Help DOD inspection - Standard temperature calipers should be calibrated at?

M

megan c

#1
Could somebody help me with standdard temperture calipers should be calibrated at???????????????
DOD requires this info in there caibration reports of inspection equipment.We are going through a pre-award inspection by the dod. WE cant afford to fail.
 
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S

Southern Girl

#2
Use 68 degrees F and you can not go wrong. Just make sure that is what you are doing and your standards are kept at the same temperature. Good luck with your audit. If that is the only thing that you have to worry about, you'll be fine!
 
K

Kevin H

#3
Caliper Calibration

Megan, I can't comment about DOD requirements, but I ran an ISO Guide 25 lab for a steel mill - we were testing per ASTM and AMS standards. We calibrated calipers at their usage temperature. For us this was between 60 degrees F and 90 degrees F (which was spelled out in our calibration procedure). They were used for % elongation measurements in tensile testing, and the usage areas were somewhat temperature controlled as they were in an older building that hadn't had its temperature sytem brought up to the best standard.
Ususally in the tests you do, something spells out calibration temperature requirements - look at the tests and see if there are requirements if so, use those. If not I'd report what you're doing, better to report reality and get a corrective action request than to tweak something and run into problems later.
Hope this helps. Kevin
Note: Gage blocks were certified by an independent calibration lab at I believe 68 F. I calcualted the effect of temperature on the gage blocks used based on published coefficients of expansion & determined over the temperature and measurement ranges used for our lab calibrations, the change in length was below the resolution for the calipers. The calculation was maintained as a record within the lab system.
Kevin
 
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Mike S.

Happy to be Alive
Trusted Information Resource
#4
68 deg. F seems popular, but I just had my gage block set done by a reputable lab and they were done at 72 F. Considering caliper resolution is limited to .0005" or .001" I'd say 68-72 would be just fine.
 
R

Ryan Wilde

#5
Standard temperature of ANY dimensional calibration is 20°C (68°F). Any variance from that temperature adds uncertainty, but does not invalidate the calibration. Your calipers, for instance, do not have the resolution to really show the variance due to temperature in any reasonable environment (example: ± 10°F, measuring at 6 inches, assuming you were using steel gauge blocks, would only bring uncertainty up about 80 µin, or 0.00008 in from doing them in a perfect environment).

But, having said that, I believe that what the DOD is looking for is for you to state your environment at the time of calibration on your certificate, regardless of what temperature it is. I worked at a DOD facility, and we did calipers at 73°F as a regular course of business, and it was well within the DOD required temperature range for doing so. You should have a specified range in your quality system that defines what temperature you have deemed adequate for performing calibrations.

Ryan
 
R

Ryan Wilde

#6
Mike S. said:
68 deg. F seems popular, but I just had my gage block set done by a reputable lab and they were done at 72 F. Considering caliper resolution is limited to .0005" or .001" I'd say 68-72 would be just fine.
Now, I would sweat if my gage block set was calibrated at 72°F. Just the uncertainty added by thermal expansion (supposedly, for steel gauge blocks it is about 6.4µin/in/°F, but NIST states that the uncertainty of that expansion is in the 10% range) would really catch my attention. If I were doing a 1" block, my uncertainty DUE TO TEMPERATURE would be in the 5µin range, which on a Class 2 set is over half of the tolerance.

Ryan
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#7
All branches of the military are currently using ANSI/NCSL Z540-1-1994. Some are also using elements of ISO/IEC 17025:1999. Neither of those Standards provide requirements for specific temperatures.

Southern Girl brought up a good point. If you cal at 20 C or 23 C you can't go wrong. If you are outside that (say 28 C) then the cal may can still be valid but your uncertainties must reflect the increased temp, thermal coefficient of expansion for the gage blocks AND the calipers (different rates by the way), if the tech holds the gage block - even with gloves - there is an expansion factor there also.

If the caliper is only going to .01" then even with all the expansion figured in, you'll be better than 4:1 so just document it under Z540 and go on with life. If you are going much more than that, then calculate the uncertainties and maintain them. You may well be greater than 4:1 anyway, but you'll need evidence.

Hope this helps.

Hershal
 

Wes Bucey

Quite Involved in Discussions
#8
As an aside:
In my high tech, tight tolerance machining environment, we frequently "normalized" sample, gages, and instruments in a temperature controlled room for 24 hours before making crucial measurements. (the actual temperature is less important as long as EVERTHING is the same)

We were careful not to inadvertently heat anything by touching with hot hands or breath once they were normalized. It took most of the question of coefficient of expansion off the table.

Of course, in most cases, this is overkill unless you are dealing in measurements to fifty millionths of an inch. Going much smaller (nanometers, for example), puts you in the world of electron microscopes.
 
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