Abstract
The triple points of oxygen samples sealed in miniature pressure cells were investigated by means of adiabatic calorimetry. The triple point of a 99.999 percent pure commercial oxygen sample was found to be 0.94(0) mK higher than that of an "ultra-pure" sample prepared by thermal decomposition of potassium permanganate (KMnO(4)). The higher value is attributed principally to argon impurity in the commercial oxygen. The results of eight sets of observations using six thermometers, calibrated on the International Practical Temperature Scale of 1968 as maintained at the National Bureau of Standards, and two sealed cells of the ultra-pure oxygen show a range of 0.17(8) mK. The capsule-type platinum resistance thermometers that have been used are shown to have outstanding stability and the multiple calibrations made on them at the National Bureau of Standards extending over six years are shown to be consistent to within 0.15 mK at 54.361 K. The results of measurements on an internationally circulated sealed cell of commercial oxygen show its temperature to be 0.58(1) mK higher than those of the ultra-pure oxygen.