Alternative fitting procedures to enhance the performance of resistive sensors for thermal transient measurements

The main concern in most of transient techniques is to obtain a controlled heat flow in a prescribed direction, so that the actual boundary conditions in the experiment agree with those assumed in the theory. Enhancements of the fitting procedures in transient dynamic techniques have been established. One of these dynamic techniques known as the extended dynamic plane source is discussed in details. This technique has been used for simultaneous measurements of thermal conductivity, diffusivity and the specific heat from a single transient recording of the temperature increase. The technique uses a resistive sensor as both, a plane heat source and temperature sensor, i.e. in similar manner as the wire in the transient hot-wire method. The sensor consists of a resistive heater pattern cut from a thin sheet of metal (Ni) covered on both sides with thin layers of an insulating material. The conducting pattern of the sensor, known as the hot-disk configuration, has the shape of spiral strips. Infrared camera images of the sensor were taken, at the beginning of the transient recording, to illustrate the deviations from the ideal conditions due to design defects within the sensor. Alternative fitting procedures have been introduced in order to meet the design criteria of heat and temperature sensor and to fulfil the isothermal boundary conditions required by experimental setup. Within the total time of transient recording, and using a sequence of computational steps, it is possible to find a correct “optimal” time interval to enhance the fitting procedures. These computational steps are based on fitting procedures that provides the selection of the optimal time interval within the total measuring time and thus to obtain more accurate and reliable results. These procedures have been tested using measurements on acrylic “Plexiglas” sample; the corresponding deviations from published data did not exceed 1.5% and 2% in thermal conductivity and diffusivity, respectively. It is another alternative to the standard fitting procedures which mainly based on linear regression and least square fitting. It is anticipated that these fitting procedures have the potential to be used in similar transient techniques.