Development and Evaluation of an Improved Apparatus for Measuring the Emissivity at High Temperatures

An improved apparatus for measuring the spectral directional emissivity in the wavelength range between 1 µm and 20 µm at temperatures up to 2400 K is presented in this paper. As a heating unit an inductor is used to warm up the specimen, as well as the blackbody reference to the specified temperatures. The heating unit is placed in a double-walled vacuum vessel. A defined temperature, as well as a homogenous temperature distribution of the whole surrounding is ensured by a heat transfer fluid flowing through the gap of the double-walled vessel. Additionally, the surrounding is coated with a high-emitting paint and serves as blackbody-like surrounding to ensure defined boundary conditions. For measuring the spectral directional emissivity at different emission angles, a movable mirror is installed in front of the specimen, which can be adjusted by a rotatable arrangement guiding the emitted radiation into the attached FTIR-spectrometer. The setup of the emissivity measurement apparatus (EMMA) and the measurement procedure are introduced, and the derived measurement results are presented. For evaluating the apparatus, measurements were performed on different materials. The determined emissivities agree well with values published in literature within the derived relative uncertainties below 4% for most wavelengths.

Rock Emissivity Measurement for Infrared Thermography Engineering Geological Applications

Infrared thermography is a growing technology in the engineering geological field both for the remote survey of rock masses and as a laboratory tool for the non-destructive characterization of intact rock. In this latter case, its utility can be found either from a qualitative point of view, highlighting thermal contrasts on the rock surface, or from a quantitative point of view, involving the study of the surface temperature variations. Since the surface temperature of an object is proportional to its emissivity, the knowledge of this last value is crucial for the correct calibration of the instrument and for the achievement of reliable thermal outcomes. Although rock emissivity can be measured according to specific procedures, there is not always the time or possibility to carry out such measurements. Therefore, referring to reliable literature values is useful. In this frame, this paper aims at providing reference emissivity values belonging to 15 rock types among sedimentary, igneous and metamorphic categories, which underwent laboratory emissivity estimation by employing a high-sensitivity thermal camera. The results show that rocks can be defined as “emitters”, with emissivity generally ranging from 0.89 to 0.99. Such variability arises from both their intrinsic properties, such as the presence of pores and the different thermal behavior of minerals, and the surface conditions, such as polishing treatments for ornamental stones. The resulting emissivity values are reported and commented on herein for each different studied lithology, thus providing not only a reference dataset for practical use, but also laying the foundation for further scientific studies, also aimed at widening the rock aspects to investigate through IRT.

AVERAGE TOTAL HEMISPHERIC EMISSIVITY MEASUREMENT IN THE LWIR SPECTRUM FOR ADHESIVE TAPES USED IN THE THERMOGRAPHY TAPE TEST

Thermographers often use comparative methods to estimate surfaceemissivity. Among the most used is the tape method. In this method a knownemissivity tape in the LWIR (Long Wavelength Infrared) spectrum is placedon the surface to be inspected. After thermal equilibrium, the temperature ofthe tape and the surface under inspection must be the same. In this case, thetemperature observed on the tape is the reference temperature. The emissivityof the surface must then be changed until the reference temperature isreached. It is common practice to admit the value of the emissivity of theadhesive tape as 0.95, there are few studies that present these data withmetrological rigor, which leads to doubts about the emissivity of thecommercial tapes. In this work, experiments were performed on Tekbond,Double A, 3M 101, Rapix, Altape, adhesive tapes for temperatures of 50, 55,60, 65 and 70ºC. An experimental apparatus was developed through which itwas possible to estimate surface reflection, transmission and atmosphericemission for one and two layers of tapes, in order to make emissivitymeasurements possible. Through the data it was possible to statisticallyestimate the LWIR average total hemispheric emissivity as well as theacceptance range to 95% certainty, being therefore equal to ε=0.94±0.03. Itis possible to arm, therefore, that the value of 0.95, usually used as emissivityof the adhesive tape, is extremely reasonable because it is 0.01 of the averagevalue of the Gaussian distribution calculated by this work.