What is a True RMS multimeter (and why does it matter?)
Content
In the past, most AC electricity was used in its simplest form, with a pure sine (sine) wave. However, as technology advances, we now have many electrical loads with different waveforms, such as actuators and even basic lighting controllers. However, it is recommended that you use a true-rms multimeter for this because, unlike other equipment such as an average response meter, it can reliably measure these waveforms.
In this guide, you will learn what true RMS is in a multimeter and how it accurately measures pure and non-sinusoidal waves.
What is True RMS in a Multimeter
True RMS, which means "RMS", is a mathematical term and one of the tools for accurately measuring alternating current (AC) or alternating current voltage. It evaluates the "heating" potential of the applied voltage. Unlike an "average response" measurement, a true RMS meter measures the power wasted in a resistor. It's also the only meter that can reliably measure both sinusoidal and non-sinusoidal AC signals. That is why it is widely used.
- sinusoidal (sine) waves are pure, distortion-free waves with symmetrical peaks and valleys.
- non-sinusoidal the waves are distorted with peaks, trains, squares, triangles, sawtooth, and other jagged or angular waves.
Most electronic loads are characterized by non-sinusoidal waves, which makes it difficult to measure them with an average sensitivity meter. Thus, it is best to use a true RMS meter to measure AC voltage that is not a pure sine wave. Depending on which DMM you are using, the waveform (pure sine or non-sinusoidal) can significantly affect readings. (1)
Average Response Meter and True RMS Meter
When measuring pure sine waves, the average meter takes pure sine waves and calculates the average of the largest and smallest portions of the wave. It is then multiplied by 1.11 or the conversion factor. Hence it is RMS, but that's fine when you only have linear loads. If it is uneven or distorted, this average will be higher or lower than it could be, 40% lower or 10% higher depending on the waveform, making it inaccurate.
However, true RMS is reliable for both non-sinusoidal and sine waves. It converts the data using mathematical calculations and provides accurate measurements of the signal type.
Example
When you check the output voltage with a real RMS meter on a portable grinder with a regulator that controls the speed of the motor, you will see 79VAC. However, when an identical measurement is made with an average meter, the result is 65 VAC. As a result, the error is 18%.
Multimeter Response Chart
| Signal type | Average response | TrueRMS |
| sine wave | Right | Right |
| square wave | 10% high | Right |
| Single phase diode rectifier | 40% Low | Right |
| Three-phase diode rectifier | 5-30% Low | Right |
RMS Calculation
Let's move on to calculating it. The RMS value estimates the DC equivalent value of an alternating current (DC) waveform. It defines the "effective" or constant heating value of any AC waveform in a more technical sense.
Formula:
- В AC+DC = √ В AC 2 + В DC 2
- V High Frequency = √ V Total 2 – V Low Pass 2
Example:
A 120VAC or 120VAC signal and a 120VDC battery connected to the same resistor should heat the resistor to the same temperature.
True RMS measurement
In recent years, the likelihood of non-sinusoidal waves in circuits has increased substantially, necessitating the use of true-rms meters. Here are some examples:
- Variable speed/frequency drives
- Solid state media
- Ballast with electronic component
- HVAC control systems
- computers
In these environments, current is drawn in short pulses instead of the smooth sine wave that a typical induction motor draws. The shape of the current can significantly affect the measurement of the current clamp. In addition, a true-rms meter is the best choice when making measurements on power lines with uncertain AC characteristics. (2)
Sample Application
HVAC is an example of using true RMS meters. Modulating systems are becoming more common as heating and cooling technologies evolve towards greater energy efficiency. If you're working with one of these modern HVAC units, you'll need a true RMS DMM to get accurate readings. True RMS measurements are required for computerized controllers, solid-state VFDs, and VFD motors to correct the problem.
We have also written some more multimeter tutorials. Check out the list below. Until our next article!
- How to measure DC voltage with a multimeter
- How to Use a Cen-Tech Digital Multimeter to Check Voltage
- How to use a multimeter to check the voltage of live wires
Recommendations
(1) waveform - https://www.sciencedirect.com/topics/
engineering/waveform
(2) power lines - https://www.sciencedirect.com/topics/earth-and-planetary-sciences/power-line
Video links
