True RMS / TRMS - when did you need to measured it last time ? - Page 1
Forum section: electronics / testgear

author: boo9
author_rank: Contributor
timestamp: August 24, 2014, 02:04:32 pm
content: - Under what circumstances you did need to measure the TRMS ?- What was the signal frequency ?- What was the measurement value (order of magitude, mV, V, 10s-V 100sV ) ?I would like to get the general idea for how necessary is to have that feature in the DMM, and what parameters for TRMS measurement are necessary (bandwidth, crest, etc..)Let's assume in this topic that we all understand what is the TRMS value. If not, then prerequisite reading is topichttps://www.eevblog.com/forum/testgear/a-few-questions-about-true-rms.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: August 24, 2014, 03:18:07 pm
content: Quote from: boo9 onTodayat 12:04:32 AM>QuoteWell,As a repair tech related to IT products repairing I would say in my trade only in UPS/Inverter repairs i needed TRMS Multimeter .But whats the problem when you spend money in buying a good multimeter its gonna be TRMS hence the question is why bother .

author: c4757p
author_rank: Super Contributor
timestamp: August 24, 2014, 03:22:50 pm
content: I'd say that the biggest use for a TRMS meter is measuring AC current. Most devices that are more interesting than a big old resistor in a glass balloon have a very non-sinusoidal current draw, so it's the only way you're going to get an accurate reading.Personally, I rarely need it. Many of my meters aren't TRMS. But when I need to measure something's AC current consumption, one of the nice ones comes out.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: August 24, 2014, 03:30:12 pm
content: Quote from: c4757p onTodayat 01:22:50 AM>QuoteHi ,I Would Say Very Few High End Multimeters Are Not TRMS

author: c4757p
author_rank: Super Contributor
timestamp: August 24, 2014, 04:00:08 pm
content: Uh... yeah. So would I.

author: boo9
author_rank: Contributor
timestamp: August 24, 2014, 04:06:20 pm
content: Guys, what about the frequency of the signal ?

author: zapta
author_rank: Super Contributor
timestamp: August 24, 2014, 04:58:04 pm
content: >> True RMS / TRMS - when did you need to measured it last time ?Never ;-)

author: David Hess
author_rank: Super Contributor
timestamp: August 24, 2014, 07:39:44 pm
content: If you are trying to check the output of an inverter that generates other than a sine wave, then you need an RMS meter to measure it accurately although even that is not sufficient in all cases. They are also needed to measure the current into any non-linear load like a ballast or switching power supply that lacks power factor correction. Phase control dimmers need an RMS meter to accurately measure the their output.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 24, 2014, 08:48:01 pm
content: Quote from: David Hess on August 24, 2014, 07:39:44 pmI would even say that it is not sufficient in most cases: if you have any doubts about the inverter's performance, what you probably really want to do a power quality analysis to see how clean/dirty the output actually is or get the raw waveforms and crunch the numbers yourself. A TRMS reading is a lumped representation of too many unknowns... something could be horribly wrong yet the TRMS reading could still seem fine.TRMS is a nice thing to have just-in-case but it has somewhat limited usefulness once your needs exceed basic AC line voltage and current measurements.

author: Artraze
author_rank: Contributor
timestamp: August 24, 2014, 09:00:08 pm
content: When it comes to frequency, I'd say don't worry about it... I think the fastest I've ever metered is 120Hz rectified mains. Anything faster usually means using an oscilloscope for measurement. (Which isn't terrifically accurate, if course, but will usually get you within a couple percent.) Airplanes use ~400Hz mains, though, so maybe there's that.Do you need true RMS? Well, I think it's worth having. Without it, you're really only able to measure the mains or a mains transformer reliably. The problem is that while there are cases for which false RMS is usable, you can't always know if they apply, nor be sure if they are all you'll ever run into. So if you're getting a good, safe meter get true RMS (though good meters without it are rare!). If you're getting a cheap meter, don't worry about it, and just save your money for a good one when you need it.

author: CosPhi
author_rank: Regular Contributor
timestamp: August 24, 2014, 09:11:44 pm
content: When you measure current, TRMS makes often a big difference up to 50%.Special when you have this "modern" energy-saving lamps or single phase inverter. Just normal 3-phase motors usually don't matter much.On work I always have my Fluke T5 with me. But the T5 is not TRMS and often when I measure current from something different than just a motor I double check it with with a TRMS clamp.Last week for example I measured with the Fluke T5 0.7A. But with a TRMS clamp it was 1.3A.Because of all these more and less cheap inverters, when you look at the waveform from the current ... uhh

author: CosPhi
author_rank: Regular Contributor
timestamp: August 24, 2014, 09:24:53 pm
content: About your question about the crest factor. For current I would say at least factor 3 or better. That is also what you get usually from a Fluke clamp meter ... but just up to 50% of the range. On 100% you have usually less than 1.5 even on Flukes.My C.A. F09 clamp can measure with a crest factor up to 3.5 but also not up to full scale.