What Does it Really Mean?
Even the most technical of people that I have met seem to get lost in this word. In our daily lives, the word has taken on so many different meanings that it is hard to keep them all straight. When I get to work in the morning and print the first piece of paper of the day and go to the copier to get it, it says “please wait, calibrating.” How appropriate is that in a calibration lab, even the copier calibrates.
We calibrate our surround sound systems. We calibrate the displays on devices. As I write this, there is a news headline about re-calibrating ATMs taking longer than expected, and right below that is a suggested article from the Ohio department of agriculture about calibrating a manure spreader.
Somewhere along the way, the word adjustment has been replaced by the word calibration; I guess someone thought it sounded more important that way. Unfortunately, this idea that the words calibration and adjustment are somehow interchangeable has managed to spread throughout our entire culture.
How many times have I heard this statement: “you didn’t calibrate it, you just verified it.” There is a fundamentally flawed view that if an adjustment doesn’t take place, then it is somehow not calibration. This term, verify, baffles me a bit. Do the users of this term intend to mean that you verified that an item met a particular metrological standard or tolerance and that we were able to assess this without calibration, or establishing an associated measurement uncertainty?
The International vocabulary of metrology – Basic and general concepts and associated terms, JCGM 200:2012, commonly referred to as the VIM is the dictionary of terms in the metrology world. The VIM defines “verification” as: “provision of objective evidence that a given item fulfils specified requirements.” So, in order to verify an item, you must first gather this “objective evidence,” which means calibrate the item.
So, What is Calibration?
If there is a word that is synonymous with calibration then, it’s measurement. Before we can even think of establishing grounds for an adjustment we need to measure the thing first. In the world of metrology, often, we don’t even care what the nominal size of an item is, we care about the actual measurement. Take for example grade “K” gage blocks. They have a specification of ±20μin, but they are used to calibrate other gage blocks that can be ±4μin. How can this be? The critical specification for these blocks is that they are nearly perfectly flat and parallel to themselves. So, given their flatness and parallelism, their actual measurements are what is used when performing analyses with them.
Calibration from a metrological point of view is the establishment of traceability. Traceability is a documented unbroken chain of measurements going back to the “International System of Units.”
A core concept in metrology is metrological traceability, defined by the Joint Committee for Guides in Metrology as "property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty." Metrological traceability permits comparison of measurements, whether the result is compared to the previous result in the same laboratory, a measurement result a year ago, or to the result of a measurement performed anywhere else in the world. (Metrology, n.d.)
This way an inch is an inch in Germany, France, China, Mexico, or here in the United States. This is the core of calibration. This is the reason that traceability is the focus of calibration, we need to be able to replicate the same measurements all the way around the world and calibration, and traceability are how we establish that.