True RMS describes how an instrument responds to a waveform within its specified operating conditions. It does not promise accurate readings at every frequency, amplitude or pulse shape. Check whether the function measures AC only or AC+DC, then check bandwidth, crest factor, peak input and minimum reading limits for the selected range.
Start with the quantity you need
An AC-coupled True RMS function commonly reports the RMS value of the changing component after rejecting DC. A total AC+DC RMS function includes both. These quantities differ when a signal has a DC offset; the word “True RMS” alone does not identify which one a meter reports.
For a steady waveform with mean VDC and a zero-mean AC component, total RMS = √(VDC² + VAC,rms²). An illustrative 3 V DC offset plus a 4 V RMS AC component gives 5 V total RMS. That identity does not guarantee that any particular meter can measure the two components correctly.
Fluke’s explanation [1] is useful for the difference between average-responding and RMS measurement. The function-specific manual is still needed to identify coupling and restrictions.
The five conditions to find in the manual
| Condition | Question to answer |
|---|---|
| Frequency response | Within what band is the stated accuracy valid? |
| Amplitude | Is there a minimum reading or fraction of full scale? |
| Crest factor | What peak-to-RMS ratio is allowed, and does it change with range? |
| Peak input | Could narrow peaks overload the input even with a modest displayed RMS? |
| Coupling / filtering | Is this AC, AC+DC, or a filtered VFD function? |
Crest factor is peak magnitude divided by RMS for the waveform under consideration. A narrow pulse train can have a high crest factor. Its repetition rate alone does not describe the frequency content the meter must handle.
Why a PWM reading can be misleading
A pulse-width-modulated signal contains a fundamental and harmonics. A meter with limited frequency response may attenuate part of that content; AC coupling can remove its mean; peak limits can also be exceeded. A steady display can consequently represent a repeatable but incomplete measurement.
A low-pass mode deliberately changes what reaches the measuring system. It may be useful for the application described by its manufacturer, but its result should not be silently equated with the unfiltered total RMS of the original waveform.
For any waveform comparison, record repetition frequency, duty cycle, peak amplitude, offset, range, coupling and filter settings. State which quantity the reference system calculates. Avoid treating a scope display or another meter as an unquestioned reference.
What published tests establish
HKJ’s frequency-response work [2] includes a 1 V RMS sinusoidal sweep, with conditions described on the source page and some reader-contributed measurements. Such a sweep is evidence about response under those conditions. It is not a complete test of arbitrary pulse shapes or every range.
Mark Hennessy’s UT210E observations [3] include behaviour with DC offset. That is a useful reason to examine coupling, not proof that every unit or later revision behaves identically.
These are external observations, not Meter & Probe experiments. We link the original work so the setup and limitations remain visible.
Sources & scope
Sources checked on 22 September 2026. Numbered references above point to the original publishers below. Their sample measurements are not Meter & Probe measurements; manufacturer material is identified separately.
- Fluke — What is true RMS? ↗Manufacturer explanation
- HKJ — Multimeter frequency response ↗External measurements; conditions and contributors vary
- Mark Hennessy — UNI-T UT210E ↗Independent observations of a particular sample
No product in this guide has been bench-tested or certified by this publication. Evidence policy · Corrections
Keep the conditions with the reading.
Use the blank CSV to record the instrument, range, reference, environment and result.
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