National Physical Laboratory

Industrial implementation of a Co-C fixed point for thermocouple calibration

NPL's new technology in thermal metrology has enabled British industrial partners to move forward with ultra-efficient jet engine.

British Airways Boeing 747-400
British Airways Boeing 747-400 on final approach
(image courtesy of NPL © Crown copyright 2004)

Eutectic fixed points are already being put into practice in an industrial setting, with the successful completion of a joint industrial project between NPL and two UK industries to reduce measurement uncertainty in the region of 1300 °C by a factor of 3.

Airlines are under mounting pressure to reduce CO2 emissions and fuel consumption. Improvement of energy efficiency of jet aircraft is achieved by operating gas turbine engines at higher temperatures. To facilitate this gas turbine engine manufacturers are continuously developing new alloys for hot zone turbine blades that will withstand the increased in-service temperatures. A critical part of the manufacture of these blades is heat treatment to ensure that they attain the necessary metallurgical characteristics. Turbine blades for the next generation of engines undergo heat treatment at over 1310 °C to attain the required properties.

In the project described here the heat treatment, which is controlled by noble metal thermocouples, is performed at Bodycote, Derby. The process has exacting control requirements (±3 °C), at the limits of the lowest accredited uncertainty currently available from thermocouple manufacturers. With conventionally calibrated thermocouples, the heat treatment is limited by available uncertainties, and additional detailed inspection is required to verify the process efficacy. Ideally, Bodycote would like to attain ±1 °C temperature uncertainty in-process, but until recently this was not possible because even the best accredited uncertainties attainable for noble metal thermocouple calibration (in National Measurement Institutes like NPL) at these temperatures are around ±1 °C.

However, thanks to a joint programme of research between NPL, thermocouple manufacturer CCPI Europe Ltd., and Bodycote, calibration of noble metal thermocouples with world-beating uncertainties of better than ±1 °C is now possible at CCPI Europe. NPL has made this possible by implementing a novel high temperature fixed point into CCPI Europe’s calibration chain. These novel high temperature fixed points mark a watershed in high temperature metrology and have extended the range of reference fixed points from the previous limit of 1084 °C (copper) to approximately 3200 °C. It took 7 years of vigorous research to develop them.

The ideal candidate for this aerospace application is the Co-C eutectic, which melts at 1324 °C, very close to the heat treatment temperatures required. It is anticipated that when UKAS accredited this will result in much lower calibration uncertainties on the thermocouples delivered to Bodycote by CCPI Europe, and through subsequent improved process control should yield an increase in effectiveness of the heat treatment process at Bodycote, allowing them to respond effectively to the increasingly stringent demands of engine manufacturers.

Outside the current application, there is strong requirement by industry for lower uncertainties at and above 1300 °C. Successful implementation of the current fixed point in an industrial setting is likely to result in rapid take-up by other companies, probably through the supply of ultra-low uncertainty thermocouples, looking to improve their high temperature processes.

NPL has established a calibration capability based on these high temperature fixed points and is now offering an ultra-low uncertainty UKAS accredited calibrated thermocouples and thermocouple calibration service

For further information, please contact Jonathan Pearce

Last Updated: 11 Apr 2017
Created: 6 Mar 2012

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