Glass scale calibration at 74 degrees F?

greif

Involved In Discussions
#1
I recently had a 125mm glass scale calibrated by an 17025 accredited lab. The report claims a k=2 uncertainty of 0.13um.
However;
-The calibration was done at 74 degees F, with no corrections noted
-the cal report does not state the path that was used (along the lines of the scale)
-the cal report does not state if right edge, left edge, or center of line was used to locate measurement.

Is this as sloppy as I think it is, considering the 0.13um claim?
Shouldn't enough info be documented that any lab could reproduce the measurement?
 
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dwperron

Trusted Information Resource
#2
I recently had a 125mm glass scale calibrated by an 17025 accredited lab. The report claims a k=2 uncertainty of 0.13um.
However;
-The calibration was done at 74 degees F, with no corrections noted
-the cal report does not state the path that was used (along the lines of the scale)
-the cal report does not state if right edge, left edge, or center of line was used to locate measurement.

Is this as sloppy as I think it is, considering the 0.13um claim?
Shouldn't enough info be documented that any lab could reproduce the measurement?
So many issues here.....
So they are claiming an uncertainty of 0.13 µm (about 5 µin). For any distance? I'm not calling it impossible, NIST can measure glass scales at that level of uncertainty with their interferometer system. But I strongly doubt it. What did your lab list as their reference standards?

Borosilicate glass has real low thermal expansion of around 3 X 10-6 , but you are correct in wondering how that is not reported as a correction value at that level of measurement uncertainty.

Let's not even get into where they located the measurement...

Is this as sloppy as you think it is, considering the 0.13um claim? YES!
Shouldn't enough info be documented that any lab could reproduce the measurement? YES!

You have a right to request to see the calibration method they used. That should include the details you are asking about.
Do I think they over-promised and under-delivered?
YES!!!

What kind of accuracy is this scale of yours?
 

greif

Involved In Discussions
#3
The lab scope says they are capable of 7.1 micro-inch 0.01" to 40" using a linear laser system.
My oops on their claimed uncertainty for my measurement- the reports says 0.31um not 0.13.
 

dwperron

Trusted Information Resource
#4
The lab scope says they are capable of 7.1 micro-inch 0.01" to 40" using a linear laser system.
My oops on their claimed uncertainty for my measurement- the reports says 0.31um not 0.13.
So at least they have the capabilities for the uncertainties they claim.
The temperature still raises an issue, depending on the delta from your lab.
As for the measurement sites - who knows there.

Depending on the accuracy of your glass scale this might all be moot.
With a typical accuracy of 0.005 mm (5 µm) for a glass scale the uncertainty and thermal expansion are not significant.
 

Brenda

Starting to get Involved
#5
One could call it sloppy when you do not agree on the measuring protocol before you send equipment for calibration.

Same for the temperature correction, you should know the thermal expansion factor of your own equipement
 
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