Improved Transient Method Measures Thermal Conductivity of Insulating Materials
Thermal conductivity of insulating materials is typically measured using steady state techniques, such as the guarded hot-plate. Such measurements are expensive, time consuming and experimentally difficult. More importantly, they can't be used in situ to measure conductivity after the insulation is installed, or on wet insulation where moisture migrates in a thermal gradient. Transient methods overcome both limitations, offering rapid and simple measurements. One such technique relies on a metal needle placed in the sample and heated at a uniform rate. Thermal conductivity of the surrounding medium is estimated from temperature recorded inside the needle via a suitable inversion scheme. The heated needle is successfully used to determine thermal conductivity in soil and rock samples. Measurements in insulation, however, remain a challenge, due to the large mismatch between specific heat of the probe and the sample. We revisited the traditional algorithm, based on a simplified model of the conduction process, and analyzed the actual response in a multilayered system, where the thermal properties of the probe, including the contact resistance, are accounted for. The analysis suggests a simple transformation of the temperature signal, which allows us to infer the response at very long times from extrapolation of early time data. The proposed method is especially beneficial with insulation, yielding accurate estimates even with measurement durations less than five minutes. Moreover, since the transform involves the time derivative of the temperature signal, the presence of the contact resistance is uninfluential, thus effectively eliminating one of the hairiest problems afflicting transient methods.
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Paolo Castiglione, PhD (Primary Presenter,Author), METER Engineering, firstname.lastname@example.org;
Gaylon Campbell, PhD (Co-Presenter,Co-Author), METER Engineering, email@example.com;