TECHNICAL REPORTS Principle, advantages, limitations and applications of two-colour pyrometers in thermal processes by Albert Book Nowadays, temperature measurements are Measuring principle not imaginable without two-colour pyrome- A two-colour pyrometer detects the thermal radiation of a meas- ters to cover a part of the many applications uring object at two different wavelengths. The ratio of the two spec- for infrared thermometers. The following article explains physical principles, advantages, functional and analytical possibilities tral radiances φ varies almost proportionally to the temperature. Connected to the spectral radiances is the respective emissivity ε of the measuring surface for these two wavelengths (Fig. 1). but also limitations of two-colour pyrometry. Practical applications demonstrate typical areas of use. Fig. 1 Two-colour pyrometers measure the radiation at two wavelength ranges and determine the temperature from the ratio of the density of the radiation. In order to minimise the wavelength-dependent influence of the emissivity emitted from the measuring surface, wavelength bands that lie close together are chosen for this purpose. This means on the other hand that the two radiances hardly differ from each other. The ratio of two nearly identical values varies only slightly in relation to the object temperature. The lowest measurable temperature of a two-colour pyrometer is therefore limited to approximately 300 °C. A large amplification factor is needed to be able to interpret these small signal changes at all. For this reason, highest demands have to be applied to the quality of

Table 1 Influence of an emissivity-dependent attenuation for the one-colour and two-colour method of measurement. the sensors, the electronic amplifiers and the A/D converters to achieve a high signal to noise ratio, i.e. a small NETD (noise equivalent temperature difference) and thus a high temperature resolution that is required for a precise measurement. To verify the NETD value, it is necessary to adjust the pyrometer’s lowest temperature threshold to the shortest response time while checking the stability of the measuring signal. Advantages of a two-colour pyrometer The big advantage of...

TECHNICAL REPORTS Fig. 2 Influence on the displayed temperature with a changing emissivity ratio of the target object at different wavelengths, based on an object temperature of 800 ° C. vapour in the air may lead to significant measurement errors caused by atmospheric absorption bands when pyrometers Fig. 3 The emissivity of metals decreases with an increasing measuring wavelength. Even more significant is a parallel recording and evaluation of the two spectral temperatures and the ratio. with long wavelengths are used. The smaller the fluctuations of the temperature difference beWhen a signal...

TECHNICAL REPORTS ty laminated glass (2) show a significant and also irregular drop of the spectral temperature. A significant deviation of the measuring result is also visible for the ratio temperature. When measuring through inspection glasses it is therefore imperative to use glasses that have a neutral transmission curve in the wavelength range of the two-colour pyrometer. It is easy to check whether a glass is suitable by holding it in front of the pyrometer during a measurement. The two-colour temperature measured through a suitable glass may only vary slightly. Operation of the two-colour...

TECHNICAL REPORTS produce a fluctuating measuring value. Under these conditions, pyrometer manufacturers recommend a pyrometer with a very high optical resolution of >200 : 1 to obtain the smallest possible field of measurement. The peak picker function detects the highest temperature at the spots not covered by scales. But how does a two-colour pyrometer react on an inhomogeneous temperature distribution in its measurement area? The behaviour of a two-colour pyrometer under inhomogeneous temperature distribution conditions is by all means more complex. It depends on the total area of Fig. 6...

TECHNICAL REPORTS diation of the hot furnace wall is reflected by the measuring object and consequently also detected by the pyrometer, and the temperature displayed by the pyrometer is therefore always too high. The closer the temperature of the workpiece approaches the furnace temperature, the smaller are the interferences that affect the measurement. The most effective solution to eliminate background radiation is to use water-cooled sighting tubes. The costs for such an investment, however, are high and entail continuing operating costs. In addition, the installation of a tube within a furnace...

TECHNICAL REPORTS eters are very sensitive to flames within their field of view. It is absolutely essential to take this into account when choosing the place of installation. The display of the signal strength serves to check the reliability of the measurement. Due to the often very small furnace openings with a diameter of only 20 – 30 mm and a wall thickness ranging from 200 – 400 mm, high-resolution devices with good imaging qualities should be used to avoid a constriction of the measurement area. To prevent it from "squinting", the pyrometer must be parallax-free; therefore, the geometrical...

TECHNICAL REPORTS ability of two-colour pyrometer measurements used for applications with varying emissivities of the targets. From a pyrometer manufacturer’s point of view, we can only recommended to make use of the additional protection and analysis features the two-colour pyrometer offers to increase process safety and to gain insights from the additionally provided temperature information. Author Fig. 10 Compact two-colour pyrometer with LED spot light to display the exact size, position and focal distance. Dipl.-Ing. Albert Book KELLER HCW GmbH ∙ Germany Infrared Thermometer Solutions tion...