What should I expect when I use a clamp meter rated for 60Hz to measure current that is not 60Hz (Eg. 100Hz) ? Will the RMS current value be higher/lower accordingly?
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asked Jan 6, 2015 at 15:38
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4\$\begingroup\$ You'd have to measure and find out, unless the datasheet for your probe has frequency response specifications or a graph. \$\endgroup\$– user16324Commented Jan 6, 2015 at 15:40
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4\$\begingroup\$ If it is a "true" RMS current measuring device I'd expect it to be accurate at a 100Hz sinewave because to measure RMS with reasonable accuracy you have to take account of at least the first "several" harmonics of current. \$\endgroup\$– Andy akaCommented Jan 6, 2015 at 15:49
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1 Answer
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It depends. There are two ways to read RMS of a specific waveform:
- True RMS (expensive)
- Simple average of absolute value, then scaled by a constant to read RMS (cheap)
If your meter uses #1, you can measure any waveform that falls entirely within the bandwidth of the meter. More on that later.
If it uses #2, it'll be wrong for anything other than a sinewave, but if you know the waveform, you might be able to figure out how it's wrong and compensate yourself...maybe.
Concerning waveforms and bandwidth:
There is no such thing as just a meter. It's a signal processor. It only happens to have a single slow readout in amplitude. As a signal processor, it has a range of frequencies (bandwidth) that it will use, and it will ignore everything outside of that range. This is not a sharp cutoff, but more of a gradual fade-out. Kinda like turning down both bass and treble and listening to what's left.
Like audio, very few signals are a single frequency, even AC power. Everything is a mixture of different frequencies. Some are from the source; some are from the load; all must be captured for an accurate measurement. The limited bandwidth may cause errors even for 60Hz, depending on the meter and the waveform being measured.
For example, if you're measuring the input current to a linear power supply (AC -> rectifier -> capacitor), then you most definitely DON'T have a sinewave. It surges at the peak voltage and is "off" otherwise, unless you're looking at a power-factor-corrected (PFC) supply, which has a series of smaller surges to try and approximate a sinewave. The short time of these surges creates a wide range of frequencies that all have to be considered to get an accurate RMS or even average measurement. If your meter misses some, you won't get the whole picture.
That being said, if your meter has a wide enough bandwidth to cover complex waveforms at lower frequencies, it will probably handle simpler waveforms at higher frequencies just fine. Just know what you're dealing with in terms of both signal and instrumentation.
