Pyrometer Working Principle
When temperature being measured is very high and physical contact with the medium to be measured is impossible or impractical, optical pyrometer based on the principle of thermal radiation are used. These Types of Pyrometer Working Principle are used under condition where corrosive vapours or liquids would destroy thermocouples, resistance thermometer and thermister, if made to come in contact with the measured medium.
Radiation Pyrometer Working Principle measures the radiant (energy) heat emitted or reflected by a hot object. Thermal radiation is an electro magnetic radiation emitted as a result of temperature and lies in the wavelength of 0.1 —100 μm.
According to the principle of thermal radiation, the energy radiated from a hot body is a function of its temperature. Referring digram the heat radiated by the hot body is focused on a radiation detector. The radiation detector is blackened and it absorbs all or almost all radiation Therefore, the heat received by the detector is proportional to the fourth power of the absolute temperature of the hot body.falling on it (if the temperature is very small compared with that of hot body, then
Radiation Pyrometer are of two types.
- Total Radiation Pyrometers
- Infrared Pyrometers
Infrared Pyrometer Working Principle are partial or selective radiation pyrometers. Above temperatures of 550 °C, a surface starts to radiate visible light energy and simultaneously there is a proportional increase in the infrared energy.
Infrared principles using thermocouples, thermopile and bolometers are used. Also various types of photo-electric transducers are most commonly used for infrared transducers. The most useful transducers used for industrial application are the Photo-voltaic cells. These cells used in radiation pyrometers, respond to wavelength in infrared region and may be used to measure temperature down to 400 °C.
The infrared radiation is focused on a photo-voltaic cell as shown in picture It is necessary to ensure that the cell does not become overheated. The core of radiation passing to the cell is defined by the area of the first diaphragm.
The protective window is made of thin glass and serves to protect the cell and filter from physical damage. The filter is used on the range of 1000 °C to 1200 °C in order to reduce the infrared radiation passed to the photo cell. This help in preventing the photo cell from being overheated.
All infrared systems depend on the transmission of the infrared radiant energy being emitted by a heated body to a detector in the measuring system. The sensor head is focused on the object whose temperature is being measured and/or controlled.
The infrared energy falling on the detector either changes the detector resistance in proportion to the temperature as in the case of thermister or generates an emf in the detector such as a thermopile. The change in resistance or generated emf is then indicated on a meter.
Applications are not limited to non contact high-temperature processes, such as hot metal working, plastic film processing, and glass manufacturing. Infrared is also used in low-temperature applications. These include infrared photographs of accident victims to locate internal bleeding, aerial photographs to detect pollution of water bodies, the detection of the need for crop irrigation, and many others.
− 50 ° F to 6500 ° F ( − 46 to 3600 ° C) for Spot instruments.
Distance and Target Size:
Maximum distance (D) to target is about 100 ft (30 m); minimum target diameter(d) is about 0.05 in (1.5 mm)
The total range of costs is from $100 to $5,500