Triple Point of Water Cells - Fluke Calibration Instruments - Hart Scientific - #1

FLUKE

Hart Scientific®

The triple point of water (TPW) is not only the most accurate and funda­mental temperature standard avail­able, it's also one of the least expensive and simplest to use.

Triple point of water cells fill four critical purposes. First, they provide the most reliable way to identify unacceptable thermometer drift between calibrations—including immediately after a calibration if the thermometer has been shipped. Interim checks are critical for main­taining confidence in thermometer readings between calibrations. Sec­ond, they provide a critical calibra­tion point with unequaled uncertainties.

Third, for users who characterize probes using ratios (that is, they use the ratios of the resistances at vari­ous ITS-90 fixed points to the resis­tance of the thermometer at the triple point of water, indicated by "W"), interim checks at the triple point of water allow for quick and easy updates to the characterizations of critical thermometer standards, which can be used to extend calibra­tion intervals.

And lastly, the triple point of water is where the practical temper­ature scale (ITS-90) and the thermo-dynamic temperature scale meet, since the triple point of water is assigned the value 273.16 K (0.01 °C) by the ITS-90 and the Kelvin is defined as 1/273.16 of the thermodynamic temperature of the triple point of water.

Good triple point of water cells contain only pure water and pure water vapor. (There is almost no residual air left in them.) When a portion of the water is frozen cor­rectly and water coexists within the cell in its three phases, the "triple point of water" is realized. Hart water cells achieve this temperature with expanded uncertainties of less than 0.0001 °C and reproducibilities within 0.00002 °C.

In simple terms, water cells are made from just glass and water, but there's much more to it than that! For starters, that's not just any water in there.

Hart cells contain carefully and repetitively distilled ocean water and are meticulously evacuated and sealed to maintain an isotopic com­position nearly identical to the inter­national standard, "Vienna Standard Mean Ocean Water," or "VSMOW."

The oxygen atoms found in most water are predominantly comprised of eight protons and eight neutrons (¹⁶O). Some oxygen atoms, however, have an extra neutron (¹⁷O) or two (¹⁸O). Similarly, the hydrogen atoms

in water normally contain only a single proton (¹H), but sometimes contain a neutron also (²H), resulting in "heavy" water. These isotopes coexist in varying proportions in ocean water, polar water, and conti­nental water, with ocean water being the heaviest.