I worked a lot with torque wrenches in the Army, and always wondered how the “first” torque wrench was calibrated without another one to verify that it was accurate? Was there another tool to verify the calibration was correct and if so how did that one get calibrated. In my head it keeps asking “well how did the next one get calibrated?” Every time I think about the first precision tools of any type, not just for torque.
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There are indeed “levels” of calibration, from the tool you’re handling, through its manufacturer, probably through a contractor that does their calibration etc., until you follow the trail far enough and end up in a physics lab where someone is testing some measurement device against a stadard definition of a unit.
It used to be particularly fun when some of those “definitions” were literal physical objects, like the standard meter and kilogram. And there was a whole routine where the various physical standards around the world (there were multiple copies distributed around) were measured and compared for changes. And changes DID occur, because a physical object is a physical object, but it was though luck, because the unit was whatever the object was. So whatever that change was, everything was calibrated to say “this is a kilogram/meter now”.
That’s why all the SI units have since been redefined in terms of basic physical constants. But those are still only possible to measure to whatever precision is currently available, and so are constantly refined.
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