If you pay attention, on the electrical equipment that you buy, such as meters, ammeter devices, ... you will see the display on the device. RMS (Root Mean Square). So Root Mean Square what? Related concepts RMS What"s included? How to calculate RMS? The following article will provide all the information you need.

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RMS - What is Root Mean Square?

RMS with full English name is Root Mean SquareIt is understood as r.m.s.It is a concept used in metrology and electrical engineering jobs.

Or more simply, the rms value of the RMS can be understood as the mean squared value of the alternating current. When passing through the same fixed resistor this r.m.s.value is equal to the value of a direct current producing the same calorific value.


Formula for calculating the value of RMS

The measurement method will be based on the sine signal of the current, there are two types of signals: the standard sine signal: no distortion, the symmetrical movements between the peaks and the wave belly.

Non-sinusoidal signal: distorted waveform, without sinusoidal characteristics.

In the case of a sine waveform, there is RMS = 1A

Mean value = 0.9AMaximum value = 1.414A

We have the formula:

Waveform factor = RMS value / (mean value) = 1,11

In the case of a non-sinusoidal waveform, there is RMS = 1A

Average value = 0.55AMaximum value = 2.6A

We have the formula:

Waveform factor = RMS value / (mean value) = 1,82

The r.m.s.value on the scale may vary as it is calculated against the mean. When the waveform is no longer in sinusoidal form many errors are encountered. Therefore, many devices that directly measure the rms value of the RMS have been invented today

Below is a table comparing the deviation when the sub-wave is sinusoidal:

Deviation table when the wave is no longer in sinusoidal form

What is the difference between True RMS and RMS?

True-RMS (True Root Mean Square) with its full name is the real r.m.s.value, this is a term used to denote the accuracy of the actual current value.

Used to represent the mean measurements of the squares of other values ​​over a given time period

When calculating current or voltage values ​​to return the r.m.s. we can use methods that involve the true r.m.s.value (True-RMS) or mean r.m.s. Mean)

When measuring with non-distortion sinusoidal currents the measured value will be the same for the two modes. But in the case of current distorted with variable voltage impulses and the waveform is no longer normal sinusoidal, the measured values ​​in the two modes will be different.

For multipurpose metering equipment or ammeter that only use Mean method, the displayed value will be deviated.

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True-RMS method- Root Mean Square

This is a measurement method invented to improve the accuracy of the RMS measurement method. The measured value using the True-RMS method will now be calculated as the square root of the mean of the sum of the immediately squared values ​​over the given time period.

We have the formula:

Formula for calculation

Why use a power meter with True RMS

As analyzed above, when calculating, if there is an electric meter True-RMS will give higher accuracy, always with lower error when the waveform is non-sinusoidal.

Because there are superior advantages, the products use the measurement method True-RMS There will be a higher price, so you need to consider choosing and buying the product

Some devices use True-RMS

Multimeter HIOKI

This is the product line with the brand originating from Japan. A digital multimeter with wide range measurement capabilities. Both RMS and True-RMS accurately measure waveforms.

Performing both methods of measurement to bring different measurement results, to measure accurately requires the use of True-RMS

The metrological product line is designed with the rigorous manufacturing processes coming from corporate headquarters.

Products are manufactured using the most advanced technology capabilities with high quality, always creating the most beneficial products possible for customers.

Multimeter HIOKI

HIOKI pliers

High-end measuring products with modern and accurate True RMS measurement function. Product design with high durability, withstand temperatures from minus 25 degrees C to about 65 degrees C.

With dust and water resistance, it is possible to work in confined spaces. Besides, this is a device capable of measuring currents with voltages up to 1700V. Used in the measurement of electrical equipment as well as solar energy.

HIOKI pliers

APECH multimeter - Measure Root Mean Square index

This is a line of watches originating from Taiwan. Various functions such as measuring current, measuring capacitors, resistors, testing diode, ...

Using True-RMS feature, the accuracy is extremely high. The degree of error reduction 40% compared to other types of watch equipment

Root mean square - root mean square

True-RMS measurement place - Root Mean Square

Currently, the circuits have a lot of non-sinusoidal waves, so to measure the parameters, it is necessary to have meters with True-RMS function. Such as:

Variable speed drivesElectronic ballastsComputerSolid state environment

In these environments, currents in the circuit occur in short and distorted pulses rather than in standard sinusoidal waveforms. The waveform affects the current reading, the accuracy of the measured electrical values.

In addition, a machine using the True-RMS function would be the best choice when measuring currents with indeterminate AC characteristics.


The above article is information that Ocean Corp has shared with you about the definition root mean square what, as well as the concepts related to True-RMS, and how to calculate RMS parameters in the cases. Hope to help you understand the characteristics of RMS. If you have questions and are interested in this matter, immediately contact Dai Duong Corp for the best consultation support