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During the 1990s, the BIPM developed a new balance (FB-2) based on the use of flexure strips instead of knives and flats. This balance can operate in air or in vacuum. Its mass exchanger has eight positions. Two of the positions are normally used to determine air density based on the measured difference in mass between two air buoyancy artefacts.
The mass exchanger is designed to operate as quickly as possible (consistent with other constraints), to improve the efficiency of weighing. The time required to exchange any two masses is less than 2 min. To reduce air convection, heat sources inside the enclosure are minimized. The beam and the pans are suspended from Cu-Be flexure-strips, which are designed to allow mass comparison between objects up to 2 kg. An electromagnetic servo-control maintains the beam at constant position and provides the imbalance reading.
The on-scale range is about 200 mg and the standard deviation of the balance is generally within 0.1 µg.
Detail of the mass exchanger of the FB-2 balance
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Summary
