RMS - what didn't I get?

[bdunham7]

Wouldn't the easiest way to verify all of these things be to use one of those (expensive) thermal converter gizmos? I thought that's what they were for.  RMS defined as "equivalent heat?"

You'd still have the issue of whether or not the input amplifier is AC or DC coupled.  I have a Thermal RMS meter and it give me both options--so I have to choose.  RMS is not defined as equivalent heat, it is just commonly explained that way.  RMS is a term that defines itself (root of the mean of the squares) but it works out to be the equivalent of DC for heating a perfectly resistive load.

[TimFox]

As mentioned above, rms has a simple mathematical definition.
If you square an rms voltage, you have the mean of the voltage squared.
Since power into a resistive load is proportional to the square of the voltage, you then get the mean power into that load, which heats it.
Note that this is the mean (or average) power, not the rms power (despite popular usage).

[horo]

But I've not yet seen one of these meters that doesn't make it quite clear on the face of it, without referring to the manual, that it is measuring AC volts.

My Fluke has two ranges: AC and DC, while my Gossen Metrahit29s offers AC, ACDC and DC. So for the Fluke I know that I have to add both values (quadratic) w/o reading the manuals.

[garrettm]

[...]
Note that this is the mean (or average) power, not the rms power (despite popular usage).

Correct. The quantity calculated by multiplying RMS voltage and RMS current is *average* power. Or, using the squared RMS voltage or current, is *proportional* to the average power (assuming a purely *resistive* load). While it can be computed, the RMS value for a given power signal has no physical meaning and should never be used in practice. So if anyone ever says "RMS power" be wary: they are either confused, or were taught wrong. Also, and I believe I saw someone mention this possibly by accident, DC is measured/computed as an average value not RMS. Hence there is no such thing as "DC RMS" (as that would simply compute the true (AC+DC) RMS value -- at least when using the sampling method).

For those who don't believe me about RMS power not having physical meaning please see ADI's Analog Dialogue article "RMS Power vs. Average Power":

https://www.analog.com/en/analog-dialogue/raqs/raq-issue-177.html

[r6502]

Hello All,

Bandwidth is very importand. I took just a look on the specs of my hendheld multimeter Fluke 179, that has a badwidth of only 45Hz to 500Hz in the AC rasnges till 600V and is avertised at true RMS meter.  My HP34401 bench top multimeter as a bandwidth of 3 Hz to 300kHz, but error is increasing in the low and very high frequencys, see page 218 of the linked manual.

A good explanation on "True RMS AC Measurements" can be found in the manual of the HP34401a, chapter 7, page 206ff,, see link below. Here everything you need is explained.

When you want to measure a PWM signal you need a meter that ia able to measure signals with an high Crest factor, like the good old HP 3400a or the HP3403 series as TimFox wrote in his post. Modern DSO may do it as well, when set up correct.

link to HP34401 manual

Guido