Need to understand difference between an actual Calibration vs. a Calibration check

chuck111hardy

Registered
I need a better understanding here with calibration. Specifically, calibration of hand held torque wrenches used for automotive equipment installations. Some background, the association I work for has dealers across the US who perform equipment installations. Our quality program requires the tools, in this case the torque wrenches, are calibrated and traceable to NIST. The majority of our members hire a thrid-party calibration lab to perform the calibrations. But, i have a company who would like to do calibrations in-house. They asked me if a certain piece of equipment (a SnapOn QCDTC3250 electronic Torque Checker) would be sufficient to perform the calibrations. My undertstanding is that the 'checker' is just that, it checks the tool to verify if it is still in calbration, but that to calibrate the tool you would need a different piece of equipment (something at least 10x more precise). But my knowledge of calibration is weak in this area. Can someone help explain the differrence between a calibration versus a calibration check, or are they actually the same thing? thanks in advance for your knowledge...
 

dwperron

Trusted Information Resource
I step lightly into this one.... some painful memories here.

Calibration is comparing a tool to an instrument (standard) that is calibrated, whose results are traceable to a national body such as NIST, and is more accurate than the tool you are calibrating. Very brief and incomplete answer here, but good enough for this discussion.

A calibration check is a confidence test of a tool, verifying that is is still accurate prior to it being used. Frequently the equipment used in these checks can be as accurate as the standard used for the calibration.

Because of that, there are some organizations that will use these calibration checks in place of a full calibration program, especially for torque. What they do is verify the accuracy of a tool at a given setting just prior to it being used. They record these results as objective evidence that they performed a traceable check every time the tool starts being used.

Myself, I'm a belt and suspenders sort of guy. I like to see torque tools get a full calibration at a set interval, and then to go through the verification/calibration check prior to use each day to maintain confidence that the tool did not drift since the last calibration. And yes, you can do all of this in house, only sending the torque tester(s) out for calibration.
 

Tidge

Trusted Information Resource
In a past life at a business that did a lot of metrology with hand-held tools, our tools (and items like gage blocks) were regularly calibrated (traceable to NIST) but we would do calibration checks before each use. For example: in the case of things like micrometers and calipers, we'd do a 3-point (not using zero) check for linearity (using gage blocks) before each use. For other instruments (e.g. pi-wrap tapes) we couldn't always do a 3-point check (using gage cylinders) but typically we could do a 2-point check on each one.

I don't want to write an entire thesis on the appropriateness of any particular calibration check for any specific instrument, but you do have to be aware of the appropriate range of the instrument, and have some idea of how it (electro-)mechanically does its job, to know how representative the check-points are.... blah blah blah "linear over what range" fishcakes.
 

dgriffith

Quite Involved in Discussions
I agree with both above.
We use two sets of Snap-on torque testers, one in in-lb, one in ft-lb, in 2 hangars here at work.
They are there for mechanics and technicians to verify torque settings before use on aircraft. The fastener torque requirements are already established, and the tester is to help ensure the wrench is capable at the torque of interest.
Otherwise, all torque wrenches are calibrated at established intervals in our on-site calibration lab with a Snap-on m.2000 torque calibrator.
 
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