Measurement Resolution - How far to carry the decimal place out

Miner

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#11
Is it acceptable to use a "for reference only" sticker, along with a calibration, if your caliper does not have adequate resolution to pass a gage R&R?
I am talking about steel tooling here, calipers are used "on the bench" but all final decisions are based on CMM / Micrometer (higher res) measurement results.
I know that the obvious answer is to buy higher res calipers, but this is currently not an option.
The "bottom line" on this is whether the gauge is ever used to make a decision on the acceptability of the product or process. If it is used to make a decision, you must address the resolution issue. If it not used to make a decision, why do you use them?
 
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George Weiss

#12
I think it was the M3003 GUM that expressed a need to not overdo the number of digits beyond the decimal point in scientific notation.
I think it was 2 places. This is a good rule to keep. 2 places give you between .1% and 1% of reported value in resolution. So a 1Vdc measurement with a 1ppm accuracy might be reported to 1.000,000,00 8 places or +/- .01ppm, (this is not the accuracy, but resolution of reported value).
 
#13
Those are questions I am working to answer, this is not "my lab", but one of our toolshops. Great question.
For the sake of getting to the bottom of it without answering that question though, let me pose the scenario that the calipers are used just to "get a feel" for the direction and severity of any cuts of a tool, before using a more accurate system to make final checks/qualifications.

Acceptable to use "for reference only" along with calibration cert?
I don't believe I have ever seen the "ref only" sticker on anything that we bothered to calibrate.

These will not R&R well, and I have to come up with a solution other than "buy new calipers with higher res".
As always, appreciate everyone's help greatly.
 
G

George Weiss

#14
If you are going to check the item later anyway, then place a NCR, no-cal-required, or as you said, reference, or indication only label on it. You can also test the caliper to any degree of accuracy you wish. Just write a small procedure that inicates your test method and accuracy. You can call that a Derated calibration/check if you want. Some call it a Limited calibration/check. In other situations, odd items are placed into procedures, which indicate the range of display needed to satisfy the process point. Write what you want, then do as you write. Getting the better caliper is an option for when you want to do it. It is likely the simplest course, which many people end up taking. A few minutes shopping on EBAY.
 
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