The following Application Highlight features the use of the Transient Line Source method in the thermal conductivity testing of granular materials.
Granular materials (such as polymeric pellets, mineral aggregates, etc.) can be challenging or simply not viable to test using planar style sensors. The surface area relative to void space present in the active zone can introduce errors related to poor heat transfer from the sensor into the sample, ultimately resulting in a biased measurement result. The Transient Line Source (TLS) is a needle probe style sensor which provides a means for the testing of these types of samples through the extended surface area available around the probe’s length. The TLS is also applicable for the testing of other material formats ranging from slurries to geological core samples. Additional information on sample viability and testing requirements can be found in the ASTM D5334 and ASTM D5970.
Figure 1. Crushed rock aggregate
Completely sheathed in stainless steel, the standard TLS measures 150 mm in length. The sensor probe must be inserted into the sample medium, ensuring complete coverage of the probe. For soft solids, liquids and unconsolidated materials, the needle may be inserted directly into the sample, taking care to avoid any bending during insertion. For more dense materials, a guide hold may be pre-drilled to closely match the diameter of the sensor and a contact fluid may be used to improve contact with the probe. Where applicable, a containment vessel may be employed to contain the test sample. Once assembled, as with all transient techniques, the setup must be allowed to reach thermal equilibrium prior to measurement initialization. Thermal gradients within the sample and temperature differences between sample and probe can contribute to large errors in measurement values and inconsistencies between replicate tests.
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