A pyrometer measures surface temperature by radiant heat, commonly used in situations where the surface cannot be touched. The first pyrometer was invented by Josiah Wedgewood, and modern ovens use infrared pyrometers. Accurate measurements require correct judgment of a surface’s emissivity. Optical pyrometers have been replaced by infrared pyrometers, but are still used for measuring small, hot objects.
The word pyrometer comes from the Greek words pyros, which means “fire” and metro which means “to measure”. A pyrometer is a device that determines a surface temperature by measuring radiant heat. It is generally used in situations where the surface to be measured cannot be touched, either because it is moving or because it would be dangerous to do so. Common varieties include the infrared pyrometer and the light pyrometer.
The first pyrometer was invented by Josiah Wedgewood, an 18th-century English potter. He used the shrinkage of porcelain under heat to monitor the approximate temperatures inside Wedgewood kilns. Firing ceramics and monitoring the temperature inside kilns remains a major application of pyrometry today. Modern ovens generally use infrared pyrometers, also known as radiation pyrometers, to monitor their temperature.
Infrared pyrometers use infrared and visible light radiating from an object to heat a thermocouple, a device that creates an electric current that powers a temperature gauge. Focal distance – the point at which the instrument has the smallest spot reading size, and field of view – the angle at which the pyrometer optics operate, are very important to the proper functioning of an infrared pyrometer. The device determines an average temperature for the region it is measuring, so if the object being measured does not fill the pyrometer’s field of view, a measurement error will occur.
Accurate measurements also require a correct judgment of a surface’s emissivity. Infrared light from a surface is actually the sum of three factors: reflectivity — the proportion of the radiation from elsewhere that is reflected by the surface being measured; transmissivity — the proportion of radiation coming from behind the measured object and passing through it; emissivity — the proportion of infrared radiation actually emitted by the measured surface. These three values range from zero to one and together total one. Infrared pyrometers work best if the emissivity is close to one and are very difficult to calibrate for reflective metals and transparent surfaces with emissivity of 0.2 or less.
The other variety in common use is an optical pyrometer. First patented by Everett F. Morse in 1899, the optical pyrometer passes a current through a filament connected to a thermometer. An operator looks through an eyepiece at the filament and the surface to be measured. As the current through the filament varies, the temperature of the filament also varies. When the glow of the filament matches the glow of the surface, the temperature can be read from the gauge. In most applications, light pyrometers have been replaced by infrared pyrometers, which offer greater accuracy over a wider temperature range, but light pyrometers remain in use, particularly when measuring the temperatures of relatively hot objects and small, such as the annealing of tungsten wires.
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