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Accuracy of strain measurement

I read in an NI whitepaper that the accuracy of my strain measurement is calculated by summing gain error, offset error, and noise uncertainty. I'd like to understand how these factors impact information I'm monitoring in real time. Noise seems like an error source that would vary with time. Do the gain error and offset error vary with time or are they constant run to run or point to point.

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@Testr00 wrote:

I read in an NI whitepaper that the accuracy of my strain measurement is calculated by summing gain error, offset error, and noise uncertainty. I'd like to understand how these factors impact information I'm monitoring in real time. Noise seems like an error source that would vary with time. Do the gain error and offset error vary with time or are they constant run to run or point to point.


Wellcome in the world of metrology Smiley Happy

 

What that (bold part of your text)  tell you is the expected uncertainty of your bridge voltage aquisition part of the measurement.

There are many more parts in a real uncertainty budget for a $unitofinterest measurement.

Just to name some: temperature influence, nonlinearity, drift , creeping, ... (more noise, due to cable EMI, ..) 

Many books and papers ....   one source with a more detailed uncertainty budget for your task should be noted in the calibration standard(s) for your $unitofinterest sensors.

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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The strain gauge manufacturer HBM features a few good white papers and information about strain measurement and uncertainty/noise. As far as I remember, they can be found under the menu "Services & Support".



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Hi Testr00,

To add to Henrik's great answer, the noise specs in NI device manuals are RMS values, meaning they're a sort of average over time. It's possible (though very unlikely) to see a spike beyond that range, but over time the RMS noise value will be within the specifications in the device manual/datasheet.


@Testr00 wrote:

Noise seems like an error source that would vary with time. Do the gain error and offset error vary with time or are they constant run to run or point to point.


Yes, the noise varies with time, but we can't predict how it will do so.  

As such, we just slap some error bars onto the measurement knowing that the value measured will be within those error bars. Sometimes the actual signal-minus-noise value will be right in the center, sometimes right on the edge, mostly somewhere in between, but if we can't accurately predict it, all we can do is report a range of possible values.

Nick Smith
Product Support Engineer
C Series Controllers
National Instruments
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Thanks for all the great replies. Would it be correct to say that the gain and offset errors have little impact on variation in the data in the minute to minute time range?

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Well, gain and offset errors  have a direct impact  on you measurement uncertainty.

Depending on your measurment task, changes due to drifts in gain and offset and maybe temperatur  (again only the drift) maybe have a minor impact.

(Asuming no correlation (usually at least questionable Smiley Wink ) a simple error budget calculation end up in a squareroot of  sum of squares of rel. errors)

 

EDIT:

Hard to say, unless you provide a complete description of your measurement task.

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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An example would be measuring up to 1000ustrain at room temperature on using a cDAQ 9188 with NI9236 modules.

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The aquisition part is already done ... well known, easy investigated Smiley Wink .... major part is gain,offset (error/drift) + drift +tempcos

what about the physical unit (usually measured in SI units Smiley Wink ) you want to measure?*

the transducer (sensor) (again gain,offset,drift,noise,tempcos... 

it's mounting

the operator

...

 

*)voltages and currents are booring unless quantities of these units don't tell us something else Smiley Wink   (well, apart from some physics ... but then these quantities are often even harder to measure .... Smiley Very Happy).

Voltmetering (and time) is the hammer  (you onlyeasy have) in measurements, so all(most) your measurement problems are now a nails voltage(time) measurements.

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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