.

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:
)
1
(
0 T
V
V 



6. Structure

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

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
)
1
(
)
( 0 T
R
T
R 



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:
cmK
T 2896
.
0
max 


19. Planck's law
1
2
1
)
(
2
5


kT
hc
e
hc
F



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:
4
0 T
a


0


 
r

22. Cavity effect
 For a single reflection, effective emissivity
is
 Every reflection increases the emyssivity by
a factor (1-ε)
b
b
r 

 )
1
(
0






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

h
E 

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
1
5
5
ln
1
1




















y
x
x
y
x
y
c C
T







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
4
4
s
c
A
T
T





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?