The triple point of water (TPW) is not only the most accurate and fundamental temperature standard available, it is also one of the least expensive and simplest to use.
Water cells are essential
Triple point of water cells fill four critical purposes. Firstly, 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. Secondly, they provide a critical calibration point with unequalled uncertainties. Thirdly, for users who characterise probes using ratios (that is, they use the ratios of the resistance 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 characterisations of critical thermometer standards, which can be used to extend calibration intervals. Lastly, the triple point of water is where the practical temperature scales (ITS-90) and the thermo-dynamic temperature scale meet, since the triple point of water is assigned the value 273. 16K (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 vapour. (There is almost no residual aid 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 realised. Hart water cells achieve this temperature with expanded uncertainties of less than 0,0001°C and reproducibility within 0,00002°C.
In simple terms, water cells are made from just glass and water, but there is much more to it than that - for starters, that is not just any water in there.
Heavy water
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 compromised of eight protons and eight neurons (16O). Some oxygen atoms, however, have an extra neuron (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.
The ITS-90 recommends that water cells be made from water with 'substantially the isotopic composition of ocean water'. Research has shown that TPW errors associated with isotopic composition can be as large as 0,0025°C. That is seven micro-Kelvin!
Hart offers two options for verifying the isotopic composition of any purchased water cell, both at nominal costs. It can submit to a testing laboratory a sample of water taken from the user's own cell (after it was completely manufactured, so the user gets a valid comparison) and present the user with the test report. Alternatively, Hart can send that water sample to the users in a sealed ampoule for them to conduct their own tests. Multiple samples from the same cell (virtually as many as desired) so users can check for changes over time.
Impurities
Further, the potential for errors owing to water impurity is even greater than the errors from isotopic composition. Hart cells undergo multiple distillation processes and utilise special techniques to retain water purity. Among other things, Hart's primary standards scientists are able to connect quartz cells directly to the glass distillation system without using coupling hardware that may invite contamination.
Glass versus quartz
Most Hart water cells may either be purchased with borosilicate glass or with fused silica ('quartz') housings. What is the difference? Glass is less expensive than quartz, but it is also more porous, allowing impurities to pass through it over time. Research indicates that cells made from glass typically drift about 0,00006°C per year.
Many sizes
Hart cells come in four general sizes. Models 5901A, 5901C and 5901D each come in either quartz or glass shells and include 265 mm of thermometer immersion depth. The primary difference between these models (other than the arm on the 5901A) is the inside diameter of the probe well. A variety of baths is available, which can maintain the triple point within these cells for many weeks. Accredited (NVLAP) test certificates are available with any cell under our Model 1904-TPW. The 9210-5901B combination is suitable for both calibrating thermometers and providing periodic checks of sensor drift.
Accessories
For simplest realisation of the TPW in our larger cells, the Model 2031 'Quick Stick' Immersion Freezer uses dry ice and alcohol to facilitate rapid formation of an ice mantle within the cell without requiring constant intervention while the mantle forms.
Insurance is also available for each water cell purchased from Hart. Water cells are not difficult to handle, not only is the TPW difficult to realise, but they are delicate and accidents do happen. For a nominal fee Hart will insure its cells in one-year increments. After that if something goes wrong, the company will replace cells, no questions asked.
There is no tool available to temperature metrologists more valuable than a reliable triple point of water cell. Hart cells use the right water, right enclosures and right manufacturing methods to ensure cells of the highest standard available in the world.
For more information contact Val Verwer, Comtest, 011 254 2200, [email protected], www.comtest.co.za
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