The Pilot Program for mass measurement is the result of a consideration in which the values produced are thought of as the products of a mass measurement process. The collective performance of elements of the mass measurement process results in establishing the process precision which, under certain conditions, can be described quantitatively by pertinent performance parameters. The uncertainty attached to the product of the process, the measured value, is computed from these parameters and reflects the total performance of the process rather than the immediate measurement which might have produced the value. Interpretations of uncertainty and surveillance tests are discussed. The Pilot Program in mass measurement, whereby suitable process performance parameters can be established for precise mass measurement processes in other facilities, is discussed.
This paper gives a review of the concepts and operations involved in measuring the mass of an object. The importance of viewing measurement as a production process is emphasized and methods of evaluating process parameters are presented. The use of one of the laboratory's standards as an additional unknown in routine calibration provides an accuracy check and. as time goes on, the basis for precision and accuracy staterrients.
It has always been assumed that the measurement of the difference in mass between two objects would be the same in all laboratories. Recent National Bureau of Standards measurements involving dissimilar objects (effective density ranging from 2.7 to 16.6 grams per cubic centimeter) at a wide variety of pressures (0.5 to 2 atmospheres) have been made with sufficient precision to test this assumption. The results show unsuspected discrepancies which may approach 1 milligram in a kilogram in the assignment of mass values when dissimilar materials are involved. These discrepancies have not been noted in the past because precision comparisons of both like and unlike materials have nearly always been made in a relatively restricted range of environmental conditions. The worldwide mass measurement system is therefore consistent, because similar materials have been used in the construction of weight sets, but possibly offset with respect to the mass unit as embodied in the platinum-iridium defining artifact.
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