Pyrometers

Pyrometers are also popularly known as Radiation thermometers. Pyro and thermo are basically words of Greek origin where the former means fire and the latter signifies hot. Hence, Pyrometers or Radiation thermometers are referred to as the devices employed to measure thermal radiations and surface temperatures.

 

Working

A pyrometer is typically made up of an optical system and a detector. The optical system (a lens) is used to focus the infrared (IR) energy naturally emitted by an object onto a sensor or detector. This sensor is responsive to the infrared radiation and hence transforms IR energy into electrical energy. The electrical output signal of the detector can be displayed in terms of temperature. To do this, standard ambient temperature need to be considered. Emittivity of an object plays a significant role in conversion of the electrical signal into a correct temperature signal.
Since these devices can capture the invisible infrared radiation from all objects, they are also called Infrared radiation thermometers. “Infrared radiation is part of the electromagnetic spectrum which includes radio waves, microwaves, visible light, ultraviolet, gamma, and X-rays”.1For precise measurement of temperature, it should be ensured that the thermometer doesn’t make any physical contact with the surface whose temperature is to be measured.

 

Main Features

Following are the major characteristics of infrared radiation thermometers:

  • Infrared radiation thermometers or Pyrometers exclusively measures IR energy being radiated from an object in the range of 0.7 to 20 micron wavelength.
  • Unlike other temperature sensors, no physical contact is required between the IR radiation thermometer and the surface of the object. They are specifically made to measure radiation from a distance.
  • IR radiation thermometers are usually designed into the shape of a pistol as shown in the figure below. They can also be provided with a laser pointer so that precise areas could be identified.

IR radiation thermometer

Selection Criteria

Following factors must be considered while selecting an infrared radiation thermometer for a particular application:

  • Field of view considering target size and distance
  • Surface conditions from emissivity point of view
  • Spectral response taking into account atmospheric effects and transmission via surfaces
  • Temperature range
  • Mounting Considerations i.e. handheld portable or fixed mount
  • Time of response
  • Environmental conditions
  • Viewing port or Window applications
  • Required signal processing

 

Field of View

Field of view of an IR radiation thermometer is defined as the angle of vision at which the device will work. It is usually established by the optics used in the system. An infrared radiation thermometer can detect the surface temperature of all objects which fall within its field of view. Hence, an absolutely correct temperature measurement could be accomplished only if the object under measurement entirely fills the instrument’s field of view. If the background temperature remains different from the object temperature, a measurement error will take place.

 

Applications

Following are the major application areas of an infrared radiation thermometer:

  • Areas where conventional temperature sensors like thermocouples or RTDs cannot be employed or their use doesn’t result in accurate measurements due to some problems.
  • Applications where the target under consideration is moving. For example in rollers, moving machinery, or a conveyor belt. They are frequently employed for monitoring products on a movable production line where temperature measurement is required.
  • Areas where the object is enclosed by an Electro magnetic field such as in Induction heating.
  • IR thermometers are also apt for areas where the object is restricted in a vacuum or other controlled atmosphere.
  • Applications needing quick and fast response also employ IR radiation thermometers.
  • IR radiation thermometers are basically non-contacting temperature measurement devices; hence they are frequently employed in areas where non-contact measurements are desired. For example, in contaminated or hazardous areas involving high voltages.
  • They are also found suitable for applications where large distances and high temperatures are involved
  • They are also employed in the calibration of many heating devices used for cooking purposes. For instance, furnaces, ovens etc.
  • Since they are capable enough to measure temperature from a distance, they are particularly employed in areas where direct temperature measurement is complicated. For example, “in large electrical components and arrays and the inside of car engines where parts are blocked from contact by other mechanical and hydraulic devices”.1
  • They can also be used for weather forecasting and research and can determine the temperature of clouds at high altitudes.
  • IR thermometers are employed in variety of manufacturing processes such as metals, glass, cement, ceramics, semiconductors, plastics, paper, textiles, coatings, etc.

 

Benefits

Following are the major benefits offered by Pyrometers:

  • Light in weight
  • Compact in size
  • Simple and convenient to use
  • Proficiency to measure hot, hazardous and contaminated surfaces without causing any harm to the object.
  • Fast response: They can give several readings per second in contrast to conventional sensors which takes several minutes to give readings.

 

Limitations

  • Higher cost as compared to conventional thermocouples or resistance temperature detectors.
  • Regular maintenance is required to keep the optical system clean.
  • Advanced radiation thermometers involve extra complicated design and optics.
  • Unlike thermocouples and RTDs, no calibration standards and curves are available for radiation thermometers.

 

References

  1. Allqa
  2. IRthermometers.applications

 

Sources

Omega.literature

Omega.prodinfo

Temperatures