Multimeter Reading VS Oscilloscope Discrepancy

[otpowell]

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? 

[dom0]

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.

[rs20]

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.

[otpowell]

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

[ModemHead]

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.

[otpowell]

Well, I checked my bandwidth on my multimeter and it's rated for 200kHz

[otpowell]

Wow thanks ModemHead .. That's strange Amazon says 200kHz

[otpowell]

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! 

[ModemHead]

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.

[otpowell]

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.

[otpowell]

Well, thanks everyone I feel a lot better now and my measurements look much better using the scope measurements

[ModemHead]

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.

It 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.

[miguelvp]

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.

You are right, it does say that:

http://www.amazon.com/Mastech-MS8268-Digital-Manual-Multimeter/dp/B000JQ4O2U

Product Features

• AC/DC 1000V/10A 200KHz 200uF 40Mohm Relative Measurement hFE Diode Check Continuity
  
• LED / Sound Warning when incorrect banana jacks are used relative to function switch setting
  
• Auto and Manual Ranging with Relative measurement (all ranges except frequency)
  
• All range fused (resettable) with 1 Year Warranty
  
• Auto power off (could be disabled) & Blue LED Back-lit LCD display
  

Looks 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

[Wytnucls]

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 TRMS current range is 40Hz to 1kHz on all ranges.

[otpowell]

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 TRMS current range is 40Hz to 1kHz on all ranges.

Makes 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! 

[otpowell]

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 TRMS current range is 40Hz to 1kHz on all ranges.

What 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. 

[Wytnucls]

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.

[The Electrician]

What kind of multimeter would I need to accurately measure a 10kHz signal in RMS?

You 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.

[otpowell]

What kind of multimeter would I need to accurately measure a 10kHz signal in RMS?

You 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.

Thank you that's quite the spreadsheet!