A Robust Pd-C Fized-point for Calibration of Thermocouples
We describe the construction and performance of a palladium-carbon (Pd-C, melting temperature 1492 ℃)fixed-point for the calibration of thermocouples. The fixed-point was assessed by performing repeated melts and freezes with a constant furnace offset, above and below the melting temperature, of 8 ℃. The temperature was measured with a Pt/Pd thermocouple and a Type B thermocouple. The standard deviation of the emf at the melting point was σ= 0.73 μV (0.058 ℃, k=2) as measured with the Pt/Pd thermocouple and σ= 0.84 μV (0.145 ℃, k=2) with the Type B thermocouple. The freeze repeatability was σ= 1.44 μV (0.114 ℃, k=2) for Pt/Pd and tr= 0.71 I~V (0.124 ℃, k=2) for Type B. In general, the robustness of the Pd-C fixed point has been somewhat problematic, perhaps because the diffusion of C in Pd is considerably higher than that in comparable fixed-point metals such as Fe, Ni, and Co. Also, Pd has a high vapour pressure. These factors appear to result in more rapid diffusion of the Pd and/or Pd-C through the graphite crucible,leading to mechanical fragility and rapid contamination of the thermocouple sheath. A novel crucible design, whereby the thermowell consists of concentric sleeves, has overcome this problem, making the fixed-point much more robust. So far the current Pd-C fixed-point has performed over 60 melt/freeze cycles (from room temperature, to emulate real-world calibration conditions) without any structural problems nor any appearance of bulk fixed-point material in the thermowell.
Metrology Metal-carbon eutectic fized points thermocouples Pd-C, contact thermometry
Jonathan Pearce Graham Machin
National Physical Laboratory, United Kingdom
国际会议
2008年国际温度与热物性测量会议(TEMP BEIJING 2008)
北京
英文
224-228
2008-10-20（万方平台首次上网日期，不代表论文的发表时间）