Pyrometers A typical Radiation pyrometer Principle used for

Pyrometers

A typical Radiation pyrometer

Principle used for radiation pyrometer There are two principle used for the consrution of radiation temperature measuring devices 1. Total radiation pyrometer 2. Selective radiation pyrometer (Infrared pyrometer)

Total Radiation Pyrometer

Infrared Pyrometer

Optical Pyrometer Any metal, when heated, emits radiation of different wavelengths Color Temperature Dark Red 540 deg. Celsius Medium cherry Red 680 deg. Celsius Orange 900 deg. Celsius Yellow 1010 deg. Celsius White 1205 deg. Celsius

The disappearing filament principle used in Optical Pyrometers

The disappearing filament principle—optical system - Eye lens Field Lens Field Stop Red Erecting Lamp Range Filter Lens Filter Exit Stop Objective Lens Entrance Stop Figure - Optical system of DFP 2000 pyrometer

The disappearing filament principle-1. An operator sights onto a hot target, adjusts the range until its image is seen in red. The lamp filament is initially cooler than the target and its image appears as a darker red or black superimposed on the target’s image. Image of Hot Target Image of Filament (Cooler) Pointer indicating the center of the filament. What the operator sees when looking into the eyepiece; the target in red, its surroundings in black (cooler) or red (hot) and superimposed on the target, the filament. The view is circular because the optical system is made up of circular lenses, apertures etc.

The disappearing filament principle-2. The lamp current is raised until the image of the filament becomes hotter than the target and it appears as brighter red than the target. Image of hot target Image of filament (Hotter) Pointer indicating the center of the filament.

The disappearing filament principle-3. The lamp current is adjusted until the lamp filament’s brightness* temperature equals that of the target. The filament’s image blends into the image of the target. The filament “disappears”. * Brightness or radiance temperature is the temperature that a blackbody would have when it looks as bright as the target. It is almost always a lower temperature than the true temperature because of the effect of the target’s emissivity. However if the target is an object in a furnace or oven of about the same temperature, the true and brightness temperatures are very close to the same value. Also, if the target is in a cooler surroundings and has a relatively high emissivity, the difference between the true and brightness temperatures may be small.