What is a True RMS Multimeter (And What it is Important?) 

In the past, most AC electricity was utilized in its most basic form, with a pure sinusoidal (sine) wave. However, as technology has progressed, we now have many electrical loads with different waveforms, such as drives and even basic lighting controllers. With that, it’s advisable to use a true RMS multimeter for this since, unlike other equipment like an average response meter, it can reliably measure such waveforms.

In general, A true root means square (RMS) is an advanced measurement technique that gives precise AC or AC voltage measurements for sinusoidal and non-sinusoidal AC waveforms.

In this guide, learn what is true RMS in a multimeter and how it accurately measures pure and non-sinusoidal waves.

What is True RMS in Multimeter

A true RMS, which stands for “root mean square,” is a mathematical term and one of the tools for measuring alternating current (AC) or AC voltage accurately. It evaluates the “heating” potential of an applied voltage. Unlike the ” average responding ” measurement, a genuine RMS meter quantifies the power wasted in a resistor. Also, it’s the only meter that can reliably measure both sinusoidal and non-sinusoidal AC waveforms. That’s why it is extensively used.

  • Sinusoidal (sine) waves are pure, distortion-free waves with symmetrical peaks and troughs.
  • Non-sinusoidal waves have distortion with spikes, pulse trains, squares, triangles, sawtooths, and other ragged or angular waves.

Most electronic loads are characterized by non-sinusoidal waves, making it challenging to measure them with an average responding meter. Thus, it’s best to use a true RMS meter to measure AC voltage that isn’t pure sinusoidal waves. Depending on what digital multimeter you are using, the shape of the waveform (pure sinusoidal vs. non-sinusoidal) might significantly impact the reading. (1)

Average Responding Meter vs. True RMS Meter

When measuring pure sinusoidal waves, an average responding meter assumes the pure sinusoidal waves and calculates the average value of the greatest and lowest parts of the wave. Then, it’s multiplied by 1.11, or the conversion factor. Hence, it’s an average sensing RMS display, but it’s okay when you only have linear loads. If it’s unequal or distorted, this average measurement will be higher or lower than it could be, up to 40% lower or 10% higher depending on the waveform, making it inaccurate.

With that, a true RMS measurement is reliable for both non-sinusoidal waves and sinusoidal waves. It converts data using mathematical calculations and provides accurate waveform type measurements.

Example

When you test the output in voltage with a genuine RMS meter on a portable grinder with the control that regulates the motor speed, you’ll see 79 VAC. However, when the identical measurement is made using an averaging meter, the result is 65VAC. As a result, it has an inaccuracy of 18%.

Multimeter Response Chart

Signal TypeAverage RespondingTrue RMS
Sine WaveCorrectCorrect
Square Wave10% HighCorrect
Single Phase Diode Rectifier40% LowCorrect
Three Phase Diode Rectifier5-30% LowCorrect

RMS Calculation

Let’s go on to its computation. RMS estimates an AC waveform’s equivalent direct current (DC) value. It determines the “effective” or DC heating value of any AC wave shape in a more technical sense.

Formula: 

  • V AC+DC = √ V AC 2 + V DC 2
  • V High Frequency = √ V Total 2 – V Low Pass 2

Example: 

A 120VAC or 120-volt signal AC and a 120VDC battery applied to the same resistor should heat the resistor to the same temperature.

Measuring True RMS

In recent years, the likelihood of non-sinusoidal waves in circuits has substantially increased, necessitating true RMS meters. Here are several examples:

  • Drives with variable speeds/frequency
  • Solid-state environments
  • Ballasts with an electronic component
  • HVAC control systems
  • Computers

The current is pulled in brief pulses in these environments instead of the smooth sine wave drawn by a typical induction motor. The shape of the current can significantly impact the current clamp measurement. Furthermore, a true RMS meter is the superior option when performing measurements on power lines with uncertain ac characteristics. (2)

Example Application 

HVAC is an example of where true RMS meters are used. Modulating systems are becoming more common as heating and cooling technologies progress toward greater energy efficiency. If you’re working on one of these modern HVAC units, you’ll need a true RMS digital multimeter to get an accurate reading. True RMS measurements are necessary for computerized controllers, solid-state VFDs, and VFD-controlled motors to debug an issue.

We’ve also written some more learning guides on multimeter. Check the list below. Until our next article!






References
(1) waveform – https://www.sciencedirect.com/topics/
engineering/waveform
(2) power lines – https://www.sciencedirect.com/topics/earth-and-planetary-sciences/power-line

Video References

About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gI realized early on carpentry was a huge passion for me and I’ve stayed in the industry for over 20 years now. This gives me a unique ability to really be able to tell you what the best tools and recommendations are. I’m not only a carpenter but I also like machinery and anything to do with electrics. One of my career paths starting off was as an apprentice electrician so I also have a lot of experience with electrical products and anything related.