Precision is a key attribute in virtually all processes conducted by scientists and engineers. For measuring temperature, a task essential for maintaining and understanding a test sample’s many properties, professionals can depend upon a simple thermometer under relatively mild conditions. However, when heat rises in intensity, such as in kilns and furnaces, thermometers are surpassed in their capabilities, necessitating the need for pyrometers.
Today’s pyrometers are commonly automatic and digital-based, being used or different purposes, such as the manufacture of aerospace components, and current practices in pyrometry incorporate a system of sensors and equipment for measuring surface temperatures that cannot be reached or touched. In the past, however, different types of equipment and techniques have been utilized to achieve these results.
The first pyrometer was invented by Josiah Wedgwood in the 1780s, after he tasked himself with producing a thermometer which could measure high temperatures inside a kiln. He designed his initial device so that it would match fired clay cylinders with those fired at known temperatures. This led to cylinders that ranged in color from yellow-brown to red, which indicated low to high temperatures, respectively. He later upgraded the pyrometer to measure the shrinkage of clay. Taking advantage of the principle that porcelain shrinks when fired, Wedgwood used the shrinkage of small pieces of fired porcelain to measure kiln temperature.
The late Nineteenth Century saw further advancements in pyrometry, particularly the invention of the manual optic pyrometer. This device measured high temperature, at a safe distance, by comparing the radiation a hot object produced with the radiation generated by a hot filament.
To operate these pyrometers, which have remained in use until the present day, users observe the object to be measured beyond a red filter, through which a dull red glow from the heated object with a line of brighter light from the filament running through it should be visible. After adjusting the electric current passing through the filament, observations can be made on the temperature of the filament, and it will soon effectively disappear after reaching the temperature identical to that of the object. The meter will then display the temperature, a value derived from calibration that converts the filament’s electric current to temperature.
Despite remaining the standard pyrometer style for the majority of the Twentieth Century, the manual pyrometer fell out of use in the 1980s, following the popularity of microchips and compact electronic equipment. Since this time, the simpler and quicker alternatives have been the different variants of digital pyrometers, which, for the most part, compare the heat radiation from the measured object with the radiation produced by an internal heat source. There are differences in how exactly these devices measure heat, with some performing the task by absorbing light with semiconductor-based, light-sensitive photocells.
Some digital pyrometers are intended for making quick one-off measurements, and these devices are shaped and operated like pistols, making use of built-in detectors, signal amplifiers, power sources, and temperature meters after being aimed and operated towards the measured object. However, many industrial processes are dependent on continuous, precise measurements and require multiple optical fibers permanently fixed to the machine to keep track of the temperature.
These systems are the focus of SAE AMS 2750E-2022 – Pyrometry, which details pyrometric guidelines for thermal processing equipment used in heat treatment. Specifically, it covers temperature sensors, instrumentation, thermal processing equipment, system accuracy tests, and temperature uniformity surveys. Through the comprehension of these components, the user of the standard receives assurance that the parts or raw materials being heat treated are done so with optimal thermal processing equipment.
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