Primary standards are expensive and complex to use. There is, however, an exception to that and it’s the Triple Point of Water Cell. Fixed-points, sometimes called intrinsic standards, are used to define the International Temperature Scale of 1990 (ITS-90). The most commonly used fixed-point is the Triple Point of Water. A Triple Point of Water Cell is used to create a thermal equilibrium between the three phases of pure water: liquid, solid and gas. This thermal equilibrium occurs at 273.16 Kelvin, or 0.01°C.
Triple point of water cells fulfil 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 characterise 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 characterisations 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 thermodynamic 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 vapour. (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 realised.
Comtest has on offer Fluke Calibration’s 9210 Triple Point of Water (TPW) maintenance apparatus that provides built-in programming for the simple supercool-and-shake realisation and maintenance of the 5901B Mini TPW Cell. Simply insert the cell, enter the ‘freeze mode’ through the front panel buttons, and in no time at all the 9210 will audibly alert the user that the mini cell may be removed. Shaking it will initiate the freezing portion of the water. On reinsertion of the cell, the programme can be changed to ‘maintain mode’ and then it’s 0.01 °C for the balance of the day, with an uncertainty of only + 0.0005°C.
- Easy preprogrammed realisation
- Inexpensive fixed-point solution
- Training complete in less than an hour