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Non-contact temperature measurements

Temperatures can be determined without touching the object under investigation by measuring the heat it radiates (pyrometry). However, accurate temperature determination requires knowledge about the object emissivity. For aluminium, it is very difficult to determine the emissivity since it is small and depends on various characteristics such as surface roughness, oxides, and alloy composition.

We have developed a 4-wavelength pyrometer for measuring the temperature on aluminium. The pyrometer has been tested in 100 different measurements on realistic samples with various alloy compositions, temperatures, oxides and degrees of surface roughness. By proper calibration, we were able to obtain an accuracy of about 6°C for temperatures in the range 400-600°C. The pyrometer also produces estimates of the temperature accuracy.

Potential applications of the pyrometer is found in the extrusion and hot-rolling of aluminium, but also in other applications requiring accurate, absolute temperature measurements.

The material emissivity is known to depend on many factors such as wavelength, temperature, surface roughness, oxidation, direction of observation, etc. Errors in emissivity introduce errors in the temperature measurements, limiting the precision of the methods. Another non-contact temperature measurement method that has been investigated at SINTEF is based on both radiance and reflectometry measurements. The purpose is to measure the effective emissivity, to ensure a better temperature determination.

A third method that may be used for metals with inter-band transitions is the spectral-shift method. The reflectivity of aluminium has a dip in the 800-900 nm range due to an inter-band transition in the Al electronic band structure. This dip shifts to longer wavelengths with increasing temperature. Theoretically, the dip is approximately 200 nm broad and shifts 100 nm over 300 K, i.e. about 0.3 nm/K. The figure below shows the recorded spectra of aluminium at increasing temperature. The method is limited to temperatures below 300°C, but may be used at low temperature with high accuracy (± 5 °C at 200°C and ± 1 °C at 0°C).

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The figure shows that an absorption feature in the Aluminium reflection spectrum shifts to longer wavelengths when the temperature increases.

Key Factors

Project duration

01/01/2005 - 31/12/2005