Working of Infrared Pyrometers

Working of Infrared Pyrometers Points : Working of Infrared Pyrometers These are practical or selective radiation pyrometers. Infrared energy is invisible to the human eye but can be felt. There is a proportional increase in infrared energy as the temperature of the surface radiating body increases. Above temperatures of approximately 550°C, a surface starts to radiate visible light energy and simultaneously there is a proportional increase in the infrared energy. This proportional increase in infrared energy with surface energy makes infrared pyrometer possible by combining a suitable detector, electronic circuitry, and means of indication and or control. The infrared spectrum ranges from 0.22 µm to 17µm and commonly used portion is 2 to 7µm. Some manufacturers use only 5 to 7µm portion of the range for the purposes of the pyrometery.

Infrared principles using vacuum thermocouple, thermopiler and bolometers have been successfully employed in infrared spectrometers as well as in total radiation pyrometers. Various types of photo-electric transducers are most commonly used for infrared transducers. The most photo-voltaic cell. The photo-voltaic cells have high speed of response.

The infrared radiation is focused on to a photo-voltaic cell and therefore, it is necessary insure that the cell does not become overheated. The cone of radiation passing ultimately to the photocell 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 in the range of 1000°C to 1200°C in order to reduce the infrared radiation passed to the photocell. This helps in preventing the photocell from getting overheated.

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 temperature, as in case of a thermistor, or generates an emf in the detector, such as a thermopile. This change in resistance or generated emf is then indicated on a mete or a digital display or is use to operate a controller for the process.