Measurement of the specific heat capacity (cp) using the transient heating wire method
The specific heat capacity is a fundamental thermophysical material property and helpful for the assessment of materials and their areas of application. It can be determined using differential scanning calorimetry (DSC).
Another way to determine the specific heat capacity is to use the transient heating wire method (ASTM C1113-99). The time-dependent method provides exact and precise results for the determination of the thermophysical parameters thermal conductivity λ, thermal diffusivity a and specific heat capacity cp of gases and liquids. The temperature increase is measured in a sample that is heated by a thin heating wire at a known distance. The wire serves as a Joule heat source and at the same time as a resistance thermometer. As the temperature rises, the resistance of the wire changes and can therefore be used for temperature measurement.
As early as 1931, Stålhane and Pyk had measured the thermal conductivity of powders with an apparatus that was very similar to today’s method; they found the following empirical connection:
Taking into account the heat conduction equation, the temperature T at a distance r from the wire at a specific point in time t can be calculated as follows for the heating wire as a linear heat source:
This allows the thermal conductivity λ to be calculated from the temperature change at two different points in time t1 and t2 (Equation 3):
Since thermal conductivity λ, thermal diffusivity , specific heat capacity cp and density ρ are related as follows,
the specific heat capacity cp can be determined using the Equation 3:
Transient hot bridge method (THB)
A further development of the transient heating wire method is the transient-hot-bridge of the Physical-Technical Federal Institute in Brunswick (Germany), which was presented on the European market in 2005 (see THB).
The multi-patented sensor is the key component. While only one heating strip is used with the heating wire method, the THB uses four meandering heating strips. The asymmetrical transverse division of each strip and the different distances between the strips, in conjunction with a Wheatstone bridge circuit, enable an effective compensation for significant systematic measurement errors.
The time-dependent transient hot bridge method enables simultaneous measurement of the thermophysical material properties λ, a und cp of numerous materials and geometries. The method delivers results for solids and liquids as well as powders and pastes with high measuring accuracy and a short expenditure of time.
The area of application is broad, so the measuring method is used, among other things, to evaluate heat storage materials and insulating materials as well as to analyze electronic components and materials used in nanotechnology.