What is an RMS Multimeter?

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Troubleshooting any electrical equipment or circuit can be challenging if you do not have the proper tools. A multimeter can measure a wide array of properties like conductance, resistance, inductance, and frequency. Many engineers and technicians who are exposed to any field of electronics understand the value of a multimeter.

Multimeters save a huge amount of time by detecting electrical faults. You will find a lot of things you should know about these devices. One of their features is RMS. A root means square (RMS) multimeter is a measurement tool measuring the effective level of AC or voltage.

Root mean square is a mathematical word that describes effective level. Other multimeters in the market today are created to measure only the average level. For any power measurements, the true RMS multimeter offers the proper current or voltage level. 

The Fundamentals of True RMS 

As mentioned earlier, RMS refers to the root mean square, coming from a mathematical equation that measures the “effective” value or the heating value of an alternating current wave shape. Technically writing, the AC rms value is equal to the DC heating value of a specific waveform—current or voltage.

For instance, suppose a resistive heating element in an electric furnace is valued at 15 kilowatts of heat and 240 volts AC rms. You would then receive the same amount of heat if you applied 240 volts of DC rather than AC. 

You see, electrical power system components like thermal elements of circuit breakers, bus bars, conductors, and fuses are rated in RMS current. That’s because their major limitation has to do with heat dissipation. 

Would you like to test an electrical circuit for overloading? Then you need to check the RMS current and compare the computed value to the rated value for the component in question. 

What about if the current lamp is rated and designated to respond to the true RMS value of current? It then suggests that the internal circuit of the clamp measures the heating value based on the RMS formula. That strategy will offer you the proper heating value irrespective of the current wave shape.  

Specific low-cost current clamps that do not have true RMS circuit normally utilize a shortcut tactic to locate the RMS value. Such meters are designated to “average responding-RMS indicating.” Further, those multimeters obtain the rectified average of the AC waveform and scale the amount by 1.1 to measure the RMS value. 

In short, the value they show isn’t the true value. Instead, it’s a measured value according to the hypothesis about the wave shape. The standard responding way works for pure sine waves but can result in massive reading errors of about forty percent. That’s especially true when a waveform is slanted or inaccurate by non-linear loads like computers or adjustable speed drives.  

Considering the Differences Between RMS and True RMS 

Take note that a digital multimeter that uses the averaging method to measure the RMS value of current of voltage is referred to as RMS measurement. The RMA value of the AC voltage or current has a similar heating influence that is equivalent to the heating effect of DC voltage or current of the same scale. 

You can express the alternating current or voltage for its effective values. You see, the square root of the average square of alternating current or voltage is what we call the RMS value. RMS’s ratio to the average value for an impeccably sinusoidal waveform is 1.11. Therefore, the RMS value is equivalent to 1.11 times the average value. 

Remember that the peak value for a perfect sinusoidal voltage waveform is 1.414 times the RMS value. You can assume that the RMS value of the perfect sinusoidal waveform is 0.707 times the peak value. Hence, the average responding multimeter offers the RMS measurement properly for a pure sinusoidal waveform. 

Nonetheless, you need to remember as well that the true RMS multimeter measures the healing effect of the voltage or current waveform by sampling the waveform itself. Further, the True RMS multimeter features a high sampling rate.

The numerous samples for a waveform offer an accurate heating effect. Hence, it’s considered the most accurate and efficient way for measuring distorted voltage or current waveform. 

Here’s Why You Should Start Using RMS and True RMS Multimeters

Take note that the standard responding multimeter can measure the current and voltage. These multimeters also offer exact readings for linear loads. In case you didn’t know, those linear loads are composed of:

  • incandescent lamps 
  • induction motors
  • heaters 

The linear load attracts current in phase along with the applied voltage. Hence, the current attracted by non-linear loads grows in proportion to the voltage. In short, the current acquired by linear loads is sinusoidal. 

In regard to linear loads, both average responding and true RMS multimeters can be utilized to measure current or voltage.

But there’s one thing you need to remember, as the average responding multimeter reads quite low. You use a true RMS multimeter to measure the current of non-linear loads, including electronic equipment, variable frequency drive, and DC drive.

The semiconductor devices such as IGBT, SCR, and diodes feature non-linear characteristics. The current drawn by such devices isn’t linear to the applied voltage. That’s why the current waveform is one-sided. 

Semiconductor devices create harmonic current within the electrical network. An average multimeter can’t measure the non-sinusoidal current precisely. Hence, non-linear loads require an RMS or true RMS multimeter for testing voltage and current.

What about if the multimeter is specified and categorized to respond to the current’s true RMS value? It suggests the meter internal circuit measures the current or voltage’s heating value. That technique offers exact and real measurement regardless of the current wave shape. 

Final Thoughts

A true RMS multimeter reads accurately for all types of wave shapes. That’s especially true, whether it be a triangular wave, square wave, or sine wave. So, there you have it! We hope we have provided you with the information you need and answered your question on “what is an RMS multimeter.” What are your insights about this post? Share your thoughts with us by leaving your comments below.

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