Principle, advantages and possible applications of innovative panorama pyrometers
Introduction
Pyrometers detect the thermal radiation on the surface of a measuring object in a defined measuring field and determine the temperature from this. The size and shape of the measuring field are determined by the lenses, the optical design and the sensor technology. Due to the geometry of the lenses, the aperture system and the sensor technology, the devices available on the market to date usually have a round measuring surface. Based on an innovative optical structure and high-quality lenses, devices with a rectangular measuring field are now also available. The following article explains the design, function, advantages and possible applications of pyrometers with a rectangular measuring field.
Temperature measurement of moving objects
The idea of developing a pyrometer with a rectangular measuring field was conceived more than 30 years ago, as there are applications in non-contact temperature measurement technology that can be solved more easily and, above all, more reliably. A significant advantage of pyrometric temperature measurement in contrast to contact measurement is that pyrometers are ideally suited for detecting moving objects. The prerequisite is, of course, that the measuring object is located in the pyrometer's measuring field. As the example of wire production shows, it becomes problematic when the measuring object oscillates transversely to the production direction and does not always fill the measuring field (Fig. 1).
Fig. 1 Correct measurement is possible as long as the wire oscillates within the measuring field.
Until now, single-channel pyrometers with a very small measuring field have been used in conjunction with an oscillating mirror mounted in front of the pyrometer to solve such application-related measuring problems. The rotating or oscillating mirror deflects the measuring spot periodically. A peak picker in the pyrometer records the temperature at the time when the measuring spot is completely filled by the object. In addition to the disadvantage of a moving mechanism that is susceptible to faults, the detection time is limited. Due to the scanning movement, the object temperature is not recorded continuously, but only cyclically.
For this reason, many years ago there were already experiments with devices that generated a rectangular measuring field purely optically. A special cylindrical lens spread the measuring field in the direction of an axis, as known from a mirror cabinet. In principle, a solution has been found. The uneven sensitivity distribution on the measuring surface of the sensor proved to be a problem. Another disadvantage was the high cost of this special lens. In addition, the devices could only be used for a fixed measuring distance. Another difficulty was that the optical image in the through-the-lens sighting was distorted, making it difficult to align the device.
The use of a rectangular measuring field is particularly interesting in conjunction with a two-colour pyrometer. 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 changes in proportion to the temperature. This measuring principle allows the measuring object to be smaller than the measuring field. In contrast to a single-channel pyrometer, the correct temperature is still determined for a hot measuring object against a cold background.
For this reason, many years ago there were already experiments with devices that generated a rectangular measuring field purely optically. A special cylindrical lens spread the measuring field in the direction of an axis, as known from a mirror cabinet. In principle, a solution has been found. The uneven sensitivity distribution on the measuring surface of the sensor proved to be a problem. Another disadvantage was the high cost of this special lens. In addition, the devices could only be used for a fixed measuring distance. Another difficulty was that the optical image in the through-the-lens sighting was distorted, making it difficult to align the device.
The use of a rectangular measuring field is particularly interesting in conjunction with a two-colour pyrometer. 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 changes in proportion to the temperature. This measuring principle allows the measuring object to be smaller than the measuring field. In contrast to a single-channel pyrometer, the correct temperature is still determined for a hot measuring object against a cold background.
Design and functioning
In contrast to the solution described above with a cylindrical lens, the rectangular measuring field of the new panorama pyrometer is realized by a highly precise aperture, which is positioned in the measuring branch of the detector between the aperture (3) and the deflection mirror with sensor (4) (Fig. 2). This solved the two fundamental problems. The device does not require a specially shaped lens and the measuring object is displayed in focus at the usual distance in the through-the-lens sighting or on the monitor screen of devices with an integrated video camera.
Fig. 2 Block diagram of the optical design of the panorama pyrometer: measuring object (1), focusable interchangeable lens (2), aperture system (3), deflection mirror and sensor (4), measuring field marker (5), eyepiece or video camera (6)
A further advantage of this new optical design is that the measuring field marking in the viewfinder or on the monitor is displayed correctly both in the exact position and in the actual size of the rectangular measuring field. This is the only way to check and ensure correct alignment of the devices.
Another optical challenge had to be solved during the development of the panorama pyrometer. Optical aberrations and an inhomogeneous distribution of sensitivity on the measuring surface usually have the effect on two-colour pyrometers that temperatures are measured quite differently depending on the position of the measuring object in the measuring field. At the edge of the measuring field, the display can rise by more than 30 °C at an object temperature of 1,000 °C (Fig. 3).
Conventional two-colour pyrometers may also display varying temperatures when the measuring object diameter changes due to production-related processes and the measuring field is differently filled out as a result.
Another optical challenge had to be solved during the development of the panorama pyrometer. Optical aberrations and an inhomogeneous distribution of sensitivity on the measuring surface usually have the effect on two-colour pyrometers that temperatures are measured quite differently depending on the position of the measuring object in the measuring field. At the edge of the measuring field, the display can rise by more than 30 °C at an object temperature of 1,000 °C (Fig. 3).
Conventional two-colour pyrometers may also display varying temperatures when the measuring object diameter changes due to production-related processes and the measuring field is differently filled out as a result.
Fig. 3 Erroneous temperature rise with two-colour pyrometers if the hot object is located in the edge area of the measuring field.
To minimize this physical effect, precision lenses were developed for the optical systems. These lenses have a consistently good imaging quality over the entire surface of the entrance aperture (minimum spherical aberration). Moreover, the lenses come with a minimum longitudinal chromatic aberration to achieve an equally sharp imaging for both measuring wavelengths as well as for the visible range. In addition, it was necessary to develop an optical design consisting of precision lenses and to use high-quality sensors. The result is the new panorama pyrometer that delivers a constant measured value regardless of the position and diameter of a wire in the measuring field, for example.
A variety of optical options
The modular design of the optical and electric components offers the option also for the panorama pyrometer to choose from several interchangeable lenses with focal adjustment. A number of supplementary lenses that can be attached to the front thread of the objective lens are also available to decrease the measuring field. This results in numerous optical imaging variants with regard to both the desired measuring distance and the required measuring field size (Fig. 4). This means that even wires from a diameter of 0.1 mm can be detected.
Fig. 4 Modular design of the pyrometer consisting of electronics, interchangeable lenses and optional supplementary lenses.
Simple alignment and high operational reliability
The optical alignment of a pyrometer to a small measuring object or at a large measuring distance requires high-quality mechanics for adjustment. It is self-explanatory that a device with a rectangular measuring spot is much easier to align under these conditions (Fig. 5). This advantage is particularly noticeable with a portable pyrometer if the operator holds the device in his hand when aiming, as the width of the rectangular measuring field is 2 to 3 times greater than that of a comparable device with a round measuring field. This ensures safer handling and temperature detection.
Fig. 5 Aligning the panorama pyrometer with a rectangular measuring field to small objects and large measuring distances is extremely simple.
Fig. 6 Reliable temperature measurement even if the position of the weld seam fluctuates.
Fig. 7 A portable panorama pyrometer measures the temperature during the pouring process.
Typical applications
In production processes where the position and size of the hot object can change or in heat treatment systems where the heating zone of the workpiece fluctuates, the panorama pyrometer offers greater operational reliability and is much easier to align. As a rectangular measuring field is wider than a round measuring field with the same area, the risk of the hot spot moving out of the measuring field is significantly lower.
A typical example is the production of continuous pipes, in which the material is bent and welded together. Heating is achieved via an induction coil. The position of the small welding point can fluctuate, so that with conventional devices the weld seam can sometimes lie outside the measuring field and measurement is then no longer possible (Fig. 6).
During the production of glass bottles, the position and shape of the glass gob on the scissors changes. Here too, a panorama pyrometer provides greater measurement reliability. Added to this is the influence of the temperature of the material and the colour of the partially transparent glass. This influence is greatly reduced by the two-colour measuring method of the panorama pyrometer.
The wire is then subjected to heat treatment in wire drawing plants. The wire runs through an induction coil at high speed. The wire cannot be prevented from swinging between the guide rollers. With thin wires, the fluctuation can be several times the wire diameter. A punctual measurement is hardly possible under these conditions.
The manual, non-contact temperature measurement of liquid metal during pouring into the mould is carried out from a safe distance. With a conventional device with a round measuring field, it is difficult to align the pyrometer with the pouring stream, especially as the position of the stream can change depending on the tilt angle of the ladle. A device with a rectangular measuring field is much easier to handle (Fig. 7).
Measuring the temperature of the smallest objects, such as a filament or a heating element in an X-ray tube, places the highest optical demands on the devices. For the most part, such applications could previously only be solved with so-called intensity comparison pyrometers. The temperature of the devices is recorded manually by the operator visually comparing the radiance of an internal reference radiator and the measuring object.
The difficulty in using electronic measuring devices lay in the mechanical alignment of the devices to extremely small measuring objects. Such measuring tasks can also be solved much more easily with the panorama pyrometer.
A typical example is the production of continuous pipes, in which the material is bent and welded together. Heating is achieved via an induction coil. The position of the small welding point can fluctuate, so that with conventional devices the weld seam can sometimes lie outside the measuring field and measurement is then no longer possible (Fig. 6).
During the production of glass bottles, the position and shape of the glass gob on the scissors changes. Here too, a panorama pyrometer provides greater measurement reliability. Added to this is the influence of the temperature of the material and the colour of the partially transparent glass. This influence is greatly reduced by the two-colour measuring method of the panorama pyrometer.
The wire is then subjected to heat treatment in wire drawing plants. The wire runs through an induction coil at high speed. The wire cannot be prevented from swinging between the guide rollers. With thin wires, the fluctuation can be several times the wire diameter. A punctual measurement is hardly possible under these conditions.
The manual, non-contact temperature measurement of liquid metal during pouring into the mould is carried out from a safe distance. With a conventional device with a round measuring field, it is difficult to align the pyrometer with the pouring stream, especially as the position of the stream can change depending on the tilt angle of the ladle. A device with a rectangular measuring field is much easier to handle (Fig. 7).
Measuring the temperature of the smallest objects, such as a filament or a heating element in an X-ray tube, places the highest optical demands on the devices. For the most part, such applications could previously only be solved with so-called intensity comparison pyrometers. The temperature of the devices is recorded manually by the operator visually comparing the radiance of an internal reference radiator and the measuring object.
The difficulty in using electronic measuring devices lay in the mechanical alignment of the devices to extremely small measuring objects. Such measuring tasks can also be solved much more easily with the panorama pyrometer.
Metrological limits
Due to the two-colour measuring principle, the field of application is limited to applications with temperatures above 600 °C. Another limit is the degree of partial illumination up to which the two-colour pyrometer is still able to form a reproducible measured value.
This value depends, among other things, on the emissivity of the measuring object and the absolute temperature. At the start of the measuring range, a two-colour pyrometer can already provide a reliable measured value if the radiation energy is 10 % of the radiance of a blackbody radiator at the same temperature. As the measuring temperature increases, even greater signal attenuation is permissible. Attenuation is caused by the emissivity, the degree of partial illumination, the shape of the measuring object and influences that obstruct visibility such as vapour, dust and smoke in the measuring field. Let’s take a steel wire with an emissivity of 0.6 as an example. With a round measuring object, it must also be taken into account that the radiation detected by the pyrometer is sometimes emitted at a very flat angle. As an approximation, the safety factor of 1.5 is then factored in. The following formulas can be used to calculate the degree of partial illumination, the width of the measuring field and the maximum measuring distance.
Degree of partial illumination = (minimum analyzable signal strength ÷ emissivity) × safety factor
With reference to the example above, the measuring field must be at least 10 % ÷ 0.6 × 1.5 = 25 % full so that the pyrometer can determine a measured value. The signal strength as an indication of the safety of the measured value can be shown on the pyrometer display.
For a wire diameter of 5 mm, this results in a maximum width of the measuring field of 5 mm ÷ 0.25 = 20 mm for the start of the measuring range.
With a panorama pyrometer, the optical resolution is specified by the distance ratio (measuring distance ÷ measuring field size) for the width DW and for the height DH. Based on a distance ratio of, for example, DW = 40 : 1, this results in a maximum measuring distance of 40 × 20 mm = 800 mm. Or, to put it another way, for an intended measurement distance of 500 mm, for example, a lens with a distance ratio of DW ≥ 500 mm ÷ 20 mm, i.e. ≥ 25 : 1, must be used so that the measuring field is sufficiently illuminated by the measuring object.
The panorama pyrometer can also be operated so that the measuring field is aligned lengthwise to the object. This means that the pyrometer detects a larger area of the measuring object compared to a device with a round measuring field, so that it can be used for wires with a diameter from 0.1 mm.
This value depends, among other things, on the emissivity of the measuring object and the absolute temperature. At the start of the measuring range, a two-colour pyrometer can already provide a reliable measured value if the radiation energy is 10 % of the radiance of a blackbody radiator at the same temperature. As the measuring temperature increases, even greater signal attenuation is permissible. Attenuation is caused by the emissivity, the degree of partial illumination, the shape of the measuring object and influences that obstruct visibility such as vapour, dust and smoke in the measuring field. Let’s take a steel wire with an emissivity of 0.6 as an example. With a round measuring object, it must also be taken into account that the radiation detected by the pyrometer is sometimes emitted at a very flat angle. As an approximation, the safety factor of 1.5 is then factored in. The following formulas can be used to calculate the degree of partial illumination, the width of the measuring field and the maximum measuring distance.
Degree of partial illumination = (minimum analyzable signal strength ÷ emissivity) × safety factor
With reference to the example above, the measuring field must be at least 10 % ÷ 0.6 × 1.5 = 25 % full so that the pyrometer can determine a measured value. The signal strength as an indication of the safety of the measured value can be shown on the pyrometer display.
For a wire diameter of 5 mm, this results in a maximum width of the measuring field of 5 mm ÷ 0.25 = 20 mm for the start of the measuring range.
With a panorama pyrometer, the optical resolution is specified by the distance ratio (measuring distance ÷ measuring field size) for the width DW and for the height DH. Based on a distance ratio of, for example, DW = 40 : 1, this results in a maximum measuring distance of 40 × 20 mm = 800 mm. Or, to put it another way, for an intended measurement distance of 500 mm, for example, a lens with a distance ratio of DW ≥ 500 mm ÷ 20 mm, i.e. ≥ 25 : 1, must be used so that the measuring field is sufficiently illuminated by the measuring object.
The panorama pyrometer can also be operated so that the measuring field is aligned lengthwise to the object. This means that the pyrometer detects a larger area of the measuring object compared to a device with a round measuring field, so that it can be used for wires with a diameter from 0.1 mm.
Device versions
Devices with panorama lenses are available for the stationary CellaTemp PA series and the portable CellaTemp PT series. Both versions have a through-the-lens sighting for aligning and focusing the device. The stationary CellaTemp PA is alternatively available with a colour video camera. This means that the alignment and field of view of the object can be monitored at all times on the monitor in the control room. In addition to the measuring field marking, the measured value and the measuring point number are also transmitted via the video signal and displayed on the monitor screen. Thanks to the camera's special TBC function (Target Brightness Control), the intensity is only recorded in the measuring field for exposure control and not over the camera's entire field of view, as is usually the case. This displays a small hot measuring object in front of a cold background with optimum brightness and without overdriving the object on the monitor screen.
For thermal processes and temperatures above 600 °C, the new panorama pyrometer is clearly superior to previous devices with a round measuring field if alignment is difficult on small objects or at large measuring distances or if the hot spot, i.e. the hot spot to be detected, is not fixed. The additional costs of approx. 25 % are certainly money well spent due to the higher operational reliability.
Fig. 8 Compact panorama pyrometer with LED spot light.
Conclusion
For processes with temperatures above 600 °C where an alignment is difficult because the target is small or the distance is too large, or when the hot spot, i.e. the spot to be captured, is moving, the new panorama pyrometer is clearly superior to common pyrometers with a round field of view. The additional cost of approximately 25% is definitely money well spent due to the higher operational reliability.