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Long term drift characterisation of noble metal thermocouples using high temperature fixed points.

Pearce, J V; Machin, G; Ford, T*; Wardle, S* (2008) Long term drift characterisation of noble metal thermocouples using high temperature fixed points. Acta Metrologica Sinica, 29 (5A). pp. 86-89.

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In a previous paper1 we described the implementation of a Co-C high temperature fixed point (HTFP) at thermocouple manufacturers CCPI Europe, for the assessment of the calibration of noble metal thermocouples such as type R, S, and B. In that activity, a set of alumina sheathed bare wire type S thermocouples was constructed by CCPI and calibrated in their UKAS laboratory using the Au ingot and Pd wire bridge. These had uncertainties of ±3 °C (k = 2) in line with CCPI¿s UKAS accreditation. The same thermocouples were also calibrated using, in addition to the Au and Pd points (accredited by UKAS), the Co-C eutectic fixed point (not accredited at that time), and had an estimated uncertainty of ±0.9 °C (k = 2).

The thermocouples were then used extensively in a furnace at Bodycote Heat Treatments Ltd. over a period of 6 months. This furnace is used for the heat treatment of aerospace engine components. The cumulative exposure to temperatures in excess of 1300 °C was about 5 months in that 6 month period. Now, the thermocouples have been returned to CCPI for assessment of their drift performance through recalibration. The recalibration was performed following the normal UKAS accredited procedure (Au and Pd points) and, again, additionally using the (not UKAS accredited) Co-C eutectic fixed point. The thermocouples were not re-annealed prior to their re-calibration after use in the heat-treatment furnace.

The results showed that the indicated temperature difference for these thermocouples between the newly constructed state and the extensively used state was considerably less than 1 °C at and below the Co-C temperature. This is a remarkable result given how extensively these thermocouples had been used. Given that the current accredited uncertainty of CCPI for their UKAS calibration is ±3 °C (k = 2), the use of the Co-C point, which has an uncertainty of less than ±1 °C (k = 2), was critical in confirming, with low uncertainties, the excellent performance of these thermocouples.

1 J.V. Pearce, G. Machin, T. Ford, and S. Wardle, Optimizing heat treatment of gas turbine blades with a Co-C fixed point for improved in-service thermocouples, International Journal of Thermophysics, 2008, 29: 222-230

Item Type: Article
Keywords: Thermocouples, high temperature fixed points, eutectic, contact thermometry
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4374

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