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In other cases such a meter may still be usable (and in fact may sometimes be better than a true-RMS meter) if one knows how its measurements are computed and can figure out from that what one wants to know about the signal (e.g.
(major) We use RMS to measure voltages and currents because for resistive loads it relates directly to average power.Unfortunately it's very dificult to build circuits to accurately square and square root a singal.So multimeter designers cheated.
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They do tend to cost more, and have more error than the simple averaging circuit. If you are doing mains voltage work you should consider a true RMS meter that is Cheaper multimeters get by with specifying RMS by measuring a pure sinusoidal input's peak voltage and then multiplying that by 0.707 and displaying the result.For a pure sinusoidal AC input with no distortion, that's fine, but for other waveforms it isn't.The reason it isn't is that the RMS value of a waveform is equal to the magnitude of the DC signal which would cause equivalent heating of the load.The measurement is done by sampling the value of the input signal many times during a single cycle, squaring each of those values, adding them, then taking the square root of their sum and displaying it.So, the extra money you pay for a true RMS instrument is for the extra silicon needed to do the extra work,(minor) the firmware required to work it all out,(minor) and the convenience provided so you don't have to gear up to figure it out all by yourself.
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Microsoft is radically simplifying cloud dev and ops in first-of-its-kind Azure Preview portal at portal.azure.com I found a multimeter that measures True RMS, and another one that knows the same but without TRMS.
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Important RMS Support Change: In an effort to ensure proper compliance with the 1979 Trade Act Agreement. In other cases such a meter may still be usable (and in fact may sometimes be better than a true-RMS meter) if one knows how its measurements are computed and can figure out from that what one wants to know about the signal (e.g. In many cases, if one has a measurement of a known type and one also knows the shape of the waveform and DC offset (if any), it will be possible to compute what the other measurements would have been (e.g.
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They measured some property of the signal that is easier to measure (often "mean magnitude"). if one has a meter that is known to measure the peak voltage and scales it by 70.7%, and one wants to know the peak voltage of an irregular signal, one could use such a meter by multiplying its displayed result by 1.414, while an RMS meter may be nearly useless).The primary advantage of a true RMS meter is that it will measure irregular waveforms in a known fashion, subject to documented frequency restrictions. EzRMS automatically calculates demand forecasts for each future use of an organization’s hotel rooms, and recommends the appropriate selling strategies, such as open/close rates, stay controls, open/close room categories, and overbooking levels. Sorry, we no longer support Internet Explorer
@PeterGreen: The Fluke Scope-Meter included readouts for DC and true-RMS AC+DC; I've noticed other "true RMS" meters apparently filtering out DC, though I'm not quite sure in what contexts such a DC-excluded RMS signal would be meaningful, though. That scale factor makes the assumption that the input waveform is a sinewave.More recently "true RMS" meters have appeared. Electrical Engineering Stack Exchange works best with JavaScript enabled
They work by sampling the signal and then calculating RMS in software (where accurate squaring and square rooting is more feasible). Featured on Meta
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)The smaller shape of the latter would be handier and it costs 2/3rd of the TRMS's price, so I ask Your advice, Experienced Electronics Experts:In what cases, and how much does True RMS matter?
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for a sinusoidal signal with zero offset, the peak voltage will be about 1.414 times the RMS voltage), but a number by itself, without information about what kind of measurement it represents, is apt to be meaningless.For many purposes, sinusoidal waveforms are reported as RMS voltage (a 120V or 240V power main, for example, will nominally have 120V RMS or 240V RMS), but cheap meters will often measure AC voltage via some other means and then scale the result in whatever fashion would be appropriate for a sinusoidal signal with zero offset.If one is measuring a sinusoidal signal with zero offset, such a meter will work just fine. Pretty much all multimeters even "true RMS" ones will block DC on their AC measurement ranges. So for high frequently input signals it's reading may not actually be an accurate RMS value. Circuits that perform 'true RMS' calculation have maximum bandwidths and dynamic ranges, but within that range they can do okay, say for things like measuring the RMS voltage from a phase controlled dimmer.
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