of Traceability

by David A. Vossbrinck

Traceability of accuracy for measuring or calibration equipment forms the very foundation of any quality program. Without traceability, all other aspects of quality control, from simple calibration procedures through statistical process control, can be worse than meaningless-they can be misleading.

Tracing the accuracy of any and all measuring equipment to a central control authority is essential for accurate communication within and between industries. One one-thousandths of an inch must mean the same in New York City as in San Diego. Without traceability to an absolute standard, there would be only chaos.

What is traceability?

Traceability is the comparison of any measuring device or system to a standard of greater accuracy. Typically, a minimum of four-to-one greater accuracy is required, although 10-to-one is not uncommon. This ultimately has a known comparison to a national standard such as those kept by the National Institute of Standards and Technology. They, in turn, periodically send their standards to Sevres, France, to the Bureau International des Poids et Measures in order to achieve international agreement.

Although a standard should be at least four times more accurate than the device it is testing, it need not have the same principle of operation. For instance, a spring scale with an accuracy of ±1 percent could be certified by a load cell with an accuracy of ±0.25 percent, which in turn could be certified by dead weights with an accuracy of ±0.0625 percent.

The load cell is a secondary standard, while the calibrated weights are considered a primary standard because of the direct link between the scale (measuring in pounds) and the known weights. Secondary standards are used for ease and convenience but must ultimately be traceable to primary standards.

Any valid certification of accuracy has all the information necessary to trace the standards used to NIST.

Sometimes it is necessary to provide traceability of two standards to qualify the calibration of one device; a torque device for example. Torque is defined as a force multiplied by a known distance from a center point. In the case of torque, both weights and calibration arms must have independent certifications traceable to NIST, and the final accuracy is a combination of each component's uncertainty. Hanging weights and arms on every torque tool would be an unmanageable procedure, thus the domination of strain gauged torque analyzers used to calibrate torque tools.

Periodically, calibration equipment needs to be certified. In some cases, the frequency of recertification is determined by the manufacturer's recommendation, but ultimately it is determined by each company's quality assurance program. Frequency of usage plays a large part in frequency of recertification, as does the difference in accuracy between the calibration equipment and the standard.

Unfortunately, recertification may be useless if the agencies performing the recertification don't follow equipment procedures. Proper procedures, while essential to any meaningful recertification, are often omitted in quality audits. For example, auditors pay attention to the traceable certification of an arm used as a primary standard for measuring torque, but often overlook errors introduced during its use. It is unrealistic for quality auditors to be experts in procedures for every type of calibration equipment used, so they rely on the agencies doing recertification work to generate and properly use procedures.


Periodic recertification, traceable to NIST, is the core of any quality assurance program when measuring devices are an essential part of a company's manufacturing or inspection process. Without it, liability increases while product quality suffers, with a resultant increase in the overall cost of doing business.

About the author . . .

David A. Vossbrinck is CEO of Torque & Tension Equipment Inc. in Santa Clara, California. For more than 25 years, Torque & Tension Equipment has supplied both primary standards and calibration equipment to all industries for the measurement of static and dynamic torque.