1. Pyrometers

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

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

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

5. Black body radiation Sefan- Boltzmann’s law W b = σ T 4 b/w two bodies W b = σ (T 1 4 – T 2 4 )

6. Emissive power of blackbody is W by = C 1 λ -5 / e (C 2 / λ T) where λ – wavelength in cm C 1 – I radiation const - 3.74 X10 9 erg/s/sq.cm/0.01µcm C 2 – 1.438 cm K T- absolute temp of black body

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

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

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

13. Here detector temp > casting temp Detector losses its temp. in all ways If detector temp. finally settles to T 2 & casting temp is at T 0 and target temp is T 1 K o (T 2 - T 0 ) = K 1 T 1 4 = heat input Value of T 0 - important for detectors

14. V o α (T 2 - T 0 ) α T 1 4 V= K v T n n b/w 3.5 to 4.5 K v - Experimental value

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

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

17. Bolometer Fast in response Good sensitivity costly

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

19. Photocell Fast Large sensitivity But limited spectral sensitivity

20. 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

21. 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

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

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

24. Application Can be used where life of thermocouple limits

27. Optical pyrometer

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

30. 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

31. working

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

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

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

35. Can also use double control pyrometers

36. Radiation pyrometer disappering filament type

39. Thank u