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UNI-T UT70D and True RMS AC measurement accuracy!

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I asked this in another post: "Lots of useful measurements but as far as I could see (late coming here) none on True RMS current reading of non sine waveforms.  
Could an owner of one please try that for me and see how it performs."  But I guess everyone was busy and it was an old thread so I have started a new one.

Bought the meter and hooked it up to a simple series circuit.  Our 240V mains supply, Plug in Bench Meter (also True RMS and cheap as chips), fused plug, 70D and load (power 480 Ohms).  Expected I = 242/480 = 504 mA.  Plug in Meter gave 0.49A.  70D gave 0.508A.  OK

Now put in series didode (IN5406).  Expect I to fall to 0.707 (1/sq root 2) of previous.

Plug in Meter gave 0.36A against expected of 0.35A  OK

70D gave 0.275A against 0.36A expected 30% low! and not OK

Measure voltage across the load with the 70D.  AC = 243.5V  HW = 132.9V against 172.2 expected.

Rang supplier Tech support who say "looks like a design fault, take it back and buy a Fluke".


That definitely sounds useless. I know my Agilent clamp meter will vary it's amp readings by a slight amount depending on which direction I clamp it over the wire (and unlike Fluke and some others, it doesn't have an arrow on the clamp), but clamp meter's aren't supposed to be THAT exact to begin with...

Just had this from a Non Forum member friend Chuck Doose

The 70D meter is only reading the AC component of the RMS value of the waveform and not the DC component

You need to measure the average DC value of the waveform in the DC mode, then take the RMS value in the AC mode. Now to get the true RMS of the waveform take the square root of the sum of the squares. So with your mains AC voltage at 243.5 Vrms, the HW average DC should be 109.2 DC, and the RMS AC component should be 132.2 Vrms, so the actual true RMS value should be sqrt(109.2^2+132.2^2) = 171.4 Vrms

For the current the DC avg = 0.227 ADC, HW AC rms = 0.274 Vrms and the true rms of both is 0.356 Arms.

Give it a shot with the meter now and see if that works out for you.

Most "true rms" meters I've seen only calculate the AC component of the waveform, in other words they AC couple the signal to the meter and calculate the rms without the average DC level.

The Fluke 287 I have, can be set up to measure AC only, DC only or the AC+DC rms values.

 So the thing to remember is RMS of AC+DC components is added in quadrature

I haven't tried it yet but it sounds good... still think I need the Fluke though this is far too cumbersome... looked throught the translated Book and didnt see anything about this.

Why doe the window I type in on this site keeping jumping gack 10mm so I cant see what I'm typping?


Yep! tried it for current

DC = 0.228A  AC = 0.280A... do the sum and you get 0.36 for the DC + AC RMS value which is OK

I can see that this is handy if you want to read RMS of say ripple voltage

Pity they dont say this in the Book and can I live with it?  Rather like the feature of the Fluke of being able to read AC or DC or Combined


The UNI-T UT71D has a AC/AC+DC button on it to change the coupling.

A lot of meters will only do one or the other, and a lot of people don't realise it.


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