AC vs. AC/DC clamp meter accuracies? - Page 1
Forum section: electronics / general

author: PetrosA
author_rank: Frequent Contributor
timestamp: February 26, 2010, 05:32:26 am
content: Hi all! I'm an electrician, so I don't need super accurate test instruments like some of you do, but I would like to get the best product for my money. I've been looking at clamp meters and I noticed that of the two types, the ones that only read AC amps seem to have better accuracy ratings over the ones that read both AC and DC amps. From my research, I understand that to get a DC current reading, you have to measure the Hall effect, whereas with AC all you need is a CT to measure the magnetic field. None of the meters I've looked at specify what's in the clamp. They're all rated for TRMS.So here's my quandary. Given two similar meters (with a $100 price difference...), a cheaper one that measures only ACA with 1% ±10d or one that measures ACA/DCA with 2% ±10d for the ACA readings, should I be willing to give up accuracy just to have the ability to measure DCA? Can someone help me understand what's causing the loss of accuracy when the ability to read DCA is incorporated into a meter?Thanks

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2010, 12:32:29 pm
content: It all goes back to what you're doing.I wouldn't buy a meter that only measures AC because it's no use to me as I often work with batteries.As you're an electrician DC might not be important to you.Unless you get a true sine wave meter, AC won't be much good unless you only use it to measure pure sinusoidal loads such as lamps and AC motors. Measuring the current though an SMPs, dimmer or variable speed motor drive will give you inaccurate results unless it's designed for measuring non-sinusoidal currents.

author: PetrosA
author_rank: Frequent Contributor
timestamp: February 26, 2010, 01:58:34 pm
content: Quote from: Hero999 on February 26, 2010, 12:32:29 pmAs I wrote, I'm only interested in true RMS meters. There are so many harmonics in the system nowadays from all the non-linear loads like electronic transformers, CFLs, fluorescent ballasts, computers and other electronic devices that a non TRMS meter is practically useless. Any meter I get would measure DC voltage, which is important to an electrician. There are three situations I can think of where an electrician would need DC measurements; industrial DC motors (I don't work with these), automotive/vehicle systems (nor with these) and low voltage LED lighting, which is becoming more common. The LED lighting I've worked with so far hasn't required any current measurements on the load side. And there's always the times when you have to check your battery because the car won't startWhat is important for me is the ability to make an accurate reading on AC current, especially at lower ranges. The more low voltage lighting and CFL systems we install, the more common it is to have circuits that need troubleshooting below the 10A-15A range, and sometimes below the 5A range. (I'm not even going to get into small HP VFD drives which are another beast altogether requiring the ability to measure voltage and current through a wide range of frequencies...)

author: Unknown
author_rank: Unknown
timestamp: February 26, 2010, 03:22:44 pm
content: >> What is important for me is the ability to make anaccuratereading on AC current, especially at lower ranges.Doesn't this answer your question? On the other hand if one was a Fluke and the other not, I'd almost certainly buy the Fluke. I assume both meters are from a reputable manufacturer and are Cat III or better?As for the price difference, the $ will soon be forgotten, but you will be reminded of the quality of the meter every time you use it.

author: djsb
author_rank: Super Contributor
timestamp: February 26, 2010, 03:43:35 pm
content: Hi,I was thinking of buying a clamp meter and came to the conclusion that a Fluke 336 or 337 is the best. I was particularly interested in a meter for measuring inrush current. Fluke explains all about this herehttp://fluke.informationstore.net/efulfillment.asp?publication=10461-engThis application note explains more general usehttp://fluke.informationstore.net/efulfillment.asp?publication=10561-engHope this info is useful.David.

author: PetrosA
author_rank: Frequent Contributor
timestamp: February 26, 2010, 05:31:51 pm
content: Quote from: Machina on February 26, 2010, 03:22:44 pmWell, it helps me make my decision, but it doesn't answer my question which is; why would a combination AC/DC clamp meter have less accuracy for AC current readings than an AC only clamp?

author: PetrosA
author_rank: Frequent Contributor
timestamp: February 26, 2010, 05:43:01 pm
content: Quote from: djsb on February 26, 2010, 03:43:35 pmFluke has some good reading on their site. Thanks for the links. Actually, I'm looking at Agilent. They are coming out with three clamp meters in March. I'm looking at the U1211A since it seems to have the best accuracy in the AC Amps range.http://www.home.agilent.com/agilent/product.jspx?cc=US&lc=eng&nid=-34618.920246&pageMode=PL

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2010, 06:15:07 pm
content: To measure DC you need a hall effect sensor, as you probably know, transformers don't work for DC.I would imagine that all true RMS meters will use a hall effect sensor because many non-sinusoidal loads can introduce a DC component into the signal.