DETERMINATION OF THE TRIPLE-POINT TEMPERATURE OF GALLIUM

We have determined the triple-point temperature of high-purity gallium to be 29.77406°C using three standard platinum resistance thermometers (SPRTs) calibrated on the IPTS-68 and using samples of gallium from three commercial sources. All data obtained on the highest purity sample have a standard deviation of ±0.00011°C. The overall systematic uncertainty is estimated to be +0.6-1.1 m°C. The day-to-day irreproducibility of a single melt of each gallium cell was found to be < ±15 μ°C, while over a period of 7 months the irreproducibility among melts of each gallium cell was ±0.00011°C. These limits on the reproducibility appear to be largely instrumental since every time an SPRT was left in a gallium cell for several days there was no change in the indicated temperature of the triple-point to within the instrumental resolution of ±1.5 μ°C. The pressure dependence was measured and determined to be dT/dP = -2.011 ± 0.001 m°C/atm.