Some recent developments at NBS in mass measurements

During the early 1970's the results of a routine circulation of some mass artifacts between several laboratories revealed unexpected systematic errors. Our interest in understanding these anomalies led us to undertake several projects that are soon to be incorporated into a new round-robin effort. We believe this new work will demonstrate that the mass unit can be disseminated through artifacts of less than ideal characteristics and the results remain free of serious systematic errors. Such a demonstration will not only increase our confidence in the calibration of mass strandards but should also point to areas where fine tuning can yield further improvements on a laboratory-by-laboratory basis. We will discuss the effects on mass measurement of artifact characteristics, i.e., geometry, thermal conductivity, density, etc. The role played by the interaction of these artifact properties with ambient conditions is important to the outcome of a mass measurement process. To minimize these interactions we have applied electronic control circuitry to the weighing process. Our next round-robin weight package will contain the developments which are discussed in this paper.