Because of the need to keep National Prototype Kilogram 20 in pristine condition, NIST now uses six 1 kg flywheel standards as intermediaries so that its working standards need not be calibrated directly against the prototype. Flywheel standards are calibrated against the prototype, and working standards are calibrated against the flywheels on a Mettler balance. This Swiss one-pan precision balance is displayed here holding two flywheels and two weights belonging to a customer of NIST from private industry. The company requested precision calibrations, and thus NIST is calibrating its customer's weights directly against NIST flywheels rather than against NIST working standards as is typically done when less accuracy is required.
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