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I am looking at getting a clamp ammeter for measuring AC and DC currents, but I will be mostly focusing on AC. I'm on a student budget, so cheap = good.

Many devices do not integrate PFC circuits, and so have non-sinusodial current waveforms, tending to draw most current near the peak of the waveform. Will I need a true RMS meter to measure these accurately? Will the crest factor of the RMS conversion be important?

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2 Answers

Yes. When you have non-sinusoidal signals and want to measure their "real" voltage or current, then you need to do true RMS calculations. Ordinary voltmeters and ammeters take the average of the absolute, assume the original was a sine, then scale the result to provide correct RMS for that (sine) case.

Note that if you want to find real delivered power, just having true RMS of the voltage and current separately still isn't enough. If you know the voltage and current have the same waveshape and are in phase, then you can multiply their RMS values to get real power. With arbitrary voltage and current waveforms, the only way to get real power is integrate the instantaneous product of the voltage and current over a whole cycle. Unfortunately for you,

Ave(Volt(t) * Curr(t))

Is not the same as

Ave(Volt(t)) * Ave(Curr(t))

Physics can be so inconvenient at times, especially when you're on a budget.

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You only need True RMS if you are measuring waveforms that are not a pure sine wave.

From Wikipedia:

When measuring the value of an alternating current signal it is often necessary to convert the signal into a direct current signal of equivalent value (known as the root mean square, RMS value). This process can be quite complex (see root mean square for a detailed mathematical explanation). Most low cost instrumentation and signal converters (for example handheld multimeters of the sort used by maintenance engineers) carry out this conversion by filtering the signal into an average value and applying a correction factor.

The value of the correction factor applied is only correct if the input signal is sinusoidal. The true RMS value is actually proportional to the square-root of the average of the square of the curve, and not to the average of the absolute value of the curve. For any given waveform, the ratio of these two averages will be constant and, as most measurements are carried out on what are (nominally) sine waves, the correction factor assumes this waveform; but any distortion or offsets will lead to errors. Although in most cases this produces adequate results, a correct conversion or the measurement of non sine wave values, requires a more complex and costly converter, known as a True RMS converter.


What the hell, it ate my entire earlier edit.

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