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What Is a True RMS Multimeter?

By Geisha A. Legazpi
Updated May 16, 2024
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A true root mean square (RMS) multimeter is a measurement device that measures the effective level of alternating current (AC) or voltage. Root mean square is a mathematical term that suggests effective level. Other multimeters may be designed to measure just the average level. For power computations, the true RMS multimeter provides the correct voltage or current level.

Several units can be used when measuring voltage. Direct current (DC) is simple because there the only unit is the volts direct current (VDC). A 12 VDC power supply will provide 12 VDC. AC refers to voltage levels of sinusoidal waveforms commonly referred to as sine waves. A 60 cycle per second (cps) or hertz (Hz) sine wave takes a 0.0167 second to make one cycle, which means the polarity peaks at opposite every 0.0083 second.

The true RMS voltage of electrical utility gives about 110 volts alternating current (VAC). This is equivalent to about 155.6 volts peak (V-Pk). The true RMS level is defined as 0.707 of the peak level for a pure sine wave. When the waveform is not a pure sine wave, the relationship between peak and RMS will not be a scale of 1:0.707.

There is a simple relationship between the VDC and the volts root mean square (Vrms). If 1 VDC and 1 Vrms were fed into a resistive load, the amount of power being delivered is the same. It should be noted that 1 VDC is steady instantaneous voltage, while 1 Vrms is referenced to a time-changing voltage. The words multimeter and multitester are used interchangeably, but a multimeter is used for metering or measurement, while a multitester is used for testing. Some manufacturers may argue that testers may be used just to affirm the presence or absence of voltage or current, while meters are used to measure the actual levels.

Besides power applications, a true RMS multimeter may be used in frequencies much higher than 60 Hz. Audio RMS meters measure sound voltages at various test frequencies in sound laboratories. Even in frequencies higher than audio, in ultrasonics, a true RMS multimeter is used for the calibration and maintenance of high-accuracy equipment, such as ultrasonic ranging used in submarines.

There are numerous radio frequency (RF) applications of RMS meters. In component-level troubleshooting, technicians use the RF RMS meter to measure voltages that are more than about 500 million cps. The readings provide performance indications or help isolate a faulty component. The true RMS multimeter is useful at a work site, at the repair workshop, and in research and development (R&D) laboratories.

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Discussion Comments
By Mammmood — On Feb 26, 2012

@NathanG - I am neither an electrician nor an engineer. However I do work for a software company that caters to the electrical utilities.

We hook up our software to relay test set equipment and have to take measurements about how the relays are doing with the different current loads.

We hook up a true rms multi meter to the test set and read the measurements off that device. I write down the results and hand them to our lead electrical engineer who understands that stuff a lot better than I can.

By NathanG — On Feb 25, 2012

I suppose multi meters are definitely the device to get if you’re measuring AC and DC volts. However, if you just needed to measure DC alone, then you can get by with a single DC volt meter that will probably be cheaper and easier to read than the multi meter.

I believe electrical technicians use the multi meters a lot in their work, as would electrical engineers to. When you’re reading the multi meter you need to know more than how to read the numbers. You should understand the waveforms and what they mean if you’re going to understand the results you get back.

As the article points out, AC voltage peaks and cycles, so you need to understand that to use the device.

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