UNI-T UT70D and True RMS AC measurement accuracy! - Page 1
Forum section: electronics / general

author: tempo
author_rank: Newbie
timestamp: November 10, 2010, 12:53:21 pm
content: HelloI 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.OKNow 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 0.35AOK70D gave 0.275A against 0.36A expected30% low! and not OKMeasure 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".Gary

author: PetrosA
author_rank: Frequent Contributor
timestamp: November 10, 2010, 01:09:19 pm
content: 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...

author: tempo
author_rank: Newbie
timestamp: November 10, 2010, 01:51:14 pm
content: Just had this from a Non Forum member friend Chuck DooseThe 70D meter is only reading the AC component of the RMS value of the waveform and not the DC componentYou 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 VrmsFor 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 quadratureI 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?cheeers

author: tempo
author_rank: Newbie
timestamp: November 10, 2010, 02:06:59 pm
content: Yep! tried it for currentDC = 0.228A AC = 0.280A... do the sum and you get 0.36 for the DC + AC RMS value which is OKI can see that this is handy if you want to read RMS of say ripple voltagePity 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 CombinedGary

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 10, 2010, 08:06:55 pm
content: 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.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: November 10, 2010, 09:20:54 pm
content: Quote from: tempo on November 10, 2010, 12:53:21 pmFew months back , I was looking for the same thing , I was looking to find any standards about True RMS ,and with those at hand, I would be able to separate the multimeter's or AC/DC clamps like :True RMSJust RMSNo-RMSAnd the bottom line are , thatno one in this planet, has an clear reference about it .Gary ?you got that point ?The bottom line are , far from the theoretical behavior of one True RMS instrument ,the practical use of this instrument .The most common need for True RMS measurements are in anything that has an inverter.DC/AC AC/AC what ever .I told you and before, that benchmarking of multimeter at "True RMS" , needs very expensive calibrators.I you need some good material for reading ... download the manual of FLUKE 8012A ,in it there is some good points about True RMS .