InstrumentationTemperature Measurement

Radiation Pyrometer – Temperature Measurement

What is Infrared ?

Infrared (IR)  portion  of  the  spectrum  spans  Wavelengths between  0. 7  t  1000  microns.  Based on  the principle  that  all objects  warmer than  absolute zero  (  0 ° K)  emit  energy  some where  in  that range. Infrared  thermometers  are  able  to  determine an  object’s  temperature  without  touching  it  by measuring  the  amount  of  infrared  energy being  emitted  from  the  object.  Modern  IR instruments  can  on y  measure  between  0. 7 and  20  microns  because  they  are  not  sensitive enough  to  measure  the  small  amounts  of energy  available  outside  this  range.

Radiation Pyrometer - Infrared Sensors

Basic Components

1. Lens  to  collect Energy   emitted  from  an  object.

2.Detector  to  convert  the  thermal energy to  an electrical  signal with  signal  conditioning ( filters
    and  amplifiers).
3. Emissivity  adjustment  to  correct for  various  surface finish s  and materials.
4. Temperature  TB  ( internal temperature )  must  be  known  or determined  through  prior
 5. Ambient  temperature compensation.
 6. TA ,  the  unknown  temperature ,  is determined  from  the  detector temperature.

Radiation Pyrometer – Fundamentals 

Pyrometry literally means “fire” (pyros) “measuring” (metron). Pyrometers manipulate the fact that all objects above absolute zero temperature 0 K (-273.15 °C; -459.67 °F) radiate and absorb thermal energy. If the relationship between the radiation intensity and wavelength and the temperature can be established, the temperature can be found from the radiation.

Two principal theories are employed by pyrometry: Planck’s law and the Stefan-Boltzmann law. Planck’s law is used in narrow-band pyrometers, where only one or a few specific wavelengths are targeted. The Stefan-Boltzmann law is used in broad-band pyrometers, where a wide range of wavelengths are measured.

Planck’s Law

This is a derived formula,from the german physicist Max Planck,that portrays the amount of radiation emitted by a blackbody as theoretically determined by its temperature. It is an equation that produces a curve,termed Planck’s blackbody radiation curve,which illustrate that the warmer a body is,the greater is its blackbody emission at each wavelength and the shorter is the wavelength at which emission peak.

Stefan-Boltzmann Law  

Stefan-Boltzmann law that relates the energy,E,radiated away from a perfect emitter,to the temperature,T,of that body.It has the form E,T,where E is the energy radiated per unit area per second.T is the temperature and is the Stefan Boltzmann constant. Its value is 5.6697 10 Wm K.The law was derived by the Austrian physicists Josef Stefan and Ludwig Boltzmann.

Radiation Pyrometer - Plancks Law
The radiation intensity is a function of wavelength I and temperature T.For a given temperature,there is a particular wavelength associated with the maximum radiation intensity. An approximate equation that relates them is Wein’s Displacement Law.
Weins Displacement Law
Infrared Temperature Sensors
Single-point Infrared Temperature Sensors point Infrared Temperature Sensors
Infrared thermometers measure the surface temperature of objects within their field of view.
Focal length of instrument is important consideration.
Non-contact infrared thermometers uses:
 Facility maintenance
 Utilities and electrical inspection
 Medical industry
 HVAC/R maintenance and inspection
 Food safety
 Automotive and diesel maintenance
 Asphalt, cement, and construction materials.
Material Emissivity  :
Material emissivity is subject to a great amount of uncertainty because it depends on surface finish, color, oxidation, aging and several other factors.
 Advantages of Infrared Sensors 
Infrared thermometers can measure 
Objects that move, rotate or vibrate.
They can measure temperatures > 1500 °C.
They do not damage or contaminate the surface of the
Object of interest (food, painted surfaces).
Response time is in the millisecond range.

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