improved Mass Measurement Accuracy using the PNB Load Cell Scale

The PNB Load Cell Scale is a preloaded, Narrow-Band calibration mass comparator. It consists of (1) a frame and servo-mechanism that maintains a preload tension on the load cell until the load an unknown mass is sensed, and (2) a null-balance digital instrument that suppresses the cell response associated with the preload thereby improving the precision, and accuracy of the measurements. Ideally, the objects used to set the preload should be replica mass standards that closely approximate the density and mass of the unknowns. The advantages of the PNB scale are an expanded output signal over the range of interest which increases both the sensitivity and resolution, and minimiEes the transient effects associated with loading and unloading of load cells. An area of immediate and practical application of this technique to nuclear material safeguards is the weighing of 0Fg cylinders where in-house mass standards are currently available and where the mass values are typically assigned on the basis of comparison weighings. Several prototypical versions of the PNB scale have been assembled at the U.S. National Bureau of Standards. A description of the instrumentation, principles of measurements, and applications are presented in this paper.