Which is the correct Measurement Point on waveform Signal?

[JCollado]

Hello

Please see image attached.

I have a labview software that is measuring load output from a load cell. I believe the reading is accurate except that with the Labview software we get up to 50 lbs less than when we have the technician read it with the oscilloscope.

What I see is that the technician measures the amplitude at the very top of the signal and when there is some noise the oscilloscope reading is even higher because the placement of the cursor is higher.

I see that LabView is more or less measuring at the center of the signal not at the top because when we place the oscilloscope cursor in the middle of the signal it it then reads about the same my labview program. I also have a low bypass filter in the labview software.

Which is the correct measurement points that the oscilloscope expect to measure? in the attached image is the white cursor placed correctly or is the red cursor correct?

That is just a sample image enlarge for better view.

Thank you all.

JCollado

[LV_Pro · Accepted solution]

If I'm understanding you correctly the width of the oscope trace represents 50 lbs. difference in measurement? If you think about how an oscope works, then the center of the displayed line is probably the most accurate. The line width is determined by a few things, noise superimposed on the signal being one. So, with noise filtering in your LabVIEW program, its "line width" will be narrower/thinner, as it will have removed some of the noise component. Placing the cursor in the middle of the oscope's displayed trace is a rough approximation of this, sort of looking for the point where the noise is "averaged" out, 1/2 above/ 1/2 below the cursor, which is approximately the "data" signal. It is hard to make precision measurements of a noisy signal with an oscope for this very reason, where do you put the trace? If you were to remove the LP filter from the program you would probably get the equivalent to the "wide trace display" which would be a series of values which plotted approximated the underlying signal, but with high and low variations equivalent to the width of the trace.

So after all that verbiage, I would read the center of the trace, perform an "eyeball" noise averaging.

[JCollado]

Thank you  LV Pro.

It is good to have some backup info in case things go wrong with teh component.

I added updated image as per what I understood from you. Well the average of the noise position. That seem to be the one that sounds good to my supervisor.

Thanks