Learning Library · Multimeters, explained · 3
True RMS vs. average-responding meters
Two meters on the same outlet can disagree, and both can be working perfectly. An average-responding meter is calibrated for a clean sine wave. A true-RMS meter measures the heating power of whatever waveform is actually there. Change the waveform and see which one keeps telling the truth.
Top: two cycles of the signal. Cyan dashes mark ± the true RMS value; pink dashes mark what an average-responding meter reports. Bottom: the two readings side by side.
What RMS means
The root-mean-square (RMS) value of a voltage is the DC voltage that would heat a resistor by the same amount. Square every point of the waveform, average the squares, and take the square root.
Squaring and averaging used to be expensive to do in a handheld meter, so many meters take a shortcut instead. They rectify the signal, measure its average, and multiply by 1.111, the ratio of RMS to average for a pure sine wave. On a sine wave that is exact. On anything else it is a guess.
Try this
- Leave the sine wave at 170 V peak, the shape of 120 V mains. Both meters read 120.2 V.
- Switch to a square wave. The average-responding meter reads 11.1% high.
- Switch to a triangle. Now it reads 3.8% low.
- Pick rectifier current: the narrow pulses drawn by a power supply's input capacitor, the load current of most electronics. The average-responding meter reads 38.2% low, and the crest factor is 2.32.
- Pick pulse and set the duty cycle to 10%. With AC coupling, the average-responding meter reads 33.4% low and the crest factor reaches 3.00. Switch to AC+DC and the true-RMS reading rises from 51.0 V to 53.8 V, because it now includes the pulse's DC level.
Where it matters
Variable frequency drives, LED drivers, switch-mode power supplies, dimmers, and computer loads all draw non-sinusoidal current. On those circuits an average-responding meter can under-read current by enough to hide an overloaded conductor or a breaker running hot. That is why electricians working on modern loads choose true RMS.
A true-RMS reading still has limits. Crest factor, the peak divided by the RMS value, says how spiky a waveform is: 1.41 for a sine, 1.0 for a square, and much higher for narrow pulses. A true-RMS meter specifies the crest factor it can handle. AC bandwidth sets how many harmonics it sees; the UNI-T UT181A and UT171B list a 100 kHz AC bandwidth. Outside those limits the reading falls, even on a true-RMS meter.
Measure the waveform that is really there. UNI-T true-RMS handheld multimeters.
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