May 23, 2016

Pyrometer - M & M ASSIGNMENT (M.P.Jithesh. Aiyappa, Rangaraju .D, Shubham.Garg)

A pyrometer is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry. This device can be used to determine the temperature of an object's surface.

The word pyrometer comes from the Greek word for fire, "πυρ" (pyro), and meter, meaning to measure. Pyrometer was originally coined to denote a device capable of measuring temperatures of objects above incandescence (i.e. objects bright to the human eye).

Modern pyrometers became available when the first disappearing filament pyrometer was built by L. Holborn and F. Kurlbaum in 1901. This device superimposed a thin, heated filament over the object to be measured and relied on the operator’s eye to detect when the filament vanished. The object temperature was then read from a scale on the pyrometer.

The temperature returned by the vanishing filament pyrometer and others of its kind, called Brightness Pyrometers, is dependent on the emissivity of the object. With greater use of brightness pyrometers, it became obvious that problems existed with relying on knowledge of the value of emissivity. Emissivity was found to change, often drastically, with surface roughness, bulk and surface composition, and even the temperature itself.

To get around these difficulties, the ratio or two-color pyrometer was developed. They rely on the fact that Planck's Law, which relates temperature to the intensity of radiation emitted at individual wavelengths, can be solved for temperature if Planck’s statement of the intensities at two different wavelengths is divided. This solution assumes that the emissivity is the same at both wavelengths and cancels out in the division. This is known as the gray body assumption. Ratio pyrometers are essentially two brightness pyrometers in a single instrument. The operational principles of the ratio pyrometers were developed in the 1920s and 1930s, and they were commercially available in 1939.

As the ratio pyrometer came into popular use, it was determined that many materials, of which metals are an example, do not have the same emissivity at two wavelengths . For these materials, the emissivity does not cancel out and the temperature measurement is in error. The amount of error depends on the emissiveties and the wavelengths where the measurements are taken. Two-color ratio pyrometers cannot measure whether a material’s emissivity is wavelength dependent.


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