Measuring for success

Measuring for Success A Calibration Overview By Michelle Garetson March 1999 From the time that we get up until the time we go to sleep, metrology affects our lives. Metrology -- the science of measurement, and calibration -- the...

The environment for Thee's Continental Testing group is a little different. CTI operates two mobile calibration vehicles in addition to its main lab. Special considerations such as change in climate and terrain have to be acknowledged for mobile units as those factors can affect calibration testing. Consistency in testing is key and every effort is made to monitor and maintain a constant for examination and verification.

Who or what controls calibration? York explains, "Metrologist's and tool calibrator's work is not necessarily governed by FAA. The standards by which people in calibration must adhere to are those set out by the National Institute of Standards and Technology (NIST)."

NIST, based in Gaithersburg, Maryland, sets the national standards that are used by manufacturers of precision instruments as well as primary and secondary calibration labs.

The FAA, however, does monitor shop practices and paperwork involved with calibration for safety compliance issues.

An example might be that an FAA inspector may want to follow up on accuracy of a self-calibrating tool such as an eddy current tester to make sure that instrument is performing properly.

Both York and Thee regard the ANSI/Z540-1 as a recommended controlling document for calibration facilities. It lists basic operating procedures for calibration labs and a copy of this document can be ordered from the National Conference of Standards Laboratories (NCSL) through their web site at

As mentioned before, the National Institute of Standards and Technology (NIST) sets the standards for accuracy, and whenever physically possible, CTI and Midcoast, as secondary cal labs, subscribe to testing to a ratio of four times greater than the unit under test. In other words, their equipment should show four times the accuracy demonstrated by the test sample. But, Thee says "regulation is placed on the airline or repair station to ensure that calibrations on any precision tool or test equipment being used to certify aircraft or components are NIST traceable."

Traceability is very important in metrology and calibration; whether it means tracking the particular tool or instrument used, the manufacturer that produced the item, or a facility where calibration was performed.

"In a Ôtree' of calibration traceability," explains Thee, "NIST is at the very top, then the primary calibration labs, the secondary labs, and finally, the end user."

York offers the abridged definition, "Traceability is an unbroken chain of comparisons from the measurement being made." He was also kind enough to furnish the proper definitions for common calibration terms from the NCSL Glossary of Metrology-Related Terms. (See sidebar)

Calibration Documentation
With respect to traceability and the maintenance organization, equipment that has been tested and calibrated, will be accompanied by documentation, providing a history for that item.

All tools submitted for testing will first be evaluated on its condition upon arrival. It then goes through an initial check on its readings and performance. That data is recorded on the Calibration Report as customers may request "in-and-out" data to be returned with the tool. This form is sent back to the customer with the equipment, and a hard copy along with a computer backup copy remains at the calibration lab.

The Significant Out-of-Tolerance Notification form also accompanies the equipment if its condition warrants corrective action. Customers receiving this form may be asked if they would review the recommendation and determine if their end products have been affected, or if they would notify their customers who might have been affected by the use of the equipment, or if they merely wish to annotate the calibration records for that item with the significant out of tolerance disclaimer. Or, they can do nothing.

There is also a Certificate of No Calibration Required which is used for "non return-to-service" equipment only. Additionally, the manufacturer of the equipment must clearly state that calibration is not required in order to qualify.

A Certificate of Calibration is returned with the tool or equipment, and a copy of th report is kept by the facility that performed the testing.

A National Calibration Certificate can also be requested if you need direct traceability to the National Standards Institutes. This document provides credibility for ISO 9000 registration and compliance.

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