I have an application for measuring phase angle on an electrical supply by measuring voltage, voltage across a resistor for current and the vector sum of both. Use of trigonometry derives phase angle. But you need to use an averaging meter for good results. When the current and volts are in phase the averaging meter will show this, but the rms meter could give a false phase.
You need simultaneous measurement setup of voltage and current (preferably an oscilloscope) to get a phase angle or power factor. I don't know what you do, but it sounds like TRMS or AVG DMM would make no difference.
Eh? The term "phase angle" implies that we're dealing with sinusoidal signals, and with sinusoidal signals, you can indeed (at least theoretically) perform the vector sum of voltage and current and compare it against voltage and current signals alone to derive phase angle. E.g.
If the phase angle between voltage and current is 0 degrees, they combine constructively, so the RMS of (voltage + IR) will be (RMS of voltage) + (RMS of IR).
Phase angle is 90 degrees, RMS of (voltage + IR) will be hypotenuse ( (RMS of voltage), (RMS of IR))
Phase angle is 0 degrees, destructive inteference, RMS of (voltage + IR) will be |(RMS of voltage) - (RMS of IR) |.
Everything inbetween is given by the cosine rule. So, with three DMMs, you can measure phase difference. Is it accurate or an advisable way to do it? No comment
The thing I don't get is why using an RMS meter would cause problems in particular. If your signals are truly sinusoidal, the amplitude ratios between the summed and non-summed signals will be the same, whether or not the DMM is RMS. Replace all instances of RMS above with mean-of-absolute-value, and it still works just fine. Any function which obeys k*f(X)=f(k*X) (i.e., double any waveform, and the reported voltage doubles) works fine.
EDIT: Just to be crystal clear about this, if you have one signal that the DMM reports as 12V, and another that the DMM reports as 1V, and they're identical modulo scaling ("in phase" in the sinusoidal case), the summed signal will report as 13V, both for an averaging meter and an RMS meter (even though different DMMs might have a different opinion on what a given signal is, they will all [at least theoretically] follow the summing rule I've just stated).