# meter

Discussion in 'General Electronics Chat' started by electronics1, Jan 25, 2010.

1. ### electronics1 Thread Starter Active Member

Aug 21, 2009
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What is true RMS ?
Did the true RMS make difference for measurement with multi meter for frequency OR voltage OR amperage ?

2. ### JDT Well-Known Member

Feb 12, 2009
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A "True RMS" AC meter will give the correct readings when the input is not sinusoidal.

Other AC meters only measure the average voltage and assume the input is a sine wave and then scale the reading to give the "RMS" value.

A common non-sinusoidal voltage is the output from a triac light dimmer. A true RMS meter will give the correct RMS voltage (the heating effect) an ordinary meter will give an incorrect reading.

This is also true for AC currents.

This has nothing do do with measuring frequency (if the meter also does this).

Last edited: Jan 25, 2010
3. ### electronics1 Thread Starter Active Member

Aug 21, 2009
42
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Thanks JDT
I have some questions
What is the words meaning of TRUE RMS ?
What parameters the measures are incorrect in other words they don't have input of sin waves ?
What parameters the measures are correct in other words they have input of sin waves?
I used the multi meter to measure consumer that used 110 volt

Jul 7, 2009
1,585
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"RMS" is an acronym for "root mean square". You can get an explanation here.

Be aware that meters that are capable of making RMS measurements can differ quite a bit in their capabilities. To get correct readings, you need to know the bandwidth of the signal you're measuring, the bandwidth of the meter, and the crest factor specification of the meter.

Sorry, I don't understand your other questions.

5. ### loosewire AAC Fanatic!

Apr 25, 2008
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Your meter can preform better than most wants to fool with.
There about 4 members that knows every thing that a meter will
do.

6. ### electronics1 Thread Starter Active Member

Aug 21, 2009
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How I can calculate RMS power in a wave that isn't a sinusoidal wave ?
JDT, Can you explain what you write ?

Last edited: Jan 26, 2010

Jul 17, 2007
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Aug 21, 2009
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9. ### thyristor Active Member

Dec 27, 2009
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Most meters measure the average value of an assumed sinusoidal wave. This equates to 1/∏Sinθ.dθ from 0 to ∏ which works out to be 2/∏ or 0.64 x the peak value. Since the RMS value is 0.707 x Vpk then the meter merely has some scaling circuitry which multiplies the average value by 0.707/0.64 = 1.11

So, for sinusoids, the average reading is taken, multiplied by 1.11, internally to the meter, and the resulting "RMS" value displayed.

But what if we are measuring a square wave? Well the RMS value and the average value of a (symetrical about zero) square wave are both Vpk. So your meter will take the average value (Vpk) multiply it by 1.11 to get 1.11Vpk and display this as the RMS value which is thus incorrect.

Even a "true RMS" meter will only read the true RMS value if it is symmetrical about the zero axis (ie: no DC component). True RMS meters are AC coupled to ensure this occurs.

If your wave does happen to have a DC component then you have to first measure the RMS value of the AC component. Then, measure the waveform on the DC scale. Then combine the AC and DC components by squaring each, adding, and then extracting the square root. Some very high end meters have a function called AC + DC which essentially do that calculation for you.

Last edited: Jan 26, 2010
10. ### electronics1 Thread Starter Active Member

Aug 21, 2009
42
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If I don't have sinusoidal wave
Are existents consumers that I can't activate after that in my calculus I receive 110 volt RMS ?

11. ### electronics1 Thread Starter Active Member

Aug 21, 2009
42
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H - e - l - p

12. ### thyristor Active Member

Dec 27, 2009
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Qué?????????????????​

13. ### beenthere Retired Moderator

Apr 20, 2004
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It's hard to parse
Can you restate the question?

14. ### electronics1 Thread Starter Active Member

Aug 21, 2009
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My question is what kind of waves I can't measure with the true rms multi meter ?

15. ### t06afre AAC Fanatic!

May 11, 2009
5,939
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If it is a proper true RMS multimeter it should measure correct RMS, as long as the signal is within the instrument specification range. I think that is the best answer you can get. Many digital scopes have a function for measuring RMS. You could perhaps compare your multimeter readings with the scope readings. They should be quite similar.