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Incoming links: Isotopic composition
Triple-point cell 2026-10-14
A triple-point cell turns phase coexistence into a reproducible reference for thermometer calibration. The NIST water-cell demonstration uses a sealed glass vessel containing water of specified isotopic composition and an open central thermometer well. Cooling this well with solid carbon dioxide and alcohol freezes an ice mantle while leaving liquid water and its vapor. The coolant is outside the sealed sample; the sample's vapor space does not communicate with room air.[ref]
Triple-Point Cell (NIST demonstration). Triple-Point Cell, preparation description, uploaded 21 December 2016
In the actual NIST cell-comparison experiment, newly frozen mantles were compared with an aged reference mantle in a controlled maintenance bath. A warm aluminium rod melted a thin inner layer around the well, allowing the mantle to rotate freely and establishing an ice–water interface close to the sensor. A standard platinum resistance thermometer and a resistance bridge measured the electrical resistance ratio to a calibrated, temperature-controlled reference resistor. Readings at two currents permitted extrapolation to zero electrical power, correcting self-heating; measured immersion depths supplied hydrostatic pressure corrections. Comparing preparations over time tested stabilization rather than assuming freshly frozen ice was ready.[ref]
Effects of different methods of preparation of ice mantles of triple point of water cells on the temporal behaviour of the triple-point temperatures by G. T. Furukawa and others. Furukawa, Mangum and Strouse, Effects of different methods of preparation of ice mantles of triple point of water cells on the temporal behaviour of the triple-point temperatures (1997), Abstract, §1 and §§3.2–3.5: aged reference, inner melt, bridge readout, zero-power and hydrostatic corrections
The inference is a reproducible fixed-point response for the thermometer, not a direct reading of three chemical potentials. The reference cell anchors the comparison; a water point alone does not determine a thermometer's response at every other temperature. The ideal argument excludes impurity and isotope variation, finite interface effects and unintended heat transfer; real cells also require adequate immersion and stable electrical references. Repeatability is evidence that these effects are controlled, not a reason to omit their uncertainty.
Since 20 May 2019 the kelvin has been defined through a fixed numerical value of the Boltzmann constant. The thermodynamic temperature of the water triple point is consequently a measured quantity with uncertainty, rather than an exact value imposed by the kelvin definition. Its continued use as an assigned reference on the International Temperature Scale of 1990 is a separate convention.[ref]
On the revision of the International System of Units (SI), 26th CGPM Resolution 1 (2018). Resolution 1 of the 26th CGPM (2018), effective date; Appendix 2, third bullet; Appendix 3, kelvin definition
[ref]The International System of Units (SI), ninth edition, SI Brochure. §2.3.1: practical temperature scales and ITS-90