Nondestructive Measurement of Emissivity of Damaged Parts of Coatings

Low Infrared emissivity coating (LIREC) is prone to generating some problems such as bulges, degumming, and abrasion. In order to study whether the performance of LIREC under different damages can meet the work needs, it is essential to timely measure and evaluate the performance state of LIREC in the application process. The existing methods for measuring the damage of LIREC have some disadvantages such as expensive equipment, complex operation, and inaccurate measurement results. In this paper, a measurement method of LIREC damage capability based on thermal imager is proposed. The radiation temperature is measured by thermal imager, the real temperature and ambient temperature of coating surface are measured by thermocouple, and the emittance of coating surface is calculated. Non-contact and continuous large-area emissivity measurements are carried out on the damaged parts of the coating and verified by experiments. The measurement results show that the different damage types and damage degrees directly affect the measurement results of LIREC. Wear damage increases the emissivity of the coating while debonding damage basically does not change the coating emissivity. Shedding damage of small diameter forms voids, which causes the increase of the damage parts of emittance. In addition, bulge damage impedes temperature transfer and reduces emissivity. This method can timely and accurately measure and evaluate the performance state of LIREC and can provide a new idea for the accurate measurement of damage emissivity of LIREC.

Emissivity measurements on reflective insulation materials

The development and use of new thermal insulation products in many industrial sectors, ranging from building insulations to power generation or satellite applications, requires the accurate knowledge of the radiative properties of the investigated material, i. e. its emissivity. A major objective of the research project “Improvement of emissivity measurements on reflective insulation materials” within the framework of the European Metrology Programme for Innovation and Research was to improve and validate reference techniques for the measurement of the total hemispherical emissivity of low emissivity foils with an absolute measurement uncertainty below 0.03. The calibration and measurement procedures developed within this project shall lead to a significant benefit for industrial manufacturers of reflective foils as well as for the end-users of the industrial instruments used to characterize them.

Assessment of uncertainties for measurements of total near-normal emissivity of low-emissivity foils with an industrial emissometer

The TIR100-2 emissometer (manufactured by Inglas GmbH & Co.KG) is an emissivity measurement device used by several producers of thermal insulation products for buildings and by some organizations certifying performance of insulation products. A comparison of emissivity measurements on low-emissivity foils involving different measurement techniques, including the TIR100-2 emissometer, gave widely dispersed results; the discrepancies were not explained. The metrological performance of the TIR100-2 emissometer and the uncertainties for measurement on reflective foils was not known, which could be detrimental to users. In order to quantify the performance of TIR100-2 devices for measurement of total near-normal emissivity of low-emissivity foils, the Laboratoire National de Métrologie et d'Essais (LNE) analyzed in detail the measuring principle and listed the associated assumptions and uncertainty sources. A TIR100-2 emissometer actually measures the reflectance and, for opaque materials, the emissivity is calculated from the measured reflectance. The parameters analyzed experimentally are the temperature stability and uniformity of the thermal radiation source, the emissivity of the radiation source, the response function linearity and the spectral sensitivity of the radiometric detection system measuring the reflected radiation, the size of the measurement area, and the measurement repeatability and reproducibility. A detailed uncertainty budget was established. The uncertainty sources taken into account are the uncertainties of the emissivities of the two calibrated standards used for calibration, the stability and uniformity of the radiation source temperature, the non-linearity and the spectral sensitivity of the radiometric detection system, the specific measurement condition related to the radiation source temperature, the uncertainties related to the temperatures of the standards and the sample, the noises on results, and the non-homogeneity in emissivity of the tested material. The combined measurement uncertainty was calculated for different types of reflective foils; the expanded uncertainty is around 0.03 for total near-normal emissivity measurements on smooth low-emissivity foils. A measurement campaign on five types of low-emissivity foils, involving four TIR100-2 emissometers, and a comparison to a primary reference setup at the Physikalisch-Technische Bundesanstalt (PTB) confirmed the uncertainties assessed.

Investigation of Pulverized Biomass and Coal Char Emissivity

Current work presents an optical setup, its calibration and reference process and the first results from single particle emissivity measurements of pulverized biomass and coal fuel particles. In contrast to earlier attempts, the setup offers the possibility of emissivity measurements during the whole particle burn-off. A laser ignites a single particle, placed in the center of the setup. Two photomultipliers observe the emitted particle radiation in the visible range (550 nm and 700 nm) for temperature calculation, using two-color pyrometry. An InSb-detector records the emitted particle radiation between 2.4 µm and 5.5 µm, which is later used to calculate particle emissivity in this range. The conclusion of multiple particle measurements lead to decreasing particle emissivity with increasing temperature. For coal particles the emissivity decreases from 0.45 at 2300 K to 0.03 at 3400 K. Biomass char shows a similar trend with a decrease from 0.18 (2100 K) to 0.03 (2900 K).