"real" RMS

[robinson]

Hi all

I use SCXI-1000 and module SCXI 1313,1125...

I want to measure "RMS" current, from square signal. I use DAQ Assistant, from which I get out voltage and current! All working, tested with constant DC.

I tried in this way:

But RMS Value is incorrect(I know that, because I measure with TRUE RMS MULTIMETER.....). The signal is square and look's like this:

Any ideas or "hint's".







 

[Dennis_Knutson]

There is no image, no mention of what function you are using, no mention of the RMS value returned in LabVIEW, and no mention of the value that the meter reports.

[robinson]

Maybe you will see it now:

[smercurio_fc]

Well, we can see the image, but you have not indicated what the actual values are that you're getting from LabVIEW vs the meter....

[robinson]

Voltage is from 0-16V, "RMS" give values about 8-12, which is incorrect, it must be about 50-70mA...(don't look at signal picture, it's just "example").

[Dennis_Knutson]

You've got three separate 'RMS' functions and you don't have an indicator on the one that is actually measuring your signal. Connect an indicator on the output of the Statistics output.

[robinson]

From 'statistics' I get 'RMS' output , which I connect into DC-RMS, and soo one, from that I can see value of signal(look at RMS value,DC avarage, RMS avarage...), so why I will need additional indicator, if I see value from that...

[Dennis_Knutson]

You don't need all of those other functions since Statistics returns RMS. You've got a lot of redundant/confusing code by having those extra functions. I also have no idea what sort of value you would get by passing a scalar to them.

Since you did not attach any actual code with default values for the output of the DAQ Assistant, impossible to 'look at RMS value,DC avarage, RMS avarage'.

[smercurio_fc]

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@robinson wrote:

Voltage is from 0-16V, "RMS" give values about 8-12, which is incorrect, it must be about 50-70mA...(don't look at signal picture, it's just "example").

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@robinson wrote:

From 'statistics' I get 'RMS' output , which I connect into DC-RMS, and soo one, from that I can see value of signal(look at RMS value,DC avarage, RMS avarage...), so why I will need additional indicator, if I see value from that...

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You know, we try to be professional and polite on this forum. We really try. Sometimes, though, we read responses like this which simply make our head shake. in disbelief.

You know what the difference is between voltage and current, right? If so, how in the world can you say that a VOLTAGE between 0 and 16V with an RMS value "about" 8-12 (whatever "about" is supposed to mean in engineering terms) should actually be 50-70 mA??? That's simply inane. If you are getting a voltage which is proportial to a current then you obviously have to perform the same proportianal calculation to convert the voltage to a current!

Also, the "RMS" output of the Statistics Express VI IS the RMS value. Why are you trying to calculate the RMS value of an RMS value?  We cannot "look at RMS value,DC avarage, RMS avarage". We don't have your VI, and you're not showing the front panel. Look at the picture you posted. Do you see the values of  "RMS value,DC avarage, RMS avarage" anywhere on that picture?

EDIT: It seems that Dennis pretty much said the same thing at the time I was posting.

[johnsold]

Are you sure your "TRUE RMS VOLTMETER" can accurately measure that signal?  The pulse width appears to be on the order of 300 us.  With the low duty cycle you probably need a bandwidth of several hundred kilohertz to accurately measure such a signal.  Does it have a bandwidth that wide? Can it handle a crest factor of 10?

Your sampling rate on the DAQ Assistant may not be fast enough also.

Please indicate the actual timing and voltage (or current) values for the signal.

Lynn