram on April 20, 2018, 10:59:56 pmThe 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.

author: notlegato
author_rank: Newbie
timestamp: April 25, 2018, 11:56:34 am
content: 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

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: April 25, 2018, 12:30:33 pm
content: 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.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 25, 2018, 07:50:56 pm
content: Quote from: glarsson on April 20, 2018, 08:37:05 pmNon-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.

author: Vtile
author_rank: Super Contributor
timestamp: April 26, 2018, 02:34:31 pm
content: 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_heatingAverage tension/current is purely mathematical function.Quote from: Brumby on April 20, 2018, 11:36:17 pm

author: harrimansat
author_rank: Regular Contributor
timestamp: August 17, 2022, 09:00:33 am
content: With the fluke 867 you can select RMS or Average

author: harrimansat
author_rank: Regular Contributor
timestamp: August 17, 2022, 09:01:12 am
content: Quote from: shobo on April 20, 2018, 07:28:53 pmWith the fluke 867 you can select RMS or Average