Two-colour pyrometers

Two-colour pyrometers are indispensable in today's infrared thermometer applications. The following article explains the physical principles, advantages, functional and analytical possibilities as well as the limitations of the two-colour pyrometry. We will present typical areas of application on the basis of practical applications.

A two-colour pyrometer detects the thermal radiation of a measuring object in two different wavelength ranges. The ratio of the two spectral radiance values φ changes approximately in proportion to the temperature. The respective emissivity ε of the measuring surface for the two wavelengths is linked to the spectral radiance (Figure 1).

In order to minimize the wavelength-dependent influence of the emissivity of the measuring surface, wavelength ranges that are close to each other must be selected. On the other hand, however, this means that the two radiation densities hardly differ. The ratio of two almost identical values changes only very slightly depending on the object temperature. Therefore, the lowest measurable temperature of a two-colour pyrometer is limited to approx. 300 °C. In order to be able to analyze these small signal changes at all, a large amplification is required. The highest demands are therefore placed on the quality of the sensors, electronic amplifiers and A/D converters in order to achieve a high signal-to-noise ratio or a small NETD (Noise Equivalent Temperature Difference) and thus the high temperature resolution required for accurate measurement. To check the NETD, operate the device at the start of the measuring range with the shortest response time and check the stability of the measuring signal.

The great advantage of the two-colour measuring method is that the correct temperature is determined with a wavelength-independent signal attenuation. If, for example, a dirty inspection glass or vapour, smoke and dust in the pyrometer's field of view reduce the signal, the ratio and thus the displayed temperature remain constant.

If the emissivities ε1 = ε2 (grey radiator) are the same for the two wavelengths, the term of the emissivity in the equation is reduced and the two-colour pyrometer displays the true temperature regardless of the emissivity of the measuring object. Even if the emissivity of the measuring object changes to the same extent for both wavebands, this has no influence on the measurement result. Deviations from the true temperature due to constant differences between the two emissivities can be corrected by adjusting the emissivity ratio on the pyrometer.

But how does a two-colour pyrometer behave if the emissivity changes differently for the two wavelengths when measuring on a so-called coloured radiator due to the surface or depending on the temperature?

The same selective effect occurs when the transmission of the inspection glass changes depending on the wavelength due to thin-layer deposits (e.g. oil films or vapour deposits). The two-colour method is also not completely independent of the radiation properties of the measuring object, as can sometimes be read in the literature.

The three examples in Table 1 clearly show the different influence of attenuation depending on the degree of emission for the one-colour and two-colour measuring methods. In relation to a temperature of 800 °C of a "blackbody radiator" with an emissivity of ε = 1, the following temperature values result from Planck's radiation law for a two-colour pyrometer with λ1 = 0.95 μm and λ2 = 1.05 μm with a different change in the wavelength-related emissivities (see Table 1).

The lower reading of the one-colour temperatures for the protective glass (1) is clearly visible. In contrast, the ratio value remains almost constant. In the case of the inferior quality laminated glass (2), the one-colour temperatures fall even more sharply and to varying degrees. This also leads to a considerable measurement deviation for the ratio.

With two-colour pyrometers, when measuring through inspection glasses, it is therefore essential to ensure that the glasses have a neutral transmission curve in the wavelength range of the pyrometer. This can be checked very easily by holding a disc in front of the pyrometer during the measurement. The two-colour temperature may only change insignificantly.

Another major advantage of two-colour pyrometry is that measuring objects can also be smaller than the measuring field. With a one-colour pyrometer, the measuring object must always be larger than the measuring field, as a one-colour pyrometer records the average value of the radiation within the entire measuring field. Otherwise, a temperature that is too low will always be determined for a small measuring object in front of a cold background.

If the measuring field of a two-colour pyrometer is not completely illuminated by the measuring object (partial illumination effect), this acts as a neutral attenuation of the infrared radiation. Therefore, a two-colour pyrometer still provides correct measured values even if the object is up to 80 % smaller than the pyrometer's measuring field. The degree of minimum partial illumination depends on the emissivity and the temperature of the measuring object. Ideally, the position of the object in the measuring field should be arbitrary and not influence the displayed temperature value. However, there are major differences in quality between the devices available on the market. In pyrometers with a simple optical design, a lower correction of the optical aberration of the objective lens and sensors with an inhomogeneous sensitivity distribution, the measured value can increase by up to 20 - 30 °C at a constant object temperature if, for example, a hot wire is located in the edge area of the measuring field (Fig. 5).

Another advantage when measuring small objects is that a two-colour pyrometer is much less sensitive to optical alignment and correct focusing. In contrast, a one-colour pyrometer must be very precisely aligned and focused on the measuring object in order to avoid measuring errors if the measuring object is barely larger than the measuring field.

The following measuring curve (Fig. 6) was recorded with a ratio pyrometer with a measuring field of Ø8 mm on a measuring object with Ø8 mm. A one-colour temperature was recorded at the same time. The fixed focal distance was 500 mm (measuring point 1). The measuring distance was then reduced to 250 mm (measuring point 2). Defocusing has only a minor influence on the two-colour temperature, whereas the one-colour temperature deviates by approx. 20 °C. The measuring distance was then set to 1,000 mm (measuring point 3). The pyrometer's measuring field is twice as large as the measuring object. Again, the two-colour temperature remains at almost the same level. In contrast, the spectral value drops sharply due to defocusing and partial illumination.

When measuring the temperature of sheet metals and slabs in the rolling stand, the question of which measuring method to recommend, one-colour or two-colour, always arises due to the extreme conditions (Fig. 7).

For design and thermal engineering reasons, the devices are mounted at a large measuring distance of several meters. Using a standard lens with an optical resolution of 100:1, for example, results in a measuring field diameter of 200 mm at a distance of 20 meters. The temperature distribution on the slab is extremely inhomogeneous due to the scale. With a one-colour pyrometer, the temperature is determined from the average value of the total radiation received in the measuring field. The measured value is therefore dependent on the temperature distribution and the scale. As the slab moves on the roller table, this would lead to a fluctuating measured value if the signal was not filtered. Pyrometer manufacturers therefore recommend using a pyrometer with a very high optical resolution of > 200 : 1 under these conditions in order to achieve the smallest possible measuring field. The peak picker is used to record the highest temperature at the scale-free points. 

But how does a two-colour pyrometer react to an inhomogeneous temperature distribution in the measuring field? The behaviour of a two-colour pyrometer is more complex with an inhomogeneous temperature distribution. It depends on the total area of the "hot spots" and the temperature differences between the hot and cold spots in the measuring field. Due to the partial illumination effect described above, a two-colour pyrometer determines the temperature of the hottest point in the measuring field provided there is a significant temperature difference of > 200 °C between the hot and cold areas. 

When measuring on a slab, several hot spots can occur in the measuring field due to the scale. If the temperature difference is small, the two-colour pyrometer also determines the temperature from the average value of the received radiation. It is therefore also recommended to use devices with high optical resolution and good imaging quality for a two-colour pyrometer in order to minimize the influence of inhomogeneities by means of maximum value detection. 

If water vapour and contamination are to be expected during the hot rolling process, a two-colour pyrometer should preferably be used. Using the contamination monitoring function of the two-colour pyrometer also increases the operational reliability of the measured value acquisition.

The issue of measuring the temperature of colder objects inside a hot furnace is often discussed. Cold forged parts are placed in hot furnaces for heating or cold slabs are passed through the various heating zones of a pusher furnace. Due to the high so-called background radiation of the hot furnace wall, which is reflected by the measuring object and thus also detected by the pyrometer, the pyrometer always indicates a temperature that is too high. The closer the temperature of the workpiece is to that of the furnace, the lower the disruptive effect. The most effective solution for eliminating background radiation is the use of water-cooled sighting tubes. However, this is associated with high investment and permanent operating costs. In addition, the installation of a pipe inside a furnace that extends almost to the workpiece could be difficult or impossible for structural reasons.

For this reason, the devices are often used without a sighting tube, probably because they are aware of the greater or lesser degree of faulty measurement. The influence of background radiation can be reduced if the temperature of the radiation background is measured separately using a thermocouple or second pyrometer and the reflected interference radiation in the pyrometer is corrected by calculation. This correction can be subject to uncertainty, especially if the emissivity of the object is small, fluctuates or is not precisely known.

If, for physical reasons, the rule of thumb "measure as short-wave as possible" applies to metallic objects in order to minimize the influence of emissivity, this approach is exactly the opposite when measuring colder objects in a hot atmosphere.

The background radiation has less of an effect with a longer wavelength measuring device. On the other hand, with a longer wavelength spectral sensitivity, the emissivity ε of metals is smaller and therefore the degree of reflection σ is greater (ε + σ = 1). This in turn leads to a greater dependence of the interference influence of the hot furnace radiation with changing emissivities. Manufacturers therefore recommend using devices with a spectral sensitivity in the 1 - 2 μm range in order to achieve the best compromise.

This also raises the question of how a two-colour pyrometer behaves when measuring colder objects in a hot atmosphere. In principle, a two-colour pyrometer behaves in a similar way to a one-colour pyrometer. It detects both the object radiation and the reflected radiation from the furnace wall. A two-colour pyrometer reacts less sensitively if the inspection glass is dirty or if dust and smoke are in the pyrometer's field of view. The reaction to changing emissivities is extremely dependent on local conditions and is therefore difficult to estimate. It is advisable to record and evaluate both the two-colour and one-colour temperatures in parallel during commissioning or permanently in order to be able to carry out any analyses. Modern two-colour pyrometers offer two analogue outputs for this purpose, enabling the measured values of the two-colour and a one-colour temperature to be recorded directly by the control system. Another advantage of the two-colour pyrometer is the possibility of analyzing the signal strength as an indication of the radiation properties of the measuring object (Fig. 8).

Due to the extreme measuring conditions caused by dust, vapour and smoke, two-colour pyrometers are advantageous for use in power plants and combustion plants in terms of measurement technology and safety. A pyrometer detects the radiation of the objects in the measuring field. In a combustion plant, the energy received is radiated both from the hot particles in the air flow and from the opposite wall. The measured value depends on the density of the particles, the inhomogeneity of the temperature distribution and the temperature of the opposite wall. If the wall is significantly cooler than the particles in the air flow due to heat exchanger pipes, a one-colour pyrometer detects a temperature that is too low and fluctuates depending on the load condition due to averaging. This is where the advantage of the two-colour pyrometer in terms of the partial illumination effect and maximum value detection comes into play again. Two-colour pyrometers are therefore a real alternative to the thermocouples normally used, as they are not subject to wear or age-related drift. However, two-colour pyrometers are very sensitive to flames in the field of view. This must be taken into account when selecting the installation location.

The reliability of the measurement can be checked by displaying the signal strength. Due to the often small furnace openings with diameters of 20 - 30 mm and wall thicknesses of 200 - 400 mm, optical high-resolution devices with good imaging properties must be used in order to avoid constriction of the measuring field. The geometric and optical axes should also be identical and therefore the device should be parallax-free to prevent the device from "squinting". Depending on the equipment required and the accessibility of the installation site, compact devices or pyrometers with a sighting aid in the form of a through-the-lens sighting or a video camera are used in order to be able to check the alignment and free viewing opening quickly and easily during commissioning and during operation.

From a safety point of view, the use of the contamination monitoring function of the two-colour pyrometer is also recommended here in order to automatically generate an alarm if the furnace opening becomes too dirty or overgrown.
 

In their production process, billets go through a heating furnace before they are pressed to fittings. The temperature of this process has to be controlled to reach a consistent quality and to avoid faulty parts. Pyrometers are usually installed in inductive heating systems to detect the temperature of the passing workpiece within milliseconds and from a safe distance when the billet leaves the induction furnace. The temperature is used as a control variable for process control purposes and also to discard those billets whose temperature is outside the permissible range (Fig. 9).

Both one-colour and two-colour pyrometers are used to measure the temperature. The devices are mounted at larger distances of 600 - 1,200 mm. A sighting aid in the form of a through-the-lens sighting or a pilot light is a mandatory requirement. This is the only way to set the correct focal distance and exact alignment in order to minimize any measuring errors caused by optical influences.

Particularly in the case of devices with a fixed focal distance, this cannot always be maintained exactly due to the machine design. With fixed mounting of the devices and varying bolt diameters, the measuring distance changes anyway, so that the devices are sometimes not operated at the focal distance.

In the case of devices with focusable lenses, the measuring distance is often not set correctly, as practice shows. There is hardly any readjustment with changing bolt diameters, so that these devices are always out of focus. 

A two-colour pyrometer reacts much less sensitively to changes in the measuring distance, the bolt diameter or when the devices are operated outside the focal range as described at the beginning up to certain limits and is therefore advantageous for such applications compared to a one-colour pyrometer. 

The use of compact two-colour pyrometers with pilot light (Fig. 10) is therefore recommended here in order to optimally fulfil the two essential requirements of the measuring task for a) a largely distance-independent and reliable measurement and b) a simple alignment check.

For production processes with temperatures above 300 °C, two-colour pyrometers with the advantages described are more than an alternative for achieving reliable and stable measured values due to the environment and design. The additional price of around 30 % compared to a one-colour pyrometer with comparable features is money well spent and is quickly recouped by the reduced manual inspection effort and the reduction in the production of faulty parts. The metrological advantages of the two-colour pyrometer really pay off in extreme measuring conditions with heavy steam, dirt and dust. For applications in which the emissivity of the measuring objects can change, it is advisable to check the reliability of the measurement when using the two-colour measuring method.

Device manufacturers can only recommend utilizing the additional protection and analysis options of the two-colour pyrometer in order to increase process reliability and gain insights from the additional temperature information.