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Calibration values of secondary conductivity standards obtained on a periodic basis, both for the Boeing Company and for commercial customers, have shown various drift patterns. Presently, a 100-kHz bridge used with an H-P 9825 computer compares unknown secondary standards against carefully built, National Bureau of Standards (NBS) traceable, primary bars in an oil-bath environment. Factors affecting the stability of secondary standard values of conductivity include primary conductivity bar value changes, uneven surface wear, environmental contamination, lift-off due to the presence of foreign material on the surface, and aging of the metal crystalline structure. Some secondary standards, which were large physically, exhibited significant conductivity changes across their surface due to the nonhomogeneity of the metal.
Original calibration of secondary standards was accomplished by assigning the same value to a section cut from the primary bar that was given to the primary bar itself. When new alloy metals were obtained, as older metal primary bar material was exhausted for secondary use, an H-P 65 program was developed to utilize a curve-fitting method for replacement secondary-standard material calibration. Some changes in older secondary values resulted from this method. An H-P 9825 computer program was later developed, which resulted in other secondary-value changes. During the last twelve years some significant changes occurred in the primary bars, which further altered the program and the calibration results. New primary bars have been also added from time to time to smooth out the curve-fitting program, which has some effect on the calibration values of the secondary standards.
conductivity, standard, percent IACS, titanium, aluminum, alloy, secondary, primary, bar, stability, scatter
Specialist engineer, Boeing Aerospace Company, Seattle, Wash.