In this issue focusing on new and emerging technologies, we shall speak to recent advances in the relatively old field of pyrometry. There is little information on the invention and development of pyrometers compared to better-known instruments. Common problems that often made pyrometers inapplicable in many situations now have convenient answers.



Since the origin of the word pyrometer is the non-specific pyro – “fire” and meter – “measure,” many different types of instruments can be referred to as a pyrometer. These can range from advanced fiber-optic noncontact instruments to basic thermocouples and others.

Fig. 1. The bright lines of this coned-up, inductively heated melt are cavities and folds in the liquid that enhance the emissivity. That is, since cavities and folds are more efficient radiators, more light is given off even though the temperature is the same. Conventional pyrometers see this as higher temperature. This is why the conventional pyrometer trace in Fig. 3 is so noisy. Only instruments that adjust their response for each measurement, such as SpectroPyrometers, can accommodate such changing targets.

A Brief History of Pyrometry

The first pyrometer measured heat (or really demonstrated heat) by the change in size of a substrate. This instrument, also called a dilatometer after the dilation (thermal expansion) of the substrate, was invented in the 18th century (before 1731) by Pieter van Musschenbroek (1692-1761). A 1731 edition of an Italian journal described a new instrument, the pyrometer, invented by Musschenbroek and several experiments he had done on expansion of materials caused by heat.[2] It provided more of a demonstration of heat than a measurement because there was no way to get a specific temperature reading, although it could be used for relative measurements.[1]

Credit for the invention of the pyrometer is sometimes given to Josiah Wedgwood (of the famous ceramics). He even invented his own measurement – the degree Wedgwood. The Wedgwood pyrometer is also based on the change in size of a substrate. Unlike metals, clay and ceramics shrink with heat. His pyrometer was designed for furnaces that operated from around 1000 to 3000°F (538-1649°C), which is well above the range of mercury thermometers.

In use, his pyrometer depended upon the degree of shrinkage of a small cylinder of pottery clay in the presence of heat. The cylinder was placed in a calibrated, V-shaped trough and subjected to heat. The highest temperature the pyrometer had experienced was determined by noting how far the cylinder had dropped in the trough. This was a direct measurement of the shrinkage of the cylinder’s diameter. Wedgwood thought that the hotter the furnace, the more the clay would shrink. We know now that this is not true, and while the Wedgwood pyrometer could be used to determine if a ceramic was thoroughly cured, it could not be used for temperature measurement. A good account of this instrument is given by G. Gregory.[3]

The vanishing-filament pyrometer, or optical pyrometer, is the first modern instrument in the pyrometer family. It appeared around the beginning of the 20th century. It is designed to use visible light, and in operation compares the thermal radiation from a calibrated light source, a lamp filament and the targeted surface. When the thermal-radiation intensity of the filament and the target match, the filament disappears against the target. This was thought to occur when the two were at the same temperature, and the current passing through the filament could be used to calculate the temperature.[4]

The theory behind the vanishing-filament pyrometer is that anything raised to a given temperature gives off the same thermal radiation. As use of this instrument grew, it became apparent that the material being targeted had to be taken into account. This was done through the correction factor, or the “emissivity.” This is nothing more than the radiation efficiency of the targeted material and therefore varies from zero to one.

Fig. 2. The red spot of the laser aiming indicates the area measured and was generated for this picture by connecting the detector optics to a red laser. The green ring around it is a laser projection outlining the measurement area and can be activated whenever it is desired to aim the pyrometer (it does not affect the measurement). The relationship between measurement area and projected spot does not change.

Brightness (Single-Color) Pyrometer
The vanishing-filament pyrometer is a member of the class of instruments called brightness, or single-color, pyrometers. These add up all the thermal-radiation intensity at some single wavelength or color (actually a contiguous range of wavelengths – the bandwidth of the instrument’s detector) and relate it to temperature by mechanical, electrical or computational means. The instrument’s sensitivity may vary across the bandwidth, making it difficult to use published values of emissivity, especially those of metals, which change with wavelength. For these instruments, the emissivity value is either entered by the operator or assumed to be one. Many manufacturers supply emissivity information with their instruments. These abbreviated lists are hopelessly simplified. The reality is that emissivity is affected by many things, and even the most commonly used refractory materials have emissivities that vary wildly with many factors, including temperature.

The biggest problems with single-color pyrometers are that the emissivity is often unknown or changing and anything that affects the radiated intensity (sight glass, dirt, smoke, steam, process or combustion gases) affects the temperature.

Ratio (Two-Color) Pyrometer
Due to the limitations of the brightness pyrometer, the next step was the ratio, or two-color, pyrometer. These instruments, which date from about the middle of the last century, use two detectors to add up all the intensity in two different wavebands and then relate the quotient of the two intensities to temperature. The theory is that in well-behaved materials, the emissivity cancels out for those whose radiation efficiency or emissivity doesn’t change with color. Where materials are not well behaved, the operator is expected to enter the relative emissivity.

The biggest problems with ratio pyrometers are as follows:
  • Emissivity often changes with wavelength or color (some sight glasses mimic this problem as their transmission changes with wavelength).
  • Ratio instruments lack the precision of brightness pyrometers and are susceptible to noise (differences in intensity at the two wavelengths are relatively small).
  • Relative emissivity is even less well known than the absolute value of emissivity.
  • Errors can be extremely large if one color is affected more than the other by an environmental or material variation.[5]


Fig. 3. Temperatures for several cycles of a casting process controlled by a FAR SpectroPyrometer, a modern multi-wavelength pyrometer, are contrasted with a conventional pyrometer. The intended finish temperature is 2700°F (1482°C) in both cases, but the conventional pyrometer is incorrect and reproduces poorly because the emissivity is both unknown and changing. FAR SpectroPyrometers determine the emissivity for each measurement.

Problems of Conventional Pyrometers and their Solutions

Accuracy/Confidence in Measured Value
Accuracy has always been a problem for pyrometers due to the ever-changing emissivity of the target. The emissivity of metals (targets) changes with wavelength, alloy, turbulence (of liquid) and even temperature. Brightness pyrometers require the input of an emissivity value, and ratio pyrometers require the input of a relative emissivity value. Because emissivity and relative emissivity change with so many variables and process conditions, reported emissivity values are usually incorrect. This translates directly to inaccurate temperature values.

Now, some advanced models that read at multiple wavelengths not only deal directly with unknown and changing emissivity, but they also report the emissivity as measured and the accuracy of the measurement. This feature, called the “tolerance,” is the online accuracy of each individual measurement in degrees. It is read like any tolerance, and provides the operator with instant and constant feedback. The operator instantly knows when something is interfering with the measurement and can take immediate corrective action.

Aim
Many processes require precise aim. Manufacturers attempt to provide this with visual aiming similar in operation to camera aiming. Unfortunately, in practice it is difficult to keep the two sets of optics this design requires (one for the operator and one for the detector) registered so that they view the same target. Without such registration, it is impossible, for example, to know if the target is the melt or the crucible holding the melt. An alternative scheme is more successful. Some instruments project a laser spot through the instrument lens. Since there is only one set of optics, the projected spot always indicates the target.

Process Control
Fast instruments such as modern pyrometers allow process control in manufacturing processes like metal casting where thermocouples are too slow. However, process control is only as good as the accuracy of the controlling pyrometer. Well-aimed, emissivity-independent instruments are the only choice for those applications where the process itself changes the target’s emissivity. Higher productivity, improved quality and reduced energy usage have been observed where such pyrometers have been employed.[6]

Conclusion

A process controlled by a well-aimed, accurate pyrometer is a smooth, energy-efficient process. With instant temperature and tolerance readings, manpower and energy use is reduced and quality and productivity are increased. Overheat, which not only wastes time and energy but often affects the chemical composition of the product, is virtually eliminated.

From Musschenbroek and Wedgwood through the advances of the 20th century, operators have been struggling to measure and control temperature in order to optimize the heating process. Modern pyrometers now allow us to achieve this important goal. IH

For more information: Contact Alex Oatley, publicist; FAR Associates, 1532 Newport Dr. Macedonia, Ohio 44056; tel: 330-468-0482; fax: 440-248-2688; e-mail: Alexoatley@pyrometry.com; web: www.pyrometry.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: pyrometer, dilatometer, thermal radiation, wavelength, emissivity