Author Topic: Confused about Multimeter RMS and mVdc Setting  (Read 144 times)

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Offline challie2

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Confused about Multimeter RMS and mVdc Setting
« on: February 21, 2017, 11:32:00 AM »
Good Evening or Morning

I have 2 questions regarding Digital Multimeters

1, I dont understand what is or why we use True RMS in meter am i correct in understanding that multimeters read Voltlage in RMS format which i believe to be something around the figure of .707 of the actual voltage therefore if i place my probes on a circuit and get 110VAC the circuit is actually 156VC if i understand this correctly then why do we bother using RMS to record voltage why do we not just use the actual voltage?

2, Again with measuring voltage why sometimes especially in the case of the mVdc setting why does the voltage start at 5mVdc and slowly build to 50mVdc when viewing on a multimeter i have a good fluke meter?

Thanks All

Offline helius

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Re: Confused about Multimeter RMS and mVdc Setting
« Reply #1 on: February 21, 2017, 12:02:29 PM »
1. AC is continuously changing in a sine pattern, from 0 to the positive peak, back down to 0, to the negative peak, back up to 0... over and over at the frequency of the signal. What do you propose to display as the "actual voltage"?
A. The positive peak?
B. The difference between the peaks?
C. Or the voltage that if it were held constantly, would supply the same amount of power?

RMS is option "C". It's not more correct than the others, but it is the reading that every multimeter gives you. Finding options "A" and "B" requires an oscilloscope.

"True RMS" means that this reading (the voltage that if it were held constantly, would supply the same amount of power) will also be correct for signals besides sine waves (square or pulse waves or harmonic sums of sines). It doesn't refer to anything else; there is no difference whatsoever with respect to measuring sine waves between "True RMS" and other multimeters that don't have it ("clean" AC power is a sine wave).

2. This depends on your circuit, but is often caused by the probes not making a perfect contact.

Offline naldo

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Re: Confused about Multimeter RMS and mVdc Setting
« Reply #2 on: February 21, 2017, 01:04:00 PM »
Good evening friends, see if it helps the understanding:                                                                                                                                                 - Peak to peak voltage (Vpp): It is the sum of the negative and positive peak of the sinusoid,
- Effective voltage (Vef) and RMS voltage (Vrms): they are EXACTLY the same thing,
- Effective voltage (Vef or Vrms): 70.7% of the peak voltage,                                                                                                                                         - Peak voltage (Vp): is the measurement from zero to the maximum value of a positive or negative pulse,
- Average voltage (Vmed or VCC or VDC): 63.7% of the peak voltage.                                                                                                                                 

Offline Brumby

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Re: Confused about Multimeter RMS and mVdc Setting
« Reply #3 on: February 21, 2017, 03:37:29 PM »
The first thing to be aware of is the reason why the RMS value of any quantity is taken as the number that is used in the majority of measurement and, hence, discussion - and that is because of its usefulness in so many areas.  That usefulness more often than not, comes down to making the maths manageable.

The classic explanation of RMS is an example of its usefulness - and Wiki offers an example of that:
For a cyclically alternating electric current, RMS is equal to the value of the direct current that would produce the same average power dissipation in a resistive load.

NOTE:  Much of what is discussed - including in this thread - is based on the AC waveform being sinusoidal.

With a pure sine wave, the process of determining RMS is fairly straightforward and has been pretty well mastered many, many years ago.  Getting RMS from non-sinusoidal waveforms can be more of a challenge.  Some are not at all difficult - such as a square wave - but others are.

If you want to get into the nitty gritty of this, I might suggest you start by looking up "Crest factor".

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