The Triple Point of Water (TPW) is the reference temperature for resistance ratios in platinum resistance thermometry. It is defined to be the temperature of pure water with an isotopic concentration equivalent to that of mean ocean water, with its solid, liquid, and vapor phases in thermal equilibrium. With the redefinition of the SI unit Kelvin in 2018, the TPW is no longer part of the definition of the thermodynamic temperature scale, but remains a defining fixed point on the ITS-90 with an assigned value of 273.16 K (.01°C t90). Practical realizations of this fixed point must be corrected for hydrostatic head, thermometer self-heating and isotopic composition effects. The easiest to use and most cost-effective of all the defining fixed points, the TPW cell provides the lowest uncertainty achievable for calibration of precision thermometers of many types.

Pond Engineering constructs TPW Cells of high-quality borosilicate glass (Type A and B also available in quartz) with 11 or 13mm inside diameter thermowells (other sizes available on special order). Following thorough cleaning of the cell interior, each cell is evacuated, filled with ultra-pure degassed water, and flame sealed. Isotopic concentration analysis is performed on the water used to fill the cells after the degassing process is complete. The results of this analysis are used to calculate an isotopic composition deviation correction provided with each cell. Water samples for independent isotopic analysis can be provided with each cell upon request at the time of order placement.

In 2020, Pond Engineering developed a new style of TPW cell, representing a significant leap forward in usability and transportability. The patented design of the Model K29H “Halo Cell” allows the user to re-distill the water sample at any time, virtually eliminating long-term drift issues associated with leaching of contaminants from the glass into the water sample. Unlike any other TPW cell, the K29H cell can also be transported by conventional delivery methods without damage caused by internal water hammer effects.

In addition to this new style of cell, Pond Engineering also manufactures the traditional Type A and B cells.

Uncertainty, reproducibility and stability of the temperature realized in a TPW Cell are nothing short of phenomenal. Pond Engineering TPW Cells are guaranteed to realize a temperature within +0.000 0, -0.000 1 K of the theoretical, defined TPW after correction for hydrostatic head, thermometer self-heating and isotopic composition.

Low-cost maintenance systems like Pond Engineering’s Model K29D are compatible with all 3 styles, and make TPW cells more efficient than conventional ice baths, even for routine calibration work.