True RMS or a true lie?

The two true RMS multimeters who I'm using on a daily base measure the RMS-value in the AC voltage and current ranges. The DC-ranges measure the mean value. Nothing remarkable at first sight. Still, something curious is going one here. As the 9 volt direct voltage of a battery is measured with the multimeter in the AC-range in order to determine the RMS-value, the meter will read 0 volts. The direct voltage will be blocked. This is naturally what is expected when measuring the AC-portion of a signal with the AC-range selected.

But, this is contradictory if the effective value is measured conform the RMS-calculation. According to the definition will an effective value of a voltage or current dissipate an equal great power in a resistant as a direct voltage or current with the same value. The effective value of the battery is therefore also 9 volts!

Standard deviation

The method to measure the effective value of a signal function minus its average value is called the standard deviation. Also with this method the calculation order Root Mean Square is followed, but not from the whole signal, but from the AC-portion of the signal. You could call these multimeters RMS-multimeters because the method is used, but it does not confirm the definition and therefore misleading. To dare using the word "true" before "RMS" blows my mind.

AC + DC

Yet, there is a method to get the real effective value if the direct voltages are blocked in the AC-ranges. Therefore the signal must be measured in both, the DC as well as the AC range. The next step is to quadrate the two measured values and add them together and than take the root from this sum. A very devious method for such an essential parameter. Luckily this is recognizes by the manufacturers and nowadays there are also True RMS multimeters on the market that can measure the true effective value in a direct manner. This is often referred as "RMS (AC+DC)". Unfortunately there are just a few models available who are measuring the real value and the majority measures still "false RMS".

Why is this so important?

The effective value refers to the heat development in an ohmic resistance. Incandescent lamps for instance are very sensitive for overloading. As such a lamp is driven by a PWM driver or dimmer, what could be a pulsating direct voltage, than the effective value can't be measured with a True RMS multimeter. Measuring the voltage in the DC-range is useless because it measures the mean value.

Another case is the measurement of the current in a winding of a transformer. If this is a part of a DC power supply than there will flow a pulsating current as a result of the shockwise loading of the smoothing capacitor. In case of a single sided rectifier it's impossible to measure the effective current who's responsible for the heating of the transformer.

Read more about this and other peculiarities of multimeters in the article Measurement deviation.