Abstract
Emissivity is a quantity essential to consider when assessing the measurement uncertainty in non-contact temperature measurements. This paper presents a new instrument for measuring emissivity of opaque materials from 200 °C to 450 °C in the spectral range of 2.1 to 2.5 µm. These ranges are ideal for measuring the temperature of metals, such as aluminium, during manufacture or heat-treating processes. The instrument consists of a pair of hemispherical cups coated with Vantablack® and gold respectively, a custom designed radiation thermometer, and a hot plate. This instrument enables both the direct and the indirect methods for measuring emissivity of materials. Use of two identical cups allowed for quantitative analysis of the uncertainty of the instrument to determine the most suitable emissivity measurement range. The expanded uncertainty of the instrument was lower than 0.058 (k = 2) over the entire measuring temperature range. Studies were undertaken using different materials with emissivities ranging from 0.06 to 1. These included aluminium alloy 6082, stainless steel 304, and HiE-Coat 840M paint. Relative uncertainty analysis indicated that the indirect method was more accurate for measuring low-emissivity materials, whereas the direct method was more suitable for all other materials. Our instrument, with experimentally determined measurement uncertainty, aims to offer accurate emissivity references for use in radiation thermometry applications.
Highlights
Background radiation interferenceFor each measurement, a sample was heated to the measurement temperature and stabilised for 30 min before data acquisition started
The thermocouple used in the instrument can measure temperature within an error range of ±1.5 °C over the temperature range of 0 to 375 °C and ±0.4% °C over the temperature range of 375 to 1000 °C
840M paint on an alloy 6082 (Al6082) substrate. These samples can represent materials ranging from low emissivity values to high emissivity values over the spectral range of 2.1 to 2.5 μm, according to previously published studies [28,29,30]
Summary
A sample was heated to the measurement temperature and stabilised for 30 min before data acquisition started. During this period, the black-cup and the gold-cup were exposed to the heating area of the hot plate, emitting background radiation to the sample after covering it. The thermal properties of the samples, such as heat capacity, thermal conductivity and surface condition, contribute to the vertical temperature difference. This difference was analysed using Ansys Icepak for common materials.
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