Tackling The Triple Point Shawn Carlson Scientfic American Jan 99
(build your own triple point cell, a sustainable calibration point of 0.01 C)
Calibrating With Cold Shawn Carlson Scientfic American Dec 2000
(based on the freezing point of mercury calibration point is –34.8 degrees C
also includes a proceedure to compensate for a boiling point calibration of 100C)
Homemade High Precision Thermometer Shawn Carlson Scientfic American Mar 99
(its a DIY 4 wire RTD)
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One of the horrible truths of scientific research is that simple and inexpensive techniques will get you just so far. Beyond some point, increasing accuracy can be obtained only with a disproportional rise in expense, sweat and frustration. That's partly because accurate measurements require an extremely well calibrated instrument, and providing such an exact scale can be a vexing challenge.
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Quote:
Triple point of water
The single combination of pressure and temperature at which water, ice, and water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals (ca. 6 millibars). At that point, it is possible to change all of the substance to ice, water, or steam by making infinitesimally small changes in pressure and temperature. (Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.)
Water has an unusual and complex phase diagram, although this does not affect general comments about the triple point. At high temperatures, increasing pressure results in first liquid, and then solid water (above around 109 Pa a crystalline form of ice which is denser than water forms). At lower temperatures the liquid state ceases to appear with compression causing the state to pass directly from gas to solid. It is, however, possible to melt ice by increasing pressure under specific conditions.
At a constant pressure higher than the triple point, heating ice necessarily passes from ice to liquid then to steam. In pressures below the triple point, such as in outer space where the pressure is low, liquid water cannot exist: Ice skips the liquid stage and becomes steam on heating, in a process known as sublimation.
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