Fixed Points for Secondary Level Calibrations

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Fixed Points for Secondary Level Calibrations (560.12 KB)

Fixed-point calibrations have many advantages over comparison calibrations. The unsuitability of traditional fixed-point apparatus for secondary and industrial calibrations is discussed. New kinds of fixed points have been developed for secondary and industrial calibrations.

A miniature cell for the triple point of water (TPW) and a self-operating TPW apparatus have been developed for secondary and industrial calibrations. Built-in programming in the apparatus makes its operation extremely simple. Typical freezing curves are shown. A comparison between the new device and a traditional TPW cell shows a difference between them less than 0.2 mK.

A permanently sealed gallium cell is discussed. The procedure for realizing its melting point is described in detail. Its melting plateau typically lasts from ten days to two months. Three cells were intercompared using five SPRTs. The differences among them were well within 0.1 mK.

Miniature cells for the freezing points of pure metals and a portable apparatus for these mini cells are introduced. Many freezing and melting curves were realized from the mini cells. A comparison between these mini fixed-point cells and their traditional-size counterparts shows differences within 1.0 mK for the freezing points of tin and zinc. The benefits of using the melting point technique instead of the freezing point technique for secondary calibrations are discussed. These include lack of supercool, longer plateaus, simplicity, and increased efficiency.