Temperature measurements

1. Temperature measurements Maija Ojanen Licenciate course in measurement science and technology 8.3.2006

2. Outline 1. Liquid-in-glass thermometres 2. Bimaterial thermometres 3. Electrical thermometres 4. IR-thermometres 5. Pyrometres 6. Summary 7. Other measurement methods

3. Liquid-in-glass thermometres

4. Liquid-in-glass thermometres • The “traditional” thermometres • Measurement scale from -190 °C to +600 °C • Used mainly in calibration • Mercury: -39 °C … +357 °C • Spirit: -14 °C … +78 °C

5. Functionning method • Method is based on the expansion of a liquid with temperature • The liquid in the bulb is forced up the capillary stem • Thermal expansion:

6. Structure

7. Temperature differences in the liquid Glass temperature also affects The amount of immersion (vs. calibration) Causes of inaccuraties

8. Bimaterial thermometres • Method based on different thermal expansions of different metals • Other metal expands more than other: twisting • Inaccurary ± 1 ° C • Industry, sauna thermometres

9. Bimaterial thermometres

10. Electrical thermometres

11. Electrical thermometres • Resistive thermometres • Resistivity is temperature dependent • Materials: Platinum, nickel

12. Characteristic resistances

13. Thermistor thermometres • Semiconductor materials • Based on the temperature dependence of resistance • Thermal coefficient non-linear, 10 times bigger than for metal resistor • NTC, (PTC): temperature coefficient’s sign

14. Example of a characteristic curve

15. Limitations of electrical thermometres • Sensor cable’s resistance and its temperature dependency • Junction resistances • Thermal voltages • Thermal noise in resistors • Measurement current • Non-linear temperature dependencies • Electrical perturbations • Inaccuracy at least ± 0.1 °C

16. Infrared thermometres

17. Thermal radiation • Every atom and molecule exists in perpetual motion • A moving charge is associated with an electric field and thus becomes a radiator • This radiation can be used to determine object's temperature

18. Thermal radiation • Waves can be characterized by their intensities and wavelengths • The hotter the object: • the shorter the wavelength • the more emitted light • Wien's law:

19. Planck's law Magnitude of radiation at particular wavelength (λ) and particular temperature (T). h is Planck’s constant and c speed of light.

20. Blackbody • An ideal emitter of electromagnetic radiation • opaque • non-reflective • for practical blackbodies ε = 0.9 • Cavity effect • em-radiation measured from a cavity of an object

21. Cavity effect • Emissivity of the cavity increases and approaches unity • According to Stefan-Boltzmann’s law, the ideal emitter’s photon flux from area a is • In practice:

22. Cavity effect • For a single reflection, effective emissivity is • Every reflection increases the emyssivity by a factor (1-ε)

23. Cavity effect

24. Practical blackbodies • Copper most common material • The shape of the cavity defines the number of reflections • Emissivity can be increased

25. Detectors • Quantum detectors • interaction of individual photons and crystalline lattice • photon striking the surface can result to the generation of free electron • free electron is pushed from valency to conduction band

26. Detectors • hole in a valence band serves as a current carrier • Reduction of resistance Photon’s energy

27. Detectors • Thermal detectors • Response to heat resulting from absorption of the sensing surface • The radiation to opposite direction (from cold detector to measured object) must be taken into account

28. Thermal radiation from detector

29. Pyrometres • Disappearing filament pyrometer • Radiation from and object in known temperature is balanced against an unknown target • The image of the known object (=filament) is superimposed on the image of target

30. Pyrometres • The measurer adjusts the current of the filament to make it glow and then disappear • Disappearing means the filament and object having the same temperature

31. Disapperaring filament pyrometer

32. Pyrometres • Two-color pyrometer • Since emissivities are not usually known, the measurement with disappearing filament pyrometer becomes impractical • In two-color pyrometers, radiation is detected at two separate wavelengths, for which the emissivity is approximately equal

33. Two-colour pyromerer

34. Pyrometers • The corresponding optical transmission coefficients are γx and γy Displayed temperature

35. Measurements • Stefan-Boltzmann’s law with manipulation: • Magnitude of thermal radiation flux, sensor surface’s temperature and emissivity must be known before calculation • Other variables can be considered as constants in calibration

36. Error sources • Errors in detection of the radiant flux or reference temperature • Spurious heat sources • Heat directly of by reflaction into the optical system • Reflectance of the object (e.g. 0.1) But does not require contact to surface measured!

37. Pyroelectric thermometres • Generate electric charce in response to heat flux • Crystal materials • Comparable to piezoelectric effect: the polarity of crystals is re-oriented

38. Summary • Only some temperature measurement methods presented • Examples of phenomenons used: thermal expansion, resistance’s thermal dependency, radiation • The type of meter depends on • Measurement object’s properties • Temperature

39. More temperature measurement possibilities • Thermocouples • Semiconductor thermometres • Temperature indicators • Crayons etc. • Manometric (gas pressure) sensors

40. Questions?