Which is most accurate ?

I brought 3 new sets of brander digital caliper (150mm). When I use a
25 mm +/- 0 um 20 C
length of micrometer standed bar for my verification on the caliper,
the 3 sets of caliper give me 3 different reading ; 24.99 mm, 25 mm,
25.01 mm. I conducted this testing in a room temp. 28 C
at 70 humidity. Can any person let me know which set of caliper is
reliable ? Thanks
Regards
This looks like a reasonable homework question. You start from the concept that metals expand when heated. You need to find steel's coefficient of linear thermal expansivity. You know the standard is produced for use at 20degC so it will expand at 30 degrees. Hence it will measure MORE than 25 mm. So you might well assume the 25.01 is closer to the mark. You would need to actually work out the exapansion - perhaps the 25 mm reading is still the closest?
Brian Whatcott Altus OK
(just reaching over Brian's shoulder, in case it is not a homework question.) "coefficient of thermal expansion" will let you find it easier on the internet.
The one that is entirely mechanical is more reliable! ;>)
David A. Smith
Brian Whatcott wrote in news: snipped-for-privacy@4ax.com:
Adding a touch more to Brian and NZ's responses, until you do a full, long form gauge R & R, you will have no idea of which caliper is most repeatable (reliable). Your difference in measurement may have been self- induced.
How close are we expecting a pair of calipers to read?
.001" or "maybe" .0005"
.025mm or "maybe" .013mm
Personally I'd be pretty happy with all three sets.
They all read within .0003" of the standard (not accounting for thermal expansion ~+.0001) which seems pretty good for a 6" caliper.
These particular units may have resolutions to .01mm but I'm guessing that their resolutions are beyond their accuracy.
If you want to determine "reliability" you'll need to take a lot more readings over an extended period of time.
cheers Bob

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