Everything You Wanted to Know About Calibration, But Were Afraid to Ask

Standards are as old as human society itself. During the era of the pharaohs at the time of the construction of the great pyramids at Giza calibration standards were used for measurement. The Royal Cubit was the length from the back of the pharaohʼs elbow to the tip of his middle f nger. This was established in a stable material such as granite. Then working standards were made and compared to the master. These were issued to workers and they were required to return the working standards for comparison against the Royal Cubit master every full moon. Failure to submit the working standard for verif cation was punishable by death. The accuracy obtained by the use of the working standards was phenomenal. In approximately 756 ft. or 9,069.4 in., the Egyptian builders were within 4 ½ in. This is about a 0.05% accuracy. Weights from most ancient societies have been discovered and are accurate within tenths or hundredths of a gram when compared to the required standard weight. They were very concerned with maintaining accuracy. Even within the Mosaic Law in the Bible, it was stated that, “a cheating pair of scales or false stone weights were not to be found among God's people.” Today calibration of testing equipment would appear to be a subject that is unimportant or secondary to their immediate purpose. Many companies merely send out equipment and indicate on purchasing documents, “Calibrate-Certif cation Required”. The question is, “What constitutes a proper calibration?” Another factor is the profusion of different types of equipment and equipment manufacturers, which can also add confusion to the situation. For instance some units are analog and others are digital. What rules apply? Can one type be used to verify the other? In addition, most governing specif cations in nondestructive testing only give the frequency that is expected for the calibration of certain items, rarely any limits or tolerances. This can cause quite a quandary for the inexperienced and even the experienced quality professional. This paper will attempt to address many of the questions that have caused problems over the years and even make some possible suggestions for standardizing some of the calibrations.

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