真の実効値（True RMS）と平均値応答型のテスターはどう選ぶ？

No. Mains voltage is close enough to a sine wave that an average-responding meter is correct within its own tolerance. True RMS removes the waveform-shape error, and a sine wave has no shape error to remove. The difference appears only once the load distorts the current or the voltage.

It depends on the crest factor of the waveform. A square wave reads about 11 percent high and a half-wave rectified sine about 30 percent low. A phase-angle controlled output moves with the firing angle, so the error is not a constant you can subtract. The display stays stable throughout, which is what makes the error easy to miss.

Look for three things in the AC rows: accuracy given as a percentage of reading rather than of full scale, a stated crest factor such as up to 3 at full scale, and an AC bandwidth figure. A front panel that says True RMS without any of these has not told you the limits that decide whether it works on your load.

Yes. True RMS removes the waveform-shape error and leaves two limits in place: the crest factor headroom of the converter and the bandwidth of the meter. A True RMS meter that stops at 1 kHz is still wrong on a 10 kHz switching waveform, and a signal whose peaks clip at the top of the range reads low without any warning.