Transcript Temperature measurements

Temperature
measurements
Maija Ojanen
Licenciate course in
measurement science and technology
8.3.2006
Outline
1. Liquid-in-glass thermometres
2. Bimaterial thermometres
3. Electrical thermometres
4. IR-thermometres
5. Pyrometres
6. Summary
7. Other measurement methods
Liquid-in-glass thermometres
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
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:
V  V0 (1  T )
Structure
Causes of inaccuraties
Temperature
differences in the liquid
 Glass temperature also
affects
 The amount of
immersion (vs.
calibration)

Bimaterial thermometres

Method based on different thermal
expansions of different metals
– Other metal expands more than other:
twisting
– Inaccurary ± 1 ° C
– Industry, sauna thermometres
Bimaterial thermometres
Electrical thermometres
Electrical thermometres

Resistive thermometres
– Resistivity is temperature dependent
R(T )  R0 (1  T )
– Materials: Platinum, nickel
Characteristic resistances
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

Example of a characteristic curve
Limitations of electrical
thermometres

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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
Infrared thermometres
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

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:
maxT  0.2896cmK
Planck's law
F ( ) 
2
1 2hc

5
e
hc
kT
1
Magnitude of radiation at particular
wavelength (λ) and particular temperature
(T).
h is Planck’s constant and c speed of light.
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
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
 0  aT
 In
practice:
 r   0
4
Cavity effect
 For
is
a single reflection, effective emissivity

r 
 (1   b ) b
0
 Every
reflection increases the emyssivity by
a factor (1-ε)
Cavity effect
Practical blackbodies
 Copper
most common material
 The shape of the cavity defines the
number of reflections
– Emissivity can be increased
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
Detectors
– hole in a valence band serves as a current
carrier
– Reduction of resistance
Photon’s energy
E  h
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
Thermal radiation from detector
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
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
Disapperaring filament pyrometer
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
Two-colour pyromerer
Pyrometers
– The corresponding optical transmission
coefficients are γx and γy
Displayed temperature
 1

 y
1
Tc  C    ln 
5





x 
x y
 y
5
x




1
Measurements
– Stefan-Boltzmann’s law with manipulation:
Tc  4 T 4 

A
s
– 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
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!
Pyroelectric thermometres

Generate electric charce in response to
heat flux
– Crystal materials
– Comparable to piezoelectric effect: the
polarity of crystals is re-oriented
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
More temperature measurement
possibilities
Thermocouples
 Semiconductor thermometres
 Temperature indicators

– Crayons etc.

Manometric (gas pressure) sensors
Questions?