Multimeter Reading VS Oscilloscope Discrepancy - Page 1
Forum section: electronics / testgear

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 12:18:00 am
content: Multimeters, when reading AC voltage, it's RMS right? Why am I getting such a difference between my mastech multimeter and my oscilloscope? My multimeter is a cheap one but there's thousands of reviews saying it's almost as accurate as a couple hundred dollar fluke. My multimeter is reading 660mV and my scope is measuring 1.554 RMS. Why such a discrepancy?

author: dom0
author_rank: Super Contributor
timestamp: March 24, 2015, 12:42:48 am
content: Either the oscilloscope is horribly out of calibration or (more likely) your meter doesn't measure RMS (True RMS) but is in fact an peak or average meter scaling the peak/average value so it looks like RMS (for a sine wave). Also, it might have not enough bandwidth for the signal.

author: rs20
author_rank: Super Contributor
timestamp: March 24, 2015, 12:48:30 am
content: Looks like you're using a 10kHz signal there; these multimeters are typically geared to 50/60Hz. So yeah, as suggested above, it could be a bandwidth issue. Try with 50/60Hz sine.If you want to go deeper, try feeding in square waves and sine waves and see if the measurement on the multimeter behaves like a peak or average or RMS measurement.

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 12:55:36 am
content: I tried that before with a lower frequency sine wave, the multimeter behaves like average I think, I'll double check. That makes sense with the frequency. Thanks guys, I'll try your suggestions and let you know

author: ModemHead
author_rank: Frequent Contributor
timestamp: March 24, 2015, 12:58:27 am
content: The maximum bandwidth of your meter is 200Hz in the 4V range, but your signal is 10kHz. "True RMS" is not an issue if the signal is a sine wave with no DC offset.

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 12:59:43 am
content: Well, I checked my bandwidth on my multimeter and it's rated for 200kHz

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 01:02:39 am
content: Wow thanks ModemHead .. That's strange Amazon says 200kHz

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 01:08:51 am
content: That's exactly what it was! The frequency, I changed it to 60Hz and the RMS is matching within a few mV between the scope and multimeter. So I guess anything north of 200Hz, I should use the measurements on my scope and not my multimeter. Thanks all!

author: ModemHead
author_rank: Frequent Contributor
timestamp: March 24, 2015, 01:11:08 am
content: Eh, whats a "k" among friends? If it were really 200kHz, it would be an unbelievable accomplishment on the part of Mastech. The average handheld rarely exceeds 1kHz, if even that.

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 01:14:04 am
content: I swear amazon says that. I tried to attach a screen shot but for some reason it's telling me I need to upgrade tapatalk.

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 01:19:14 am
content: Well, thanks everyone I feel a lot better now and my measurements look much better using the scope measurements

author: ModemHead
author_rank: Frequent Contributor
timestamp: March 24, 2015, 01:23:44 am
content: Quote from: otpowell on March 24, 2015, 01:14:04 amIt does say 200kHz, but I'm sure that refers to the frequency measuring range. The amplitude is less of an issue for that, all it needs is the zero crossings.

author: miguelvp
author_rank: Super Contributor
timestamp: March 24, 2015, 01:26:30 am
content: Quote from: otpowell on March 24, 2015, 01:14:04 amYou are right, it does say that:http://www.amazon.com/Mastech-MS8268-Digital-Manual-Multimeter/dp/B000JQ4O2UQuoteLooks like a mistake on their part.Edit: Then again in 4.2.8 it mentions the frequency counter can measure up to 200kHz so they are not lying.http://www.histest.com/blog/files/2012/02/Mastech-MS8268-Digital-Multimeter-Users-Manual.pdf

author: Wytnucls
author_rank: Super Contributor
timestamp: March 24, 2015, 01:29:57 am
content: You are confusing maximum frequency measurement (200kHz), which uses a counter to measure zero volt crossing occurrences and the useable TRMS AC bandwidth of the meter, which could be something like 5Hz~200Hz. Two completely different animals.(Actually 40Hz to 200Hz on the 4V range, for an average response meter calibrated to sine wave RMS).AC TRMScurrentrange is 40Hz to 1kHz on all ranges.

author: otpowell
author_rank: Regular Contributor
timestamp: March 24, 2015, 02:49:26 am
content: Quote from: Wytnucls on March 24, 2015, 01:29:57 amMakes complete sense, I forgot about the frequency measuring part, didn't even think that's what they were referring to. Thanks for all the help, research, and clarification guys!

author: otpowell
author_rank: Regular Contributor
timestamp: March 28, 2015, 04:56:30 am
content: Quote from: Wytnucls on March 24, 2015, 01:29:57 amWhat kind of multimeter would I need to accurately measure a 10kHz signal in RMS? If I'm understanding your last statement correctly, a true RMS multimeter only measures up to 1kHz? A fluke or a desk benchtop multimeter has to do it right without capping out on bandwidth. We haven't talked about bandwidth caps in class and we measure signals like this all the time with a BK precision bench top multimeter. Unless nobody mentioned that the measurements aren't accurate due to a bandwidth cap, which I doubt.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 28, 2015, 07:05:00 am
content: The multimeter TRMS frequency bandwidth and crest factor for voltage and current is often mentioned in the user manual.On the best multimeters, the high side of the bandwidth is usually limited to 100kHz, for acceptable accuracy, depending on range used.

author: The Electrician
author_rank: Frequent Contributor
timestamp: March 28, 2015, 07:07:29 am
content: Quote from: otpowell on March 28, 2015, 04:56:30 amYou need a meter with a flat response >= 10k; more than 10k if the waveform isn't a pure sine wave and you want TRMS capability for distorted waveforms. Your existing meter is not TRMS, of course.Have a look at the Multimeter spreadsheet:https://www.eevblog.com/forum/testgear/multimeter-spreadsheet/A number of meters have bandwidths of 100k.

author: otpowell
author_rank: Regular Contributor
timestamp: March 28, 2015, 01:46:34 pm
content: Quote from: The Electrician on March 28, 2015, 07:07:29 amThank you that's quite the spreadsheet!