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A robust Pd-C fixed-point for calibration of thermocouples.

Pearce, J V; Machin, G (2008) A robust Pd-C fixed-point for calibration of thermocouples. Acta Metrologica Sinica, 29 (5A). pp. 224-228.

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We describe the construction and performance of a palladium-carbon (Pd-C, melting temperature 1492 °C) 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 °C. The temperature was measured with a Pt/Pd thermocouple and a Type B thermocouples. The standard deviation of the emf at the melting point was s = 0.73 mV (0.058 °C, k = 2) as measured with the Pt/Pd thermocouple and s = 0.84 mV (0.145 °C, k = 2) with the Type B thermo-couple. The freeze repeatability was s = 1.44 mV (0.114 °C, k = 2) for Pt/Pd and s = 0.71 mV (0.124 °C, k = 2) for Type B. The robustness of the Pd-C fixed point is somewhat problematic, perhaps because the diffusion of C in Pd is consid-erably higher than that in comparable fixed-point metals Fe, Ni, and Co. Pd also 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 fragil-ity 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.

Item Type: Article
Keywords: Metal-carbon eutectic fixed points, thermocouples, Pd-C, contact thermometry
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4368

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