true RMS meter vs. oscilloscope

Guest
Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.
 
phillip.liu@fphcare.co.nz wrote:
Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.
Click to expand...

They will only be the same if they are a clean sine wave, and both
are in calibration.

--
?

Michael A. Terrell
Central Florida
 
phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
Click to expand...
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are implimented.
 
Robert Baer wrote:
phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are implimented.
Click to expand...
I must be confused...cause he said he used a "True RMS" meter. A True
RMS meter, by definition, measures the RMS value of the waveform. If
the waveform is only "slightly distorted", a TRUE RMS meter should
measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

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phillip.liu@fphcare.co.nz wrote:
Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

What's the frequency? It is within the range of the meter? How different
Click to expand...
is different? Scopes are typically not better than a few %. My HP 3457A
6.5 digit multimter has an spec of only +/- 10% of reading at 1 MHz.

If the difference is very significant, take a known scope and eyeball it.
 
<phillip.liu@fphcare.co.nz> a écrit dans le message de news:
1126843222.864518.111090@z14g2000cwz.googlegroups.com...
Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.
Click to expand...
It may or may not be your problem, but on my scope (Lecroy) when you use the
"rms measurement" function it calculates the rms value pf the signal either
through the entire memory or between the two cursors. So the result is NOT
the same than an RMS voltmeter except if the two cursors are positionned
exactly separated by an integral number of signal periods...

Hope that's helpful...

Friendly,

--
Robert Lacoste
ALCIOM - The mixed signal experts
www.alciom.com
 
mike wrote:

Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are implimented.


I must be confused...cause he said he used a "True RMS" meter. A True
RMS meter, by definition, measures the RMS value of the waveform. If
the waveform is only "slightly distorted", a TRUE RMS meter should
measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

You are fortunate to have the "read in RMS" option on your scope.
Click to expand...
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.
 
Robert Baer wrote:
mike wrote:

Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are implimented.



I must be confused...cause he said he used a "True RMS" meter. A True
RMS meter, by definition, measures the RMS value of the waveform. If
the waveform is only "slightly distorted", a TRUE RMS meter should
measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.
Click to expand...
OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike

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mike wrote:

Robert Baer wrote:

mike wrote:

Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are
implimented.




I must be confused...cause he said he used a "True RMS" meter. A
True RMS meter, by definition, measures the RMS value of the
waveform. If the waveform is only "slightly distorted", a TRUE RMS
meter should measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy
spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.


OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike
Click to expand...
I think it would be false advertising to call it RMS and then just
apply a factor to P-P.

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CJT wrote:

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.



OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike


I think it would be false advertising to call it RMS and then just
apply a factor to P-P.
Click to expand...
Try it on a battery.
 
Dave wrote:
CJT wrote:

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.




OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike


I think it would be false advertising to call it RMS and then just
apply a factor to P-P.


Try it on a battery.
Click to expand...
For sure. But then at least it wouldn't matter what factor you chose --
they'd all be wrong.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.
 
"mike" <spamme0@netscape.net> wrote in message
news:432CFBAA.6050503@netscape.net...
Robert Baer wrote:
mike wrote:

Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are
implimented.



I must be confused...cause he said he used a "True RMS" meter. A True
RMS meter, by definition, measures the RMS value of the waveform. If
the waveform is only "slightly distorted", a TRUE RMS meter should
measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy
spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.
Click to expand...
Which waveform has the greatest discrepancy between average and rms values?

Norm Strong
 
normanstrong@comcast.net wrote:

"mike" <spamme0@netscape.net> wrote in message
news:432CFBAA.6050503@netscape.net...

Robert Baer wrote:

mike wrote:


Robert Baer wrote:


phillip.liu@fphcare.co.nz wrote:


Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.


Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are
implimented.



I must be confused...cause he said he used a "True RMS" meter. A True
RMS meter, by definition, measures the RMS value of the waveform. If
the waveform is only "slightly distorted", a TRUE RMS meter should
measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy
spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike


You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.


Which waveform has the greatest discrepancy between average and rms values?

Norm Strong


What do you mean by "average?" The average of a sine wave without
Click to expand...
offset (or any waveform that is symmetric around zero) is zero.

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Dave wrote:
CJT wrote:

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.




OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike


I think it would be false advertising to call it RMS and then just
apply a factor to P-P.


Try it on a battery.
Click to expand...
TEK TDS540 reads peak and RMS as same (within A/D resolution) on a 9V
battery.
mike

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Hi Dave,

It is a 50Hz 22V rms AC sinewave.
The rms reading from my HP54600b can be upto 5% larger comparing with
my Fluke 45 meter.

I assumed that a scope would prove a more accurate true RMS reading.
Have I made a wrong assumption?

regards

Phillip
 
mike wrote:

Robert Baer wrote:

mike wrote:

Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are
implimented.




I must be confused...cause he said he used a "True RMS" meter. A
True RMS meter, by definition, measures the RMS value of the
waveform. If the waveform is only "slightly distorted", a TRUE RMS
meter should measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates the
RMS value of the waveform. I didn't bother to look up the accuracy
spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.


OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike

NOW i know something i did not previously: what make and model of the
Click to expand...
scope.
Since it digitizes, then it could calculate energy of each sample and
thus the equivalent RMS value.
If one waveform was converted this way, and if there were (say) 8
bits per sample, then about 3 bits would be used for the + half and 3
bits for the - half and one for zero; this is a bit (no pun inended)
better than measuring a peak (or Pk-Pk) and converting.
Higher bit resolution would tend toward better accuracy for complex
waveforms.
Still, no where as good as using the thermal method...
 
phillip.liu@fphcare.co.nz wrote:

Hi Dave,

It is a 50Hz 22V rms AC sinewave.
The rms reading from my HP54600b can be upto 5% larger comparing with
my Fluke 45 meter.

I assumed that a scope would prove a more accurate true RMS reading.
Have I made a wrong assumption?

regards

Phillip

Unless the A/D of the scope has more resolution than the HP or Fluke,
Click to expand...
expect less accuracy.
 
Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi Dave,

It is a 50Hz 22V rms AC sinewave.
The rms reading from my HP54600b can be upto 5% larger comparing with
my Fluke 45 meter.

I assumed that a scope would prove a more accurate true RMS reading.
Have I made a wrong assumption?

regards

Phillip

Unless the A/D of the scope has more resolution than the HP or Fluke,
expect less accuracy.
Click to expand...
Resolution and accuracy are different things.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.
 
Robert Baer wrote:
mike wrote:

Robert Baer wrote:

mike wrote:

Robert Baer wrote:

phillip.liu@fphcare.co.nz wrote:

Hi, I am trying to measure the rms voltage for a slightly
distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.

Using the scope, one can only measure the peak or peak-to-peak
voltage, then convert to RMS.
Not any different than the way AC scales on most DVMs are
implimented.





I must be confused...cause he said he used a "True RMS" meter. A
True RMS meter, by definition, measures the RMS value of the
waveform. If the waveform is only "slightly distorted", a TRUE RMS
meter should measure within the specs of the meter.

I don't know about your scope, but mine has a button that calculates
the
RMS value of the waveform. I didn't bother to look up the accuracy
spec.

It would be interesting to know the exact equipment used and the
magnitude of the discrepancy.

One common source of such problems is DC offset of the AC waveform.

mike

You are fortunate to have the "read in RMS" option on your scope.
But, i bet that it works by measuring peak or Peak-to-peak and
converting that to RMS.



OK, what are you betting? Send it to me.
It's a TEK TDS540. Suppose one could look up the spec. It can do FFT;
it's hard to imagine they'd not do a proper RMS calculation.
mike

NOW i know something i did not previously: what make and model of the
scope.
Since it digitizes, then it could calculate energy of each sample and
thus the equivalent RMS value.
If one waveform was converted this way, and if there were (say) 8 bits
per sample, then about 3 bits would be used for the + half and 3 bits
for the - half and one for zero; this is a bit (no pun inended) better
than measuring a peak (or Pk-Pk) and converting.
Higher bit resolution would tend toward better accuracy for complex
waveforms.
Still, no where as good as using the thermal method...
Click to expand...
This is going from funny to sad. Half of 8 bits is 7 bits.

Nowhere as good as the thermal method at the NIST...but probably on par
with a typical cheap commercial thermal rms converter.
mike

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Wanted, Serial cable for Dell Axim X5 PDA.
FS 512MB 45X SD Flash memory.
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
 
<phillip.liu@fphcare.co.nz> wrote in message
news:1126843222.864518.111090@z14g2000cwz.googlegroups.com...
Hi, I am trying to measure the rms voltage for a slightly distorted AC
sine waveform. I used a "True RMS" multimeter as well as an
oscilloscope. Expecting both rms readings to be the same, but
different. Any idea why? Thanks.
Click to expand...
Agilent published a few white papers on the subject. In particular they
pertained to DMM and RMS issues. The items from Agilent are: "Application
Note AN 1389-3" and "Application Note 1392". Hopefully they may explain the
difference from the DMM and the oscilloscope.

Tom
 
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