Two meters can show the same number on a wall outlet and very different numbers on a motor controller. The difference is how they measure AC, and it is the reason true RMS meters cost a little more.
What RMS means
RMS stands for root mean square. For an AC signal it is the value that produces the same heating effect as the same amount of DC. When a meter says it reads 120 V AC, it is reporting that effective value.
Average-responding meters
A basic average-responding meter measures the average of the rectified waveform and then multiplies it by a fixed factor that is only correct for a clean sine wave. On a sine wave the answer is right. On anything else, such as a chopped or distorted waveform, the same math gives the wrong result.
True RMS meters
A true RMS meter calculates the effective value of the actual waveform, so it stays accurate when the waveform is not a clean sine. Every true RMS meter has limits, so check the specification for the frequency range and the crest factor it can handle.
When it matters
- It matters on AC circuits with non-sinusoidal waveforms: inverters, variable speed drives, dimmers, switching power supplies and the AC side of many chargers.
- It does not matter for DC measurements, and it makes little difference on a clean sine wave feeding a simple resistive load.
Clamp meters: AC versus DC current
True RMS is a separate question from whether a clamp can read DC current. Many clamps only measure AC current. The BSIDE ACM91 listing states 100 A AC and DC with 1 mA resolution. The listing also notes that the related ACM81 version only tests AC current, so read the model name before you order.
Meters in our shop that are marked true RMS
- BSIDE ACM91, true RMS, 100 A AC and DC
- BSIDE U6 fork clamp meter, marked T-RMS, 600 A range
- BSIDE U1 clamp meter, marked T-RMS, 100 A
T-RMS is the label BSIDE uses on these models. See the full range in clamp meters, multimeters and infrared thermometers.
A note on safety
Use a meter and test leads that are rated for the circuit you are measuring. Look for the voltage and category rating (CAT II, III or IV) printed on the meter and the leads, and never exceed them. This guide is general information, not training. Work on mains or high energy circuits belongs with qualified people.