text: Triple-point cells are used in the calibration of thermometers.  For exacting work, triple-point cells are typically filled with a highly pure chemical substance such as hydrogen, argon, mercury, or water (depending on the desired temperature).  The purity of these substances can be such that only one part in a million is a contaminant, called "six nines" because it is 99.9999% pure.  A specific isotopic composition (for water, VSMOW) is used because variations in isotopic composition cause small changes in the triple point.  Triple-point cells are so effective at achieving highly precise, reproducible temperatures, that an international calibration standard for thermometers called ITS–90 relies upon triple-point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.