How to include machine error in uncertainty calculations?

greif

Involved In Discussions
#1
How do I state machine error in my uncertainty statement?

I have calculated uncertainty (using repeatability,temperature, user repeat, and the uncertainty of the cal standards used to calibrate the instrument).
In the 100%R to 23%R range, 451 to 700nm, the k=2 uncertainty is 0.52%

Then I read the cal standards on the instrument several times and note there is an error of around 1.5 % worst case in the 451nm to 700 nm range.

Right now (on a calibration certificate) I state the k=2 uncertainty is 0.52% with +1.5/-0.0 %R residual error.

The measurement device is a reflection spectrophotometer which reads out %R every 10nm from 400nm to 700nm.

If it makes any difference, this will eventually be in a 17025 system with ANAB accreditation.

Question: Is this a proper way to state this uncertainty and residual error?
or do I need to include the error in my uncertainty (what is the proper way to do that in this case?)

Note: eventually I hope to get a handle on the error and subtract it out if possible.

Thanks to all, this forum is a great help!
 
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dwperron

Trusted Information Resource
#2
"Right now (on a calibration certificate) I state the k=2 uncertainty is 0.52% with +1.5/-0.0 %R residual error."

It took a while for me to figure out what bothered me about this (old age creeping in).
You are claiming a "residual error" that is 3 times your measurement uncertainty.
That sounds like a problem, either with your equipment or with your process.

Normally you would combine all of the significant contributing factors in determining your measurement uncertainty. Why aren't you doing that, since both terms are stated in "%R"? ILAC says that "uncertainty arising from correction of the measurement results for systematic effects". That sounds like your "Residual error", it is a factor to be taken into account when stating measurement uncertainty.
 

greif

Involved In Discussions
#3
Yes, the equipment is more repeatable than it is accurate. Not too unusual for photometric measurements.

I see that normally known error is corrected for, so it is not included in uncertainty. I can't do this at this point (until we change the way reports are done). I can't see including the known error into the uncertainty calc., since at k=2 it ends up double.

Is just adding the known error to the calculated uncertainty kosher?
 

dwperron

Trusted Information Resource
#4
I'm afraid that I wasn't clear on this.

I meant that you need to include the uncertainty of your "known error" that you are correcting for.
 

greif

Involved In Discussions
#5
I hadn't thought of that one!

So something like this?

Uncertainty calc based on the usual things (ref std, thermal, repeatability, etc.) and uncertainty of the measurement of error to make the k=2 uncertainty, then add the measurement bias (my machine error) to that.
 
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