Leading provider of Thermal Conductivity Testing and Thermal
Conductivity Instruments as well as Thermal Resistivity,
Specific Heat Capacity and Thermal Diffusivity.
Transient Line Source- How it Works
TLS can use two different probe types: full-space probes that are fully inserted into the sample and half-space probes that are put on top. Both probe types have a rugged design, cover a wide measuring range and can be used for a great variety of sample materials and sizes. An additional field probe for in-situ testing is available.
The instrument can be controlled using any computer or notebook running MS Windows. Measurements are organized in measuring series consisting of an arbitrary number of single measurements (up to 99). After choosing the desired measuring and evaluation parameters, TLS is working completely unattended without requiring any user interaction. The thermal state of the sample is continuously monitored before and between single measurements. If necessary, a temperature drift (usually caused by instable ambient temperature) is corrected by the measuring software. Using default settings, a single measurement takes 80 seconds, while a typical measuring series consisting of 5 single measurements takes approx. 15 to 30 minutes. Prices include a multi-user license for the comprehensive software package (measuring, evaluation, graphics, quality analysis) comprising an unlimited support period.
Principles
TLS uses the widely-used transient line source method. A long and thin heating source is brought in contact with the sample and is heated with constant power, while simultaneously the temperature of the source is registered. The slower the source temperature rises, the higher is the thermal conductivity of the sample material.
There are two different implementations of the measuring principle: the full-space and the half-space method. A full-space source is inserted into the sample and hence is completely surrounded by the sample material while a half-space source is positioned on the sample surface in one-sided contact with the material. Few instruments on the market, among them TLS, are able to use both methods. We offer interchangeable probes of both types for our instrument.
TLS uses an analytical method for evaluating the data. Analytical methods for determining thermal conductivity are based on the theoretical solution of the heat equation for a constantly heated line source surrounded by sample material on all sides. The heat equation is a differential equation describing the spatiotemporal variation of temperature in the probe and the sample.
The solution of the heat equation yields a formula for the temperature rise inside the heating source which contains (among others) the thermal conductivity of the sample as parameter. The theoretical curve is fitted to the measured temperature curve and the thermal conductivity value is calculated from the fit coefficients. In contrast to empirical methods, analytical methods yield absolute values and hence do not require reference standards or calibration tests.
TLS instead uses a newly developed higher order approximation method taking into account the characteristic nonlinear curve form of real temperature curves. Evaluation with the new method is more complex, but achieves excellent accuracy (± 2%). Additionally, a comparison of the curve forms of the theoretical and the measured temperature rise permits detection of typical disturbing effects (like bad contact between probe and sample) based on the measuring data.
