RMS Voltmeter

Most I ever saw will proudly declare that they are true RMS reading meters if that is indeed what they are as it commands a higher selling price point for the 'instrument'.

Yours will give the RMS reading of 120 Volts even if it is a " cheapy". Because you are reading AC power, which is supposed to be a purely sinsoidal waveform.

A true RMS meter constantly calculates the voltage by examining the waveform of the signal under test. Pure sine wave is easy. Complex waveforms are not so easy and a regular averaging meter will mis-report the value read.

If you are only reading basic AC power voltage, you do not need a true RMS voltmeter.

If you have an oscilloscope that has a square wave output for scope calibration and probe adjustment , you can measure that with both DMM's.

Dan

Having established that it reads a sine wave correctly, set it to AC and feed it +/5V square wave (10V peak to peak), if RMS it will read 5V, if average responding it'll read high.

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I have a request.

Could someone with a UPS measure the voltage of a battery output outlet using a RMS meter or oscilloscope?

Disconnect power to UPS before the test.

Thanks. Andy

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** I have one for you.

Wake up !!!

You have not bought a "true RMS" meter.

And don't need one anyway.

..... Phil

Your angry response is uncalled for.

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** Nothing one bit angry about my words.

You presume WAAAAYY too much.

.... Phil

Grow up.

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** No you are just plain begging for a telling off.

You come here acting like a spoilt, 12 year old brat.

If you are older than that, you should be ashamed.

.... Phil

Check the other thread you started for a way to test it and be sure.

But it really is a feature so it would say so. think how hard it is to buil d it digital, well not that bad. Old analog true RMS meters actually used a n incandescent based optocoupler. In digital it must sample and analyse the waveform. Pain in the ass, I wouldn't want to try it.

waveforms are not so easy and a regular averaging meter will mis-report the value read. "

Came in handy when we were wrapping extra windings on a flyback transformer to boos the CRT filament voltage like a brightener, or to isolate it if it had a K-G2 short. I was the guy to do that, nobodyu else could grasp it. I even re-equalized the video so that if it intermittently shorted it would not cause a smear. that's why I got the big bucks. the fact that it is usel ess knowledge now is why I no longer get the big bucks.

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** Newer ones typically use a "true RMS to DC" converter IC, made by Analog Devices that computes in an analog way the RMS value of complex waves.

The AD737 has pretty good performance, but very few multimeters use it to best advantage.

** So called "Digital" multimeters " do not do that, they use AD737s or close relatives.

.... Phil

From the (simplified) datasheet it looks like all it does is average the signal.

How's that ?

By the way, in this guy's post over in basics I got a way to figure out if it is true RMS. Do you happen to know an easier way ? That is assuming you have no access to manufacturer's information. They don't all say right on t he meter, I am pretty sure my Fluke doesn't.

(I want my buddy's 8842 IIRC the model correctly, mine is only an 8050a)

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** The 10:1 stepped input attenuator is not frequency compensated ( like a scope input ) for flat response. Only expensive bench models routinely have that. So frequency response is good to a few kHz at best.

With a "true RMS" meter, measurement bandwidth, level dependency and "crest factor" are crucial specs that vary widely.

** FFS - that prick's $10 POS is NOT "true RMS". ** Wot - no Google?

Absurd.

.... Phil

The NEW one is ten bucks ? I didn't know that.

I apologize for not reining in my tongue. Andy

Do you mean to tell us that upon your tongue, you do not reign?

"But the tongue can no man tame, it is an unruly evil, full of deadly poiso n."

Cheers

Phil Hobbs