1. Pyrometers

2. A pyrometer is a type of remote-sensing
thermometer used to measure the
temperature of a surface it is a device
that from a distance determines the
temperature of a surface from the amount
of the thermal radiation it emits, a
process known as pyrometer and
sometimes radiometry.

3. Pyrometer are two types
Pyrometer are two types
Optical Pyrometer
Radiation pyrometer

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

5. Pyrometers
Measurements done by measuring
energy radiated by a hot body or by
comparison of colour

6. Wave length of thermal radiation-0.01
to 100 µ m

7. Black body radiation
Sefan- Boltzmann’s law
Wb = σ T4
b/w two bodies
Wb = σ (T1
4
– T2
4
)

8. Emissive power of blackbody is
Wby = C1 λ -5
/ e (C
2
/ λ T)
where λ – 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

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

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

12. Emissivity
ε = 0.365√(ρ/λ)
ρ - resitivity

13. They are non-contact devices, used to measure
temperature above 1500 degree Celsius, contact
devices may melt at this temperature The heat energy
emitted from the hot body is allowed passed on to the
lens. Which collects it and focused on to the detector
with the mirror and eyepiece arrangement.
Now the detector converts the radiation energy into an
electrical signal. Thermopiles and photon multipliers
are commonly used detectors. Detectors produce the
reading and shows in the temperature display.

16. Here detector temp > 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

17. V o α (T2- T0 ) α T1
4
V= Kv T n
n b/w 3.5 to 4.5
Kv- Experimental value

18. Comman detectors in pyrometers
Thermopile-
photocell
Metal bolometer
thermister

19. Thermopile
Larger o/p
Lower response time
Adaptable for industrial applications

20. Bolometer
Fast in response
Good sensitivity
costly

21. Thermister
Not used due to
Poor precision
Difficult to provide compensation
Low response time

22. Photocell
Fast
Large sensitivity
But limited spectral sensitivity

23. Lens – for focusing targets
Uses selective materials to prevents
absorption by them
1. Thallium iodide – more suitable for
high temp with shorter emission
2. fused silica -High temp with shorter
wavelength – b/w 250 to 1900 C
3. Pyrex

24. 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

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

26. disadvantages
Non linear scale
Emmisivity of target affect
measurement
Errors due to interleaving media

27. Application
Can be used where life of
thermocouple limits

30. Optical Pyrometer.
Definition: The optical pyrometer is a
non-contact type temperature measuring
device. It works on the principle of
matching the brightness of an object to
the brightness of the filament which is
placed inside the pyrometer.

32. Optical pyrometer

33. Optical pyrometer

34. Optical pyrometer
Range b/w 700 to 3000 o
C
More accurate.
Lower limit determined by human eye
Human eye compares the radiation

35. Optical pyrometer- working
disappearing 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

36. working

37. working
At Bt = Bf – milli ammeter gives a
current prop ional to temperature
tungsten is used

38. advantages
Flexible
Portable
Convient to use
Light weight
Can monitor moveable objects
Non contact type
accurate

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

40. Can also use double control
pyrometers

41. Radiation pyrometer disappering
filament type

44. Thank u