Pyrometry

Pyrometry is the determination of surface temperatures via measurement of radiation flux emitted by the surface. Pyrometric temperature measurement of solar irradiated material surfaces is a useful alternative to contact mea­surement techniques at high temperatures. Pyroelectric detectors are the most common optical thermal detectors, as they are the most sensitive to thermal radiation and are fairly inexpensive. A pyroelectric detector con­tains a sensor made from a ferroelectric material which develops a change in polarisation in response to a change in its temperature. The polarisation state of the sensor, and therefore its temperature, can be measured as an electrical signal by electrodes placed either side of it. Radiation emanating from the surface of interest passes through a window in the detector and heats the sensor; the deviation of the sensor’s temperature from ambient is then used to determine the temperature of the source material.

A difficulty with pyrometric temperature measurement is, however, that the detector responds to solar radiation which is directly reflected from the irradiated sample, as well as re-radiation. This problem is especially impor­tant in solar furnaces, where solutions have been proposed including:

• estimating the reflected radiation by varying the incident flux during temperature measurement

• determining the incident flux and the sample spectral reflectivity online (Tschudi and Morian, 2001)

• the use of pyrometry with band-pass filters centred on the atmospheric solar absorption bands of carbon dioxide and atmospheric water, which minimises or avoids this source of uncertainty (Tschudi and Morian, 2001; Hernandez et al., 2004; Pfander et al., 2006).

In addition, determination of the real temperature requires accurate knowl­edge of the surface emissivity, as the temperature is determined on the basis of the current signal generated by the radiant surface compared to the signal generated by a black-body calibrator. Use of the solar absorption band with the shortest wavelength reduces the influence of the uncertainty of emissivity on surface temperature determination compared to longer wavelengths (Rohner and Neumann, 2003). This approach is similar to that used for infrared measurements of radiation from a receiver using a solar blind camera, discussed below.

A commercial pyrometer has been tested in the wavelength band around 1.4 pm in the solar furnace at the Plataforma Solar de Almeria (PSA) in material treatment experiments with concentrated solar radiation (Ballestrin et al., 2010b). This wavelength band is an atmospheric solar absorption band due to water steam, but solar radiation absorption is incom­plete in this band. The pyrometer works well at temperatures over 800°C even through quartz windows, and has frequently been used in concentrated solar radiation experiments (Schaffner et al., 2003; Kraupl and Steinfeld, 2003; Meier et al., 2004; Osinga et al., 2004; Hirsch and Steinfeld, 2004).

Updated: August 23, 2015 — 4:12 pm