| Author |
Message |
Guest
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 Posted:
Fri Sep 16, 2005 8:35 am Post subject:
true RMS meter vs. oscilloscope |
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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.
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mike
Guest
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| Posted:
Fri Sep 16, 2005 8:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
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Robert Baer wrote:
| Quote: | 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
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Robert Baer
Guest
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| Posted:
Fri Sep 16, 2005 8:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
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phillip.liu@fphcare.co.nz wrote:
| Quote: | 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.
|
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Michael A. Terrell
Guest
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| Posted:
Fri Sep 16, 2005 8:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
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phillip.liu@fphcare.co.nz wrote:
| Quote: |
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.
|
They will only be the same if they are a clean sine wave, and both
are in calibration.
--
?
Michael A. Terrell
Central Florida |
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Dave
Guest
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| Posted:
Fri Sep 16, 2005 4:32 pm Post subject:
Re: true RMS meter vs. oscilloscope |
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phillip.liu@fphcare.co.nz wrote:
| Quote: | 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 |
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. |
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Robert Lacoste
Guest
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| Posted:
Fri Sep 16, 2005 4:35 pm Post subject:
Re: true RMS meter vs. oscilloscope |
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<phillip.liu@fphcare.co.nz> a écrit dans le message de news:
1126843222.864518.111090@z14g2000cwz.googlegroups.com...
| Quote: | 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.
|
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 |
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Robert Baer
Guest
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| Posted:
Sun Sep 18, 2005 12:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
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mike wrote:
| Quote: | 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. |
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CJT
Guest
|
| Posted:
Sun Sep 18, 2005 8:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
mike wrote:
| Quote: | 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
|
I think it would be false advertising to call it RMS and then just
apply a factor to P-P.
--
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. |
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mike
Guest
|
| Posted:
Sun Sep 18, 2005 8:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
Robert Baer wrote:
| Quote: | 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
--
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
..
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Wanted 12" LCD for Compaq Armada 7770MT.
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/ |
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Dave
Guest
|
| Posted:
Sun Sep 18, 2005 4:35 pm Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
CJT wrote:
| Quote: | 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. |
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CJT
Guest
|
| Posted:
Sun Sep 18, 2005 4:35 pm Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
Dave wrote:
| Quote: | 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.
|
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. |
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Guest
|
| Posted:
Sun Sep 18, 2005 10:05 pm Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
"mike" <spamme0@netscape.net> wrote in message
news:432CFBAA.6050503@netscape.net...
| Quote: | 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 |
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CJT
Guest
|
| Posted:
Sun Sep 18, 2005 10:43 pm Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
normanstrong@comcast.net wrote:
| Quote: | "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 |
offset (or any waveform that is symmetric around zero) is zero.
--
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. |
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mike
Guest
|
| Posted:
Mon Sep 19, 2005 12:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
Dave wrote:
| Quote: | 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.
|
TEK TDS540 reads peak and RMS as same (within A/D resolution) on a 9V
battery.
mike
--
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/ |
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Guest
|
| Posted:
Mon Sep 19, 2005 12:35 am Post subject:
Re: true RMS meter vs. oscilloscope |
|
|
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 |
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