Checking It Twice: Calibration vs. Verification
As the air cools, and we start thinking about Santa’s list, let’s consider one way we check to see if we’ve been ‘naughty or nice’.
Calibration – Its History
In 3000 BC, the Egyptians defined the Cubit used in building the pyramids as the length of the current Pharaoh’s forearm and hand span. The Master was carved into granite, and workers were given wooden or graphite copies that were calibrated every full moon. Failure to bring the copy back for calibration was punishable by death. (You don’t do that, do you?)
In the Zhou Dynasty (1046 – 256 BC), a standardized system of measuring equipment was developed, which included twice-yearly calibration by the state, acceptable tolerances and corrective actions in the case of tolerance being exceeded.
The Qin Dynasty (221 – 206 BC) produced large batches of standard measuring tools, which were distributed to all corners of the empire.
The Tang Dynasty (618 – 907 AD) added a seal to identify calibration.
Today we have calibration laboratories, with Transfer Standards that are traceable back to a Master held by National and International Standards (Nat’l Institute of Standards and Testing in the USA).
Calibration is a verification of measurement vs. a known calibrated standard:
The master blocks in any manufacturing facility are traceable through the calibration labs Transfer Standards, all the way back to the Master held by the National Institute of Standards and Testing (NIST).
Calibration requires checking that a device can measure 3 or 4 distances across the span of its capacity accurately and consistently (smallest, largest, and somewhere in between) and has a scheduled frequency for recalibration based on device wear.
Calibration is NOT the same as Verification which makes sure measurement is consistent, but lacks traceability. Verification includes:
Checking that fixtures have not worn or warped to approve output that is outside specifications.
Checking that formulae in software are calculating correctly
Checking that new or updated software is performing correctly
Resetting the Zero point on a device, or verifying it measures Zero.
Checking a device against an uncalibrated standard that may have or develop unidentified inconsistencies.
Final thought - Geometric Dimension & Tolerancing assumes that equipment is calibrated and maintained to prevent measurement errors. If final inspection occurs outside a QC lab, are the tools used calibrated?