Inside Story Issue 7

In-The-Field---Calibration
In today’s quality world, the focus of calibration has unfortunately shifted towards simply making sure the certificate is current, and away from the actual purpose of calibration.
Issue 7
2nd Quarter
2008
A Mitutoyo America Publication

Jim Salsbury, Ph.D. Corporate Metrologist, Mitutoyo America, comments, “As measuring equipment has gotten more complex and organizations more lean, one begins to wonder if the calibration certificate is really understood by the end user. Organizations spend a lot of money on calibration, but are they getting the return on that investment that they should?”

Checkmaster-calibrationThe dictionary definition of calibrate reads, “to check, adjust, or determine by comparison with a standard.” In the field of metrology, the official definition states that the purpose of calibration is to establish the relationship between the values reported by measuring equipment and corresponding standards. The use of the term “standards” in the definition indicates that part of the purpose of calibration is to establish traceability back to international standards such as to NIST (National Institute of Standards and Technology). However, according to Salsbury, “The larger issue is the exact manner in which these relationships are determined. In other words, what are the actual tests that need to be done to calibrate a specific piece of equipment?”

The relentless search for error
Calibration is all about searching for errors in measuring equipment. Good calibration procedures are based on understanding potential errors in the equipment and how those errors may impact measurements made. Calibration procedures for some equipment are well documented in standards, but as technology changes, as tolerances get tighter, or as equipment is used in new and different ways, calibration must change as well. Many organizations are not educated in the proper calibration of their measuring equipment. Even if an organization uses commercial calibration providers, they still need to know what specifications and tests are needed for their equipment. There are many different ways to calibrate certain items, and some commercial calibration providers may not follow original manufacturer specifications. Unfortunately, it is not uncommon to see specifications being changed simply to reduce the cost of the calibration, and the uneducated customer will not know that the performance of their equipment is being compromised.

Don’t fail to communicate
The definition of calibration states “to check or adjust.” It does not necessarily imply check and adjust. Terms such as calibrate, verify, and certify are often confused – and rightly so, as there is, unfortunately, no commonly accepted usage of the latter terms in the calibration field. However, it is critical for customers to communicate whether or not they want adjustments made on their equipment to bring it back within spec. The term calibrate alone does not necessarily mean adjust, and therefore it is important to mention the desire for adjustments when requesting calibration services.

Further complicating communications is the issue regarding as found or as received condition. The most important purpose of calibration, or to be more specific, recalibration, is to develop a performance history of a piece of measurement equipment over time. The as found reading documents the status of the equipment prior to any adjustments, and therefore are the critical data needed to demonstrate the long-term performance and quality of the equipment. The as found readings are also used to determine if the calibration interval needs to be increased or decreased. There is tremendous return on the little extra expenditure needed for as found readings.

Almost all commercial calibration providers today are ISO 17025 accredited. One of the many benefits of accreditation is that the measurement uncertainty of various calibration providers can easily be compared for the same calibration task. Just like an on-line shopping guide, the scopes of accreditation are available on-line for quick and easy comparison of capabilities. Many organizations require uncertainty to be a fraction of the specifications, such as the 4 to 1 rule, and the formal estimation of uncertainty required by ISO 17025 helps ensure the required quality is met at the lowest cost.

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Order what you need
The complexity of today’s measuring equipment has made it even more important for organizations to understand their calibration needs. Unfortunately, calibration is too often seen as just having the current certificate or sticker. Salsbury recommends, “At the very least, organizations need to be more clear when ordering calibration services. Instead of just stating ‘calibrate this item,’ the request should instead be put as, ‘calibrate this item to original manufacturer specifications, including as found readings, plus make any adjustments needed to bring the equipment within specs, and also provide as left readings and issue an ISO 17025 accredited certificate.’” In Salsbury’s opinion, it would be even better for organizations to become more educated with respect to their measurement needs and with methods for calibrating their measurement equipment.

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