True RMS Meters, The Value Of.

[Gary_S]

I have an Extech 330 and it doesn't have RMS capability. Now on reading all the NEETS material and studying the opening chapters of Merlin's Power Supplies book, AVG readout is not that helpful really and the RMS is the most useful and best readout to have. Why do most manufacturers not build all their meters in the RMS style? The list of instances where an RMS meter is vital or beneficial is vast yet you get all these meters that are standard.

Anyway is this meter any use to me for working on my amps in the future? as in is there any way to calculate RMS voltage from and AVG readout?

I know you can get RMS voltage from the peak voltage by multiplying peak by 0.707 and peak from RMS by multiplying RMS by 1.414 but haven't seen a way of calculating from AVG to RMS with a formula for it.

Reading between the lines this might not work anyway because i read if a waveform is not a perfectly sinusoidal wave then the readout is not gonna be accurate and might be under by up to 30% or over by as much as 8%.

Correct me if i'm wrong here guys with any of my thinking. 

[sluckey]

AC line voltage is expressed as a RMS value. So are power transformer voltages. And if you want to measure the actual output power of your amp, an RMS voltmeter is handy and will allow you to derive a meaningful power number. Those are the main reasons that make a true RMS meter important to me.

[jazbo8]

The key issue is the waveform type as you already mentioned, for a purely sinusoidal waveform, the conversion factor is 1.11 (Vrms/Vavg = 0.707/0.637 Vpk), so depends on how distorted the signal is, a rough estimate can still be made with Vavg, but if you want to have high precision, then a true RMS meter is the only way to go.

Jaz

[PRR]

> ...is this meter any use to me for working on my amps in the future?

Utterly useless. But don't throw it in a landfill. Send it to me.

> Why do most manufacturers not build all their meters in the RMS style?

I remember when it was a $200 chip. It's probably $2 now. But they still need need a way to differentiate $50 models from $100 models. So you will never find RMS on the Champ/Pinto end of the model range. It's held-back until you get to the Buick/Bandmaster end of the row.

> ...get RMS voltage from the peak voltage by multiplying peak by 0.707....
 
ONLY for Sine wave.

And if measuring Sine or Sine-like waves, you do NOT need to do the 1.414 dance: it's in there.

Seriously. You do NOT need RMS for much.

DC is always RMS.

Most audio is tested with Sines. *ALL* meters read the RMS of a Sine--  otherwise they would not give the expected number for Wall-Voltage.

When we distort audio into non-sines, the "RMS" number shown by an averaging meter is not the true heating power. But we don't really care. We can distort as little or as much as we want. On stage, whatever the moment calls for. Who cares what the silly meter says? We are not asking for "precision".

Yes, if you crank a "5.7 Watts clean Sine" amp into total overdrive, the RMS meter will correctly show the ~~10W of heat available to the speaker. And an Averaging meter will show a number so very close that the difference is insignificant.

The major "other" type is Peak (better, Peak To Peak). If aiming for clean, P-P is better than RMS because it shows if your peaks are within the amplifier's capability. If testing for overdrive, P-P may be misleading because the P-P number stops rising when clipping begins, while Ave and RMS keep rising as the waveform gets all bent and phattened. OTOH, this means a quick-test for clipping power is to wind the P-P up until it won't go any higher, and round-down.

One place where RMS may be "needed": calibrating SCR lamp dimmers. We need to know the heat in the lamp at 25% 50% and 75% fader setting. The SCR chop-wave is VERY un-sine-like. An Averaging meter will get you in a ballpark, a smooth fade; but your new-fixed dimmer won't track the factory calibrated dimmers. In a "fade to black", the mis-calibrated dimmer may ghost when the stock dimmers are essentially dark. Lighting Designer whines about that.

If you have to run lamps and heaters on UGLY waveforms, like taps on TV CRT H-sweep systems, you need the RMS number. Hope you never care. If you do, rig a lamp from ugly-power and lamp on pure DC, compare brightnesses.

[Gary_S]

Thanks PRR, so i don't need to bin this meter then    So for amps, even though this is a non RMS one, it should still be accurate enough for most things?

[The_Gaz]

As far as measuring output power is concerned, my cheapo averaging meter is less than a volt off from my Fluke when measuring 100W amps. Close enough for rock.

[jjasilli]

A meter is a meter is a meter. . .  For VTVM's 5 - 20% accuracy prevails.  So my solution is to calibrate a VTVM to the voltage range for which I expect to use the meter.  My meters (and throw me in too) wouldn't know "true" RMS from pain old, everyday, run of the mill RMS.

[LHPcope]

As far as measuring output power is concerned, my cheapo averaging meter is less than a volt off from my Fluke when measuring 100W amps. Close enough for rock.

And that's less than 0.086 db, just to illustrate the point