Nice video about RMS measurments ..    - Page 1
Forum section: electronics / general

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 09, 2010, 03:41:57 pm
content: Thats a great video ... helping in understanding of RMS power ...I do repairs , and I repair also UPS back-up units ...In some cheap UPS back-up units , the output in volts was looking truly out of specs ...I own an APC 1000 XL , the output voltage was always correct , when was measured with my ordinary multimeter ..After looking the above video , I just realized , that the quality of AC volt ( clean enough ),could effect the readings, of one ordinary multimeter , by allot !!I do not know If I must fly from joy , because I just got an Fluke 87-5 (V) ,and probably I will never see again , misleading readings from bad quality AC ( all sorts of inverters).But hey, I just found a good reason to feel good about ..

author: Simon
author_rank: Unknown
timestamp: April 09, 2010, 08:24:46 pm
content: hm thats nice to know, pity my newly aquired amprobe AM220 is calibrated and not true RMS, I suppose for non sinewave stuff it is best to use an oscilloscope which in the case of the digital ones will tell you the RMS voltage. Luckily I work in DC so I'm not too bothered about AC unless its the mains anyhow

author: RayJones
author_rank: Frequent Contributor
timestamp: April 09, 2010, 11:17:22 pm
content: One caution with true RMS meters - even the some Flukes - is the bandwidth of the true RMS measurement.We have equipment at work that uses a 20kHz filament heater voltage, that needs to be set correctly, nominally 5.5V.The waveform is not sinusoidal, so you need a true RMS meter.The budget Fluke true RMS models will read this voltage as <2V.A Fluke 189 will however read as the correct 5.5V.The true RMS bandwidth of the Fluke 189 is speced as 100kHz.The other Fluke meter is only speced as true RMS, butno BW mentionedCaveat Emptor.

author: RayJones
author_rank: Frequent Contributor
timestamp: April 09, 2010, 11:32:05 pm
content: Just found the spec for the Fluke 87VAC Bandwidth 20kHz**with low pass filter; 3db @ 1kHzHuh?3dB down at 1kHz, how much dB is it down at 20kHz?I'm thinking this would read the <2V mentioned above!And checking for the 189 - it is now discontinued - so Fluke 289 instead:Fluke 289:True-rms AC bandwidth 100 kHz

author: desolatordan
author_rank: Regular Contributor
timestamp: April 10, 2010, 12:06:24 am
content: Quote from: RayJones on April 09, 2010, 11:32:05 pmWell, the low pass filter can be enabled/disabled. With it off, it'll be -3dB at 20kHz.

author: RayJones
author_rank: Frequent Contributor
timestamp: April 10, 2010, 12:41:28 am
content: Ah OK, the difference is now obvious!Nevertheless, for our application where the signal can head slightly north of 20kHz the Fluke 87 is not to be trusted.Somebody labelled the one at work "It's a fluke if this thing reads filament volts correctly"

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 10, 2010, 01:32:09 am
content: Quote from: RayJones on April 09, 2010, 11:32:05 pmLow Pass Filter (87)The 87 is equipped with an ac low pass filter. When measuring ac voltageor ac frequency, press A to activate the low pass filter mode (K).The Meter continues measuring in the chosen ac mode, but now the signaldiverts through a filter that blocks unwanted voltages above 1 kHz, refer toFigure 2. The lower frequency voltages pass with reduced accuracy to themeasurement below 1 kHz. The low pass filter can improve measurementperformance on composite sine waves that are typically generated byinverters and variable frequency motor drives.Thats from the book ..Just email them your question.