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I am testing the material properties of some very low stiffness materials.

I'm using a force probe connected to software, sensing at about a hundredth of a gram of force.

Now, what's interesting is when my sample rate is 1/sec I get a smooth line, as expected. If I increase the sample rate the line get a more jagged, increased frequency line - still as expected.

BUT the increased sample rate also increases the amplitude of the noise/signal and this I don't understand.

I hope I explained it well enough...

But basically why is it that increased sample rate is increasing the amplitude of the signal rather than just the frequency.

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What exactly is being sampled? Is there some probe moving through a whole series of measurements each second? Or is there just one datum measured? In other words, does the measurement actually happen faster when the sampling is faster? Does the probe move faster, or are the samples just spaced closer? Could the sample behave differently at high speed? –  Colin K Mar 3 '12 at 1:22
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Could your equipment be doing averaging over the time between samples? You need to really explain how this measurement is being made. –  Colin K Mar 3 '12 at 1:26
    
Hey Colin, so its called a load cell: basically its a probe attached to the arm of this machine which slowly (.1 mm/sec) moves downward into the material while simultaneously measuring resistance from which it determines the reaction force (how this works exactly i'm not sure). The machine is attached to a computer which allows us to select the sample rate. I think the measurement happens at the same rate but the sending to the computer happens at a faster rate when sample rate increases (i think?). –  SimaPro Mar 3 '12 at 2:34
    
What you say about time averaging could be the case - that would mean that we have a lot of noise intrinsically in the collection. Then would that mean theoretically there is a sample rate if we go above which, the amplitude would stay the same? –  SimaPro Mar 3 '12 at 2:39
    
Probably not, unless the noise properties of that sensor are unusual. It would mean, however, that you aren't really being hurt by the noisier data. Fitting a line to more, but noisier, data is just as good as fitting to fewer but averaged points (making a few reasonable assumptions about your equipment). You should edit that information in to your question. I'll try to make a more detailed answer when I'm not on a smartphone. –  Colin K Mar 3 '12 at 3:45
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2 Answers

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From your description of the experiment (please correct me if my assumptions are wrong), it sounds like your apparatus consists of the application of a controlled stress to the sample (and the sensor), and the resulting strain in the sensor is measured. Whenever the stress applied by your apparatus changes, it will take some time for the system to settle to it's new equilibrium. It could be that sampling at $1 Hz$ is allowing plenty of time for equilibration, but sampling at higher frequencies you are recording the oscillations of the system as it has not yet settled.

One way to test would be to run the experiment without changing the applied force, just recording the strain at various sampling rates, and looking to see if the noise spectrum still depends on the sample rate in the way you describe. If it does, then the noise is a result of the frequency dependence of the electronics. If it does not, then the noise is resulting from the physical behavior of the sample

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Only thing I can think of is Nyquist 's theorem, sampling must be at least twice of the highest frequency component, maybe you need Aliasing filter or averaging filter?

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Thanks for the answer...The question that I'm wondering is not how to overcome this..but why it happens? Is there a reason that the amplitude should change when the sampling rate is changed? –  SimaPro Mar 3 '12 at 0:19
    
If the amplitude is changing with sample rate it is aliasing, this could mean that you have the wrong sample frequency, or the equipment has the wrong filter or sample size, it depends on the application. I've had signals look like triangle waves when they should be sinusoidal which is pretty bad aliasing –  zacharoni16 Mar 3 '12 at 19:16
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