Pyrometers

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Pyrometers
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Pyrometers

• To measure temperature of a very hot body
• Where thermometers cannot brought into contact
  or
• Where hot bodies are moving

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Pyrometers

• Measurements done by measuring energy radiated by
  a hot body or by comparison of colour

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• Wave length of thermal radiation-0.01 to 100 µ m

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Black body radiation

• Sefan- Boltzmanns law
• Wb s T4
• b/w two bodies
• Wb s (T14 T2 4)

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• Emissive power of blackbody is
• Wby C1 ? -5 / e (C2/ ? T)
• where ? wavelength in cm
• C1 I radiation const
• - 3.74 X10 9 erg/s/sq.cm/0.01µcm
• C2 1.438 cm K
• T- absolute temp of black body

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• As temperature of emitting source changes colour
  changes
• Ie, for lower temp the colour is dull

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• For practical radiating bodies
• We have to consider absorbtion, reflectivity,
  transmissibility, emissivity

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• Emissivity
• e 0.365v(?/?)
• ? - resitivity

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• Here detector temp gt casting temp
• Detector losses its temp. in all ways
• If detector temp. finally settles to T2 casting
  temp is at T0 and target temp is T1
• Ko (T2- T0 ) K1 T1 4 heat input
• Value of T0 - important for detectors

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• V o a (T2- T0 ) a T1 4
• V Kv T n
• n b/w 3.5 to 4.5
• Kv- Experimental value

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Comman detectors in pyrometers

• Thermopile-
• photocell
• Metal bolometer
• thermister

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Thermopile

• Larger o/p
• Lower response time
• Adaptable for industrial applications

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Bolometer

• Fast in response
• Good sensitivity
• costly

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Thermister

• Not used due to
• Poor precision
• Difficult to provide compensation
• Low response time

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Photocell

• Fast
• Large sensitivity
• But limited spectral sensitivity

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Lens for focusing targets

• Uses selective materials to prevents absorption
  by them
• Thallium iodide more suitable for high temp
  with shorter emission
• fused silica -High temp with shorter wavelength
  b/w 250 to 1900 C
• Pyrex

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Sources of errors in pyrometers

• Interleaving media has to be clear
• Carbon dioxide, water vapour etc absorb selective
  bans of wavelength
• Emittance errors
• Sensitive to obstruction

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Advantages

• Abitity to measure high temp
• No need for contact
• Fast response speed
• High o/p
• Moderate cost

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disadvantages

• Non linear scale
• Emmisivity of target affect measurement
• Errors due to interleaving media

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Application

• Can be used where life of thermocouple limits

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Optical pyrometer
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Optical pyrometer
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Optical pyrometer

• Range b/w 700 to 3000 oC
• More accurate.
• Lower limit determined by human eye
• Human eye compares the radiation

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Optical pyrometer- workingdisappearing filament
type

• Radiation received by objective lens- focus on
  filament- viewed through a microscope system-
  filament appears in background.
• red cadmium filters makes the radiation appear
  momchromatic

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working
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working

• At Bt Bf milli ammeter gives a current prop
  ional to temperature
• tungsten is used

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advantages

• Flexible
• Portable
• Convient to use
• Light weight
• Can monitor moveable objects
• Non contact type
• accurate

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disadvantages

• Expensive
• Human error
• Not useful for measuring temp of clear gas
• At high temp. filament erodes frequently

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• Can also use double control pyrometers

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Radiation pyrometer disappering filament type
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Thank u