True RMS/Averaging? Cant seem to see a difference. Extech EX430 - Page 1
Forum section: electronics / testgear

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 13, 2014, 03:21:14 am
content: Hey everyone! I have a Extech EX430 and it claims to have True RMS. After measuring the mains 110vac with the Extech and a cheap china averaging meter, I see no difference. (except for a few hundred mV because the china meter is not calibrated.) What exactly does True RMS do?

author: w2aew
author_rank: Super Contributor
timestamp: September 13, 2014, 03:26:28 am
content: Quote from: cybertronicify on September 13, 2014, 03:21:14 amYou won't see any difference for sinusoidal signals. You will see a difference when the AC signal is not sinusoidal. The true rms will be right.

author: Frenchie
author_rank: Regular Contributor
timestamp: September 13, 2014, 03:27:56 am
content: Averaging will assume whatever AC waveform you have is a sine wave and measure accordingly. RMS meters will actually give you the true RMS voltage.As you've observed for a clean-ish sine wave on the mains you'll get a very similar value. If you've got access to a function gen, try having a look at different wave forms and see if you still get the same result in those cases.

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 13, 2014, 09:54:01 am
content: I just tried the two meters on my 12v to 110vac inverter which is Modified Sine Wave. Didn't seem to make a difference. So as long as its a Sine wave, both meters will be pretty close. I have a signal generator on my phone which i can plug into a audio amp and maybe measure that. Would that work? It has a triangle wave mode.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2014, 05:57:20 am
content: Average reading meters are calibrated to read the RMS value on sine waves so they will read identically to an RMS meter on a sine wave source. On a square wave, they will read 11% high.Modified sine wave inverters produce the same RMS and peak voltage as a sine wave inverter so when properly working, they should read the same on an average reading or RMS reading AC voltmeter.

author: P90
author_rank: Frequent Contributor
timestamp: September 15, 2014, 08:39:59 am
content: try measuring the output of a household light dimmer with both meters...

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: September 15, 2014, 11:23:24 am
content: TRMS meters as stated in above posts will work on distorted sine waves and show the RMS value where non-TRMS meters will not. If you have a programmable arbitrary function generator you can create some distorted sine waves for experimental use and comparison, a little safer than running around tearing apart dimmers to measure the RMS value.But if you don't, then a dimmer is the next best thing, just take caution!Edit:I remembered seeing this a while back while searching for Extech products.

author: w2aew
author_rank: Super Contributor
timestamp: September 15, 2014, 01:43:35 pm
content: This might be helpful...

author: cybertronicify
author_rank: Regular Contributor
timestamp: September 15, 2014, 02:08:56 pm
content: I just measured the output of a triac with the gate turned to the lowest. The Extech showed 112 VAC and the cheapy shows 72 VAC. So there is a difference, even if they are all sine waves. Wow. Thanks everyone!

author: mcinku
author_rank: Contributor
timestamp: September 17, 2014, 12:30:26 pm
content: Another thing to consider....frequency!!Some "cheaper" True RMS meters will measure True RMS up to 1kHz signal (Fluke 175 for example). Better meters can go up to 100kHz for example.Averaging meter will usually fail,even if you measure a clean sine wave but with higher freq. These meters do just fine with 50Hz signals.Try to measure power output (True RMS voltage) of an audio amplifier at 5kHz slightly distorted signal.That will show you if your meter is a good one.

author: Wytnucls
author_rank: Super Contributor
timestamp: September 17, 2014, 12:48:27 pm
content: Another characteristic to consider is the crest factor (Peak/RMS). Most handheld TRMS meters can handle a maximum of 3, some can go up to 6, with some restriction on scale.

author: P90
author_rank: Frequent Contributor
timestamp: September 18, 2014, 03:50:01 am
content: And if you got any DC offset value in your waveform, you need a AC+DC RMS capable meter...

author: R_G_B_
author_rank: Frequent Contributor
timestamp: September 18, 2014, 09:57:01 am
content: Quote from: P90 on September 18, 2014, 03:50:01 amJust measure add the D.C component reading providing the meter is able to read True R.M.S

author: Wytnucls
author_rank: Super Contributor
timestamp: September 18, 2014, 10:15:47 am
content: No, you can't just add the DC component.Here is the formula to use:

author: R_G_B_
author_rank: Frequent Contributor
timestamp: September 19, 2014, 09:59:46 am
content: Quote from: Wytnucls on September 18, 2014, 10:15:47 amThanks for the missing part of the jigsaw.

author: David Hess
author_rank: Super Contributor
timestamp: September 19, 2014, 07:48:25 pm
content: Quote from: Wytnucls on September 17, 2014, 12:48:27 pmUsually derating the input voltage for a given range will proportionally increase the maximum crest factor within limits.