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A true transient-technique tale


U. Hammerschmidt1

1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany

Keywords: transient technique
property: thermal conductivity, thermal diffusivity
material: small specimens, coffee bean, pipe insulation, IC package

Once upon a time in 1989, a chemist told me that in Germany drying, roasting and quenching of coffee consumes more than 1 TWh/a of electrical energy. Fifteen percent of this amount could be saved with the further knowledge of the thermal conductivity of coffee beans. After a longer technical discussion, we both concluded that it is impossible to analyse a single coffee bean. Just ground coffee can be measured. This, however, does not help a lot in process control of beans. - After many years, in 2010, I recalled the above issue when reading the following statement in a paper ‘on heat and mass transfer during the coffee drying process’. ”As it is not possible to analyse the thermal conductivity of a single bean, a round plate of ground coffee had to be formed ” for the measurements [1]. – This time, the challenge was accepted.

PTB started to develop a novel transient technique to determine the thermal transport properties of small specimens with a thermometer. Meanwhile, we succeeded in measuring the thermal conductivity of a bean. For the runs, we used a commercial surface-temperature sensor of 10 mm2 measuring-grid area that is sandwiched betweeen two beans. By passing an electrical current of some milliamperes through its grid, the sensor not only determines temperature but also acts most similar to a point heat-source. That is why, we could set up a simple data reduction equation based on the known point source solution of Fourier’s heat conduction equation. The approximation of the temperature history of the self-heating sensor is reasonable accurate and furnishes both thermal transport properties. The overall uncertainty of the technique is preliminarily assessed to be 3 % for thermal conductivity and 8 % for thermal diffusivity. These values are verified against three different standard reference materials at temperatures between 23 °C and 180 °C.

The presentation illustrates theory and practice of the new hot-point method. Measuring results will be given not only for a coffee bean. Among a good many other samples, we also analysed peanuts, IC packages, pipe insulations, some powders and liquids. Among the latter class of materials, small-volume liquids and those that are electrically conducting can also be analysed effectively. Finally, it will be shown that by combining three surface temperature sensors, a special sensor can be constructed that measures the thermal transport properties of anisotropic samples simultaneously in all three directions.

References
  1. K. Burmester, R. Eggers: J. Food Eng. 99, 430-436 (2010)

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