RMS Multimeter - Page 1
Forum section: electronics / beginners

author: PsychoMaster
author_rank: Regular Contributor
timestamp: February 17, 2015, 05:43:39 pm
content: Hello AllPlease would some one tell me about RMS multimeters .Can you switch rms function off?

author: Wytnucls
author_rank: Super Contributor
timestamp: February 17, 2015, 05:56:42 pm
content: Read this:http://media.fluke.com/documents/1260729_0000_ENG_E_W.PDFTrue RMS is an integral part of a multimeter, when measuring AC. Averaging meters will only give you a proper reading for sine signals, within a narrow bandwidth. Some meters can also give you a peak to peak value.

author: Lightages
author_rank: Unknown
timestamp: February 17, 2015, 05:58:48 pm
content: Better to search on Fluke's site or search on YouTube to get a good understanding. I did not know of any multimeter that had the option to turn off trms. Some allow you to select AC or AC+DC

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 17, 2015, 06:01:56 pm
content: Why would you want to turn it off? So you can get thewrongmeasure of average signal level due to improper smoothing and estimation?

author: electr_peter
author_rank: Unknown
timestamp: February 17, 2015, 07:41:46 pm
content: Quote from: PsychoMaster on February 17, 2015, 05:43:39 pm1) Why do you want to do that?2) Just use two DMMs - one with TRMS and one with averaging.Cheap non-TRMS meters use averaging technique which worksonlywith sine wave signals.TRMS meters use AC-to-DC TRMS chipsets to do a conversion. They work for any signalprovidedsignal does have low crest factor (lower than 3 or 5 typically).Very high end (>6 digits) usually sample at high rate and compute TRMS value digitally. By playing around with the settings it ispossible to modify such multimeters to do customised measurements.

author: PsychoMaster
author_rank: Regular Contributor
timestamp: February 18, 2015, 10:42:27 am
content: I ask about turning it off because I have noticed that some rms meters that use certain processor chips are quite slow compared to other meters without rms and using the same chip.

author: electr_peter
author_rank: Unknown
timestamp: February 18, 2015, 12:11:35 pm
content: Screen update rate is independant of measurement type (TRMS or non-TRMS).Some DMMs are fast, some are slow (relatively, each person has different preferences). If you do not like update speed, just buy faster meter.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: February 18, 2015, 01:58:48 pm
content: I haven't noticed the 27/FM being any slower than the 27, but these are slow meters anyway. I haven't noticed the 83 being any faster than an 87.Hmm, maybe I should do some tests:83 vs 8727/FM vs 2776 vs 79etc.

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 18, 2015, 06:08:40 pm
content: A bit of trivia: The Tektronix DMM912, DMM914, and DMM916 handhelds have an option in their setup menus to switch between average and RMS ACV readings. This model family is discontinued and was made in Taiwan for Tek by APPA.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 18, 2015, 11:04:52 pm
content: AFAK there is only one current production hand held DMM that can be switched between RMS and average responding, the Yokogawa TY720:http://tmi.yokogawa.com/us/products/portable-and-bench-instruments/digital-multimeters/ty720-digital-multimeter/It's an expensive high end meter.

author: mjkuwp
author_rank: Unknown
timestamp: February 19, 2015, 11:42:30 am
content: I have the Yokogawa TY720 and just tested it. Indeed, the AC reading can be changed between RMS and Averaging. The result is that measuring the mains gives a slightly different reading and measuring triangle and square waves gives greatly different readings. One waveform gives the close to the same reading between the two modes - reading a sine wave out of a function generator.boring result as that is what one would expect, right? so... I don't know what the value is except possibly to crudely detect how pure a sine wave is by measuring the wave both ways.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 19, 2015, 02:25:24 pm
content: Let me give an example where having both RMS and average reading is useful.Imagine charging a 12 volt lead acid battery. I used a 12.6 VAC transformer followed by a bridge rectifier without a filter capacitor. The battery current only flows when the voltage of the sine wave out of the transformer exceeds 12 volts. This means that the current is in narrow pulses. When charging a battery, the charge received by the battery is just that--charge. We want to know how many coulombs the battery has received. If the battery is a 10 ampere-hour battery and we want a full charge, we need to apply 1 ampere (1 coulomb per second when we're charging a battery) for somewhat more than 10 hours, but we need to measure that current with an average responding meter so that 1 amp equals 1 coulomb per second. RMS is for measuring heating effect and 1 amp RMS isn't 1 coulomb/second if the waveform is peaky--it's less.I powered the transformer from a variac, which I adjusted so the RMS current out of the rectifier was 1 amp. Using an average responding meter, the current measured .72 amps; this must be divided by 1.1107 to get the true average because the meter multiplies the average current by 1.1107 to get the equivalent RMS of a sine wave. After dividing the average responding meter reading by 1.1107, the battery charging current is determined to be .648 amps, considerably less than the 1.00 amp RMS.RMS should be used where the heating effect of the current is needed, for example when we want to know if we are exceeding the current rating (which is based on temperature rise) of the transformer. Average responding should be used where the total charge transferred is important, as in battery charging or current into a plating bath or an electrolysis cell.

author: mjkuwp
author_rank: Unknown
timestamp: February 19, 2015, 06:30:47 pm
content: For charging a 12V battery, that is DC with ripple, so wouldn't you just use the DC setting and the average function? Averaging on DC is just average or mean and it will work just fine even on a meter that does TRMS for the AC functions.

author: The Electrician
author_rank: Frequent Contributor
timestamp: February 19, 2015, 08:19:09 pm
content: You could do that, of course, and it would give the measurement needed.I measured the current out of the transformer's secondary with a clamp on the secondary lead. Using a TY720 measuring AC current, I could simply switch from RMS to average and get both measurements using a clamp-on current probe that responds only to AC. The idea was to demonstrate how the ability to switch from RMS to average measurement mode could be useful.