Intercomparison of Platinum Resistance Thermometers and T68 Calibrations

Abstract

An experimental investigation has been made of the irreproducibility of the International Practical Temperature Scale of 1968 below 273.15 K. Ten national standards laboratories submitted a total of 23 capsule-type platinum resistance thermometers to NPL where, together with a further 14 NPL thermometers, they were intercompared at 50 temperatures between 4.2 and 373.15 K. Most of the thermometers submitted had been calibrated on IPTS-68 as realized in the laboratory of their origin and our measurements thus allow us not only to compare fixed point realizations and the resulting differing versions of T68, but also to separate the component of irreproducibility that results from fixed point differences from that which reflects intrinsic thermometer variation. As a result of these measurements it is clear that differences between calibrations at the fixed points are the major cause of irreproducibility in IPTS-68 and that progress towards a more reproducible scale will come most readily through a reassessment of the choice of defining fixed points and the manner of their realization.