Triple point of water (TPW)


The triple point cell is a cylindrical quartz glass container with an internal measure pit where will immersed the calibrating probe. In the cell inner it's bounded very pure deionized water with certified isotopic percentage of hidrogen and oxigen. The internal pressure is 611.73 Pa and that seems air, it's water vapour; at ambient temperature the water is in liquid phase.

To reach the triple point, must be in the peculiar balance steady situation in which will coexists the solid phase with the liquid and the vapour phases. Therefore must cool the cell to obtain ice; the tecnique is not simple. It's to remember that if all the water freeze, the cell will be irreparable damaged.

With suitable progressive freezing tecnique will'have to establish an ice case all around the measure pit. Then will'have gently heat so that present a uniform film of liquid water between ice case and glas all of the measure pit.

In such situation in the measure pit the temperature is 273.16 K which is equivalente to 0.01° C. The situation is steady until all the ice is melted.

Therefore will'have to immerse the cell into maintenance bath Hart 7312 and so we are ready to the calibration of the metal sheath SPRT. The stability of the cell will last a couple of days.


Stabilisation phase
of ice case

Cell with water at triple point

Maintenance bath steady to 0.01°C


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last update: Aug. 1 2012

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