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 comparison of measurement, are responsible for the ways that we gauge our present and future, as well as how we define our past.
We rely on our personal wristwatches and clocks to be within acceptable parameters of those wristwatches and clocks at the places where we conduct business and other pursuits. We trust that our car's speedometers are calibrated correctly when our peripheral vision spots the State Trooper's radar detector. If any or all of these items are out of sync with the accepted standards; missed deadlines, fines, and lost opportunities can result.
Why is Calibration Important?
For any business, not just aviation, having the proper tools is imperative to how business is accomplished and how customers perceive the tasks performed. However, having the proper tools isn't always enough. Many tools need to conform to industry standards in order to work properly and to be utilized specific to their design.
Proper calibration of tools such as torque wrenches, weighing scales, and non-destructive testing equipment is critical to the accuracy required in their respective functions.
Fluke Corporation, based in Everett, Washington, offers some insight on its web page (www.fluke.com) as to the importance of maintaining a calibration calendar, but the need for calibration is directly related to the specific tasks performed by your equipment.
According to Fluke, the "measuring instruments are the Ôheartbeat' of your company . . . they control the quality of your products and in the end, responsible for the success and the profitability of your business."
Further, "A regular check of your Ôheartbeat' with traceable calibration equipment is essential."
What this regular check does for your operation is to ensure the quality expected by your customers — this is especially important where ISO 9000 certification is involved.
Other benefits of regular calibration include:
• Establishment of a quality system
• Consistency of quality of your production output
• Meet production control criteria
• Lower operating costs due to enhanced reliability
• Organized records for quality audit and reviews
• Establishment of calibration traceability
"The calendar for calibration can vary," says Michael Thee, president of Continental Testing Inc. (CTI) in Watkins, Colorado. "But, most items, unless otherwise stated by the manufacturer of the equipment, is one year. FAA inspectors can require more frequent calibrations such as for pitot/static testers. Due to the sensitive nature and critical importance of that instrument, a six month calibration is recommended. Also, most NDT equipment is on a six month cycle."
Thee adds, "Customers can start a cal cycle that has all calibrations due at certain intervals at one time a year or two times per year, and so on. Instead of tracking tools at all different times a month, customers can have the bulk completed in one on-site calibration, which reduces tracking significantly."
Like the examples of the wristwatch and speedometer, Don York, metrologist for Midcoast Aviation in Cahokia, Illinois, stresses that operations need to be able to trust the measurements of their tools and machinery, regardless of where that tool lies on the calibration timeline.
"If there is doubt," says York, "then the calibration of that equipment should be checked. If a tool is dropped, if there is possible salt water contamination or excessive humidity in the environment of the equipment, a re-calibration would be warranted."
Environment is very important. Midcoast's calibration lab is constantly monitoring humidity and temperature levels in an effort to maintain consistency for calibration testing. Even the type of lighting used in labs has to be considered as different lamp categories vary in heat output.
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.
Regulations
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 www.ncsl-hq.org.
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.
Within the global marketplace, this ISO 9000 issue will become increasingly important in the future. Developing a documentation program for traceability now, the road to international compliance will be possibly less rocky.
Evaluation and implementation of calibration is an exact exercise. Calibration is necessary for any precision tool or equipment used to certify an aircraft or component, and should be accomplished by metrology professionals.
