Design, construction and validation of a guarded hot plate apparatus for thermal conductivity measurement of high thickness crop-based specimens

The search for sustainability achievement in the building sector led to the use of crop-based insulation materials. Among these, raw products like plant straws bales can present elementary representative volumes in the size order of several square centimeters. Fibers orientation may play an important role on material thermal behavior. Measuring their thermal conductivities with standard steady-state equipment can thus lead to some inaccuracies due to the necessity of resetting lower thickness samples. This paper presents a guarded hot plate apparatus designed to test high thickness samples, up to 40 cm, with an accuracy of 2 %. The different parts of the machine are described in details along with design process and challenges encountered. Temperature and heat flux measurements in the device represent critical design stages where optimal accuracy is needed in order to optimize the resulting global error. Regarding this, necessary numerical and experimental analyzes were conducted emphasizing on intrinsic systematic error assessment. To complete the study, some validations tests are performed on a reference material and other widely-spread polystyrene slabs. These tests take into account the optimal computation parameters resulting from the numerical analysis and the apparatus proved to be in accordance with accuracy requirements.