NMI and Designated Institutes (DI) maintain national standards. Their role at the top of the calibration hierarchy is to provide traceability to the SI units and to offer the best measurement capability within nations. Most NMI and DI use the MBW 373 Model for verification of generated values from primary humidity generators. In laboratories without primary realization, the 373 Model is the most commonly used transfer standard to maintain traceability.
Most of the International Intercomparisons for humidity have used the 373 thanks to its fundamental performance and its unique capability to provide traceability to the Kelvin through its second mirror PRT feature.