= Triple-point cell
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.<Triple-Point Cell (NIST demonstration)>[Triple-Point Cell, preparation description, uploaded 21 December 2016]{ref}
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.<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]{ref}
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 $F=0$ 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.<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]{ref}
Back to article page
