True RMS ? - Page 1
Forum section: electronics / testgear

author: SG-1
author_rank: Regular Contributor
timestamp: May 21, 2018, 11:38:09 pm
content: Why do DMM manufactures claim "True RMS" on the AC settings when they strip out the DC component ?In order to get close to the correct reading two measurements must be made & summed or if so equipped the meter must be in AC+DC mode.It seems to be an industry standard that the DC is NOT included & some assembly required.

author: helius
author_rank: Super Contributor
timestamp: May 21, 2018, 11:59:41 pm
content: When you put a meter on a AC setting, you would be greatly surprised if it was including DC in the measurement. TRMS is simply a description of the measurement technology, which involves measuring the heat dissipated by a resistor. This permits correct readings for non-sinusoidal waveforms, up to a specified crest factor which will be in the DMM's spec sheet. Meters without TRMS converters perform Average measurement of AC signals, and then apply a correction factor thatassumesthe signal is sinusoidal, \$ \pi \over {2 \sqrt 2} \$ times the average.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: May 22, 2018, 01:54:55 am
content: From auseful Wikipediaarticle:Quote

author: Tomorokoshi
author_rank: Super Contributor
timestamp: May 22, 2018, 02:55:36 am
content: This application note has some useful information:http://www.analog.com/media/en/technical-documentation/application-notes/an106f.pdf

author: SG-1
author_rank: Regular Contributor
timestamp: May 22, 2018, 05:05:41 am
content: My main point is that when someone is measuring a signal with the meter in ac mode they my or my not be reading the RMS value of the signal. The manuals do not mention this. The only way to know is to test the meter. I do see an advantage with it being done this way, in that you can see the contribution of the AC & DC separately. It would be nice if this were part of the instructions.Can the true rms technology actually measure a voltage signal rich in harmonics correctly ? (Provided they are within the stated frequency range of the meter.)I have a functional RMS meter built in the late 50s in a wood case. It does not strip out the DC component. As a matter of fact it does not have separate AC & DC voltage settings.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: May 22, 2018, 05:33:06 am
content: I think most True RMS meters say so on the outside since it's such a strong selling point. The two I have do (a Fluke and an Extech).

author: bugi
author_rank: Regular Contributor
timestamp: May 22, 2018, 07:01:37 am
content: Also, it seems to be common to advertise it if the meter has any DC+AC -modes (there can be multiple such), that is, modes which show both the DC and AC components with the dual display.As an example, brymen's 860-series datasheethttps://brymen.eu/wp-content/uploads/biall/102092/102092.KARTA_EN..2015-07-09.1.pdfIt touts DC+AC true rms quite a few times.Manual, on the other hand, is less clear about whether it has true RMS or not and when or how it might be in use. But on the other hand, it includes quite good general explanations on page 4:https://brymen.eu/wp-content/uploads/biall/102092/102092.INSTRUKCJA_EN..2014-11-04.1.pdf

author: LazyJack
author_rank: Frequent Contributor
timestamp: May 22, 2018, 10:24:09 am
content: Every reputable meter says in the instruction manual, how it measures RMS. Fluke even gives examples of how to calculate the combined values.It also specifies the frequency limits for the harmonic content where the measurement is within specs.In most of the usual use cases it is more useful to have the separate DC and AC RMS values. You can still easily add them in the few cases where it is needed.