Pyrometers

1. Pyrometers

2. Pyrometers • 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 • Wb = σ T4 • b/w two bodies • Wb = σ (T14 – T24)

6. Emissive power of blackbody is • Wby = C1λ-5 / e (C2/ 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

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 T2 & casting temp is at T0 and target temp is T1 Ko (T2- T0 ) = K1 T14 = heat input Value of T0 - important for detectors

14. V oα(T2- T0 ) αT14 V= Kv T n n b/w 3.5 to 4.5 Kv- 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 • Thallium iodide – more suitable for high temp with shorter emission • fused silica -High temp with shorter wavelength – b/w 250 to 1900 C • 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

28. Optical pyrometer

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

30. 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

31. working

32. working • 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