Pyrometers

Pyrometers

• Simplest & oldest non-contact way of estimating the temperature of a radiating body by observing its color

 • radiation thermometer (e.g. infrared thermometer)

 • radiation wavelengths: visible & infrared radiation bands (0.4 ~ 20 μm)  

• Automatic pyrometers - optical system concentrating the radiation on radiation detector

- radiation detector which may be either a thermal or a photoelectric sensor

- signal converter, conditioning the detector output signal before being displayed

 - measuring part, which may have an additional analogue or digital output

A pyrometer is a type of remote-sensing thermometer used to measure the temperature of a surface. Various forms of pyrometers have historically existed. In the modern usage, it is a device that from a distance determines the temperature of a surface from the spectrum of the thermal radiation it emits, a process known as pyrometry and sometimes radiometry.

Design

A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature T) is related to the thermal radiation or irradiance j* of the target object through the Stefan–Boltzmann law, the constant of proportionality σ, called the Stefan-Boltzmann constant and the emissivity ε of the object.

This output is used to infer the object's temperature from a distance, with no need for the pyrometer to be in thermal contact with the object; most other thermometers (e.g. thermocouples and resistance temperature detectors (RTDs)) are placed in thermal contact with the object, and allowed to reach thermal equilibrium.

Pyrometry of gases presents difficulties. These are most commonly overcome by using thin filament pyrometry or soot pyrometry. Both techniques involve small solids in contact with hot gases

Applications

Pyrometers are suited especially to the measurement of moving objects or any surfaces that can not be reached or can not be touched.

Temperature is a fundamental parameter in metallurgical furnace operations. Reliable and continuous measurement of the melt temperature is essential for effective control of the operation. Smelting rates can be maximized, slag can be produced at the optimum temperature, fuel consumption is minimized and refractory life may also be lengthened. Thermocouples were the traditional devices used for this purpose, but they are unsuitable for continuous measurement because they melt and degrade.