ITS-90: the instability of the Al point

摘要

Experiments, described below, revealed certain shortcomings of the current definition of the Al point: pressure in sealed type Al cells made of quartz was observed prior to their sealing and was found to steadily decrease. Consequently, the realized Al points using sealed quartz cells are in a state of drift. In addition, the initial Al point values yielded by open cells are higher than the subsequently realized ones: five different gases were used to apply the initial 1 atm pressure over the Al samples (air, N_(2), Ar, He, CO_(2)). Each of them appeared to contaminate slightly the Al samples, lowering the initial value of the Al point by 1 or 2 mK. However, sealed quartz triple point cells remained stable at their initial value indefinitely. Thus, the triple point of Al is a more reliable fixed point than the defining Al freezing point itself.