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NIShow and tell: NIST physicist Howard Yoon holds up the triple point of water, the point at which water exists simultaneously as a solid, liquid and gas in thermal equilibrium (when the water has no net gain or loss of heat).
For many years all around the globe, the triple point of water defined the metric system’s unit of temperature (the kelvin). But there were slight differences from one lab to the next. Concentrations of the hydrogen and oxygen isotope in the water changed depending on where in the world the measurements were happening.
Then, scientists agreed upon a fixed value for the triple point of water. They based it on the Vienna Standard Mean Ocean Water (a specific chemical composition of distilled ocean water, a standard formulation established by @iaeaorg in Vienna). No more changes.
Now, the triple point of water is one of multiple fixed points that we use to ensure the accuracy of standard platinum resistance thermometers, the basis of the international scale for temperature. Their accuracy trickles down through calibration labs and comparisons to the thermometers that measure, for example, your room temperature or the heat behind manufacturing processes.
#Temperature #Thermometry #Thermometer #Metrology
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