When an ordinary electric meter measures alternating current, it actually measures the average value, but the scale on the table is an effective value, which is accurate for measuring the standard sine wave, and the AC signal encountered in real life may not be a sine wave of the scale, and using the average value to detect this voltage or current is not an accurate indication of the effective value. For example, a 1KΩ resistor with 220V direct current or 220V alternating current at both ends will emit the same amount of heat regardless of whether the alternating current is a rectangular wave or a sine wave. However, because the same power waveform is not the same, the average meter indicates a different value. This requires an AC meter that can really measure the effective value, without different errors due to the waveform. The result of true RMS is measured and calculated using a special true RMS chip. Because the measurement methods have been greatly different, the accuracy of effective value measurement has been greatly improved. The cost is also much more expensive, so it is called true RMS voltmeter, ammeter (multimeter), in order to distinguish the ordinary voltage measurement expressed by the average value.
Now SCR rectifier, inverter application (such as household frequency conversion air conditioning), it is through the phase shift to adjust the voltage, it is out of the voltage is not sine wave, if the original meter scale read is not accurate. Now it can only be measured with a true RMS meter.
