What's the deal with "True RMS?"

[TaylorFade 10+ year member]

I know there are some electrical engineers on here or someone who actually knows the difference between a "True RMS" clamp meter and whatever a regular ol' clamp is called. And I am speaking of being used specifically for clamping amplifier output here.

Just reading through a few different articles, it seems that the rms is the peak current multiplied by .707 (assuming a perfect sine wave). There is no way that TRMS clamps are that much more expensive just because they do that calc for you. or maybe I'm way off on that little nugget?

Somebody school me because Harbor Freight has clamp meters for 10 bucks. Lol.

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[matt_bennett05 10+ year member]

I know there are some electrical engineers on here or someone who actually knows the difference between a "True RMS" clamp meter and whatever a regular ol' clamp is called. And I am speaking of being used specifically for clamping amplifier output here.
Just reading through a few different articles, it seems that the rms is the peak current multiplied by .707 (assuming a perfect sine wave). There is no way that TRMS clamps are that much more expensive just because they do that calc for you. or maybe I'm way off on that little nugget?

Somebody school me because Harbor Freight has clamp meters for 10 bucks. Lol.

I never knew of harbor freight until yesterday... I saw that meter for $9.99 too and was like wtf...

kinda like --->

 

[ciaonzo 5,000+ posts]

There is no spoon.

What is RMS power?

"RMS power" is a misnomer derived from the usual method of calculating speaker power, which is to divide the RMS voltage squared by the impedance (V²/Z). RMS is the acronym for root-mean-square, which relates to the mathematical equation that obtains the average of the modulus of an alternating signal (that is, with positive and negative cycles). The correct expression for "RMS power" is average power. The ratio between RMS and peak values is different for each signal.

 

[helotaxi 5,000+ posts]

An RMS meter actually uses sampling to determine the RMS voltage instead of taking the peak and multiplying by .707. This accounts for different waveforms.

 

[ciaonzo 5,000+ posts]

An RMS meter actually uses sampling to determine the RMS voltage instead of taking the peak and multiplying by .707. This accounts for different waveforms.

Did not know that, thank you!

 

[Haunz 10+ year member]

A cheap clamp meter like the one in question uses a primary and secondary wire wound around the clamp jaws to measure the current of an AC wave.. it is calibrated as the OP mentions displaying .707 the peak value of a sinusoidal wave (at 60hz)...

A real clamp meter will use a hall effect sensor, and will measure DC and any AC or pulse waveform as true RMS...

Plan on spending at least a couple hundred bucks for a decent clamp....

 

[DBBOOM 10+ year member]

Fluke 336/337 are good choices