Cool control

A more accurate method for monitoring the heat treatment of jet engine components could lead to aircraft that produce fewer emissions.

Scientists at the

(NPL) have reduced the uncertainty of thermocouple temperature sensors used to track heat treatments to less than 1°C.

Aircraft engines work most efficiently and produce fewer emissions when they run at high temperatures, but this requires thermal treatment of their components at temperatures in excess of 1,300°C. If the treatment temperature deviates too much from the optimal temperature, it may be inadequate and a component might have to be scrapped.

Treatment progress is controlled by thermocouple temperature sensors, which are calibrated using materials with known melting or freezing points, known as fixed points. The difficulty up to now has been that there were no reliable low uncertainty fixed points in the high temperature region.

NPL scientists have established a new type of reference fixed point using a material made from a mixture of cobalt and graphite in a composition known as metal carbon eutectic. Graham Machin, head of temperature standards at NPL, said that it was selected for the application because the temperature of the process was around 1,300°C.

A thermocouple consists of two dissimilar metals, joined together at one end. When it is heated or cooled along its length a voltage is produced that can be measured with a voltmeter. To calibrate the thermocouple the voltage output at known temperatures, or fixed points, needs to be determined. Once calibrated the thermocouple can then be used to determine the temperatures of industrial processes.

Before Machin and his team took on the challenge they had to consider several factors. 'We were fairly sure we could achieve better than 1°C using these new fixed points,' he said. 'but the problem was all low uncertainty measurements up to that point had been done using non-contact thermometry methods, and thermocouples are a contact device.' So to make full benefit of the low uncertainties the team made two cells of the new fixed-point material.

One cell was made for thermocouple measurements, while the other was made from the same materials for non-contact thermometry measurement.

Machin said the temperature of the cell using non-contact thermometry was determined with an uncertainty of less than 0.5 °C. That temperature was then assigned to the thermocouple cell.

Following that, he said the calibration of the thermocouple was fairly straightforward. The procedure was similar to that for any thermocouple. These are usually calibrated at conventional fixed-points such as aluminium (660°C) or silver (962°C). 'The important innovation here was that the new cobalt-carbon fixed-point melts close to the process temperature allowing us to precisely determine the thermovoltage output very close at that temperature, and hence achieve low uncertainties.'

Machin said NPL is currently working with CCP International, an industrial temperature sensor supplier, and Bodycote, a supplier of specialist testing and thermal processing services for products such as engine components and in particular turbine blades, to prove the concept in an industrial setting. Bodycote currently has a batch of thermocouples, calibrated by the new NPL method at CCPI, in their heat treatment ovens.

If testing proves positive then the calibration method using high temperature fixed-points could be applied in a wide variety of industrial settings wherever improvements in high temperature measurement would be beneficial.