Pyrometers
Manually operated / Automatic
Sim Soo Young
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• Simplest & oldest non-contact way of estimating the temperature of a radiating body
by observing its color
• radiation thermometer (e.g. infrared thermometer)
• radiation wavelengths: visible & infrared radiation bands
(0.4 ~ 20 μm)
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• Classification of pyrometers by wavelength and operating method
Previously on “Pyrometers” by
• Manually operated pyrometers
- operator’s eye acts as a comparator
1) Disappearing filament pyrometers
2) Two-color pyrometers (= Ratio pyrometers)
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• Automatic pyrometers
- optical system concentrating the radiation on radiation detector
- radiation detector which may be either a thermal or a photoelectric sensor
- signal converter, conditioning the detector output signal before being displayed
- measuring part, which may have an additional analogue or digital output
1) Total radiation pyrometers
2) Photoelectric pyrometers
3) Two-wavelength pyrometers
4) Multi-wavelength pyrometers
Manually Operated
Pyrometers
1. Disappearing Filament Pyrometers
• History
The first pyrometer by the potter Josiah Wedgwood (1780s)
Modern pyrometers became available when the first disappearing filament
pyrometer was built by L.Holborn and F.Kurlbaum. (1901)
1. Disappearing Filament Pyrometers
• Principle of operation
The brightness of a lamp filament is changed by adjusting the lamp current
until the filament disappears against the background of the target.
Eye of the observer is the detector. à lower limit of temperature range ≈ 700˚C
1. Disappearing Filament Pyrometers
1. Disappearing Filament Pyrometers – Red filter
• Red filter (Scholl RG2, Jena 4512)
Comparison occurs at one wavelength è The effective wavelength, λe =0.65μm
λe is nearly constant at all measured temperatures. è (1300Kà3600K, Δ: 0.003μm)
Comparison occurs at one color è subjective estimation of color cannot influence
the measurement results.
1. Disappearing Filament Pyrometers – Red filter
• Red filter
Steepness of the curve W0,λ=0.65 = f(T) is greater than that of W0 = f(T).
The spectral radiance difference at λ=0.65 is greater than that of total radiance.
L0,λ = CW0,λ (Weichert, 1976) (L: spectral radiance, C: constant, W: radiant intensity)
1. Disappearing Filament Pyrometers – Red filter
• Reasons for the application of a red filter
1) Comparison takes place only at one wavelength è eliminating the influence of
subjective color estimation by different observers
2) At λe = 0.65μm, the lowest possible temperature can be measured.
3) At λe = 0.65μm, pyrometer sensitivity is higher than for the total radiation.
4) Easy to produce good filters of λe = 0.65μm which are stable in time
5) The smallest color changes as a function of wavelength are observed.
1. Disappearing Filament Pyrometers - Indicator
• Scale defining equation for black bodies
(Weichert, 1976)
(Wien’s law)
(L: spectral radiance, W: radiant intensity, λ: wavelength, Tt: true temperature)
1. Disappearing Filament Pyrometers - Indicator
• Black body: physiological feeling of brightness through a red filter of
spectral transmissivity (τλ)
(Vλ: relative spectral sensitivity of a standard human eye)
• Filament: physiological feeling of brightness through a red filter of
spectral transmissivity (τλ)
(εf λ : spectral emissivity of the filament)
• The brightness of the filament and of the target are equal & λ=λe
1. Disappearing Filament Pyrometers - Indicator
(Tf:
filament temperature, I: lamp current)
• Direct calibration of the ammeter of the pyrometer in temperature units.
è Radiance temperature of a target at the wavelength, λe
• Scale divisions of the temperature scale is not linear.
1. Disappearing Filament Pyrometers - Indicator
• For a non-black body…. How to calibrate? (Equation)
ε
( λe:
non-black body of spectral emissivity, Ti: pyrometer readings of black body)
(black body)
C2: 1.4388 x 10-9 m•K
λe: 0.65μm
(non-black body)
1. Disappearing Filament Pyrometers - Indicator
• For a non-black body…. How to calibrate? (Diagram)
1. Disappearing Filament Pyrometers – Grey filter
• The tungsten filament can only be used up to 1400˚C.
• Dark deposit on the glass è changing the lamp characteristic
• To extend the pyrometer measurement up to 2000˚C
a grey filter is used.
• A grey filter: reduce the target radiance.
1. Disappearing Filament Pyrometers – Grey filter
• For a black body
(w/o grey filter)
(w/ grey filter, τ'λe: spectral transmissivity)
Denote:
A: Radiance reducing factor of the grey filter
With grey filter, it’s possible to calibrate the pyrometer above the maximum
filament temperature.
1. Disappearing Filament Pyrometers – Applications
• Typical applications
- Comparison measurement in calibration of total radiation pyrometers
- Temperature measurement of small size targets ( 0.1 mm)
- Temperature measurement in research laboratories
- Comparison measurement of temperature of non-black bodies
- Measurement of temperature uniformity inside furnace chambers
• The application of disappearing filament pyrometers in industry
has become less frequent.
2. Two-Color Pyrometers
• General information
= Ratio pyrometer
The ratio of spectral radiances at 2 wavelengths is estimated by the human eye
The observer adjusts the filter position so that the target appears to be grey.
Target temperature increases è the percentage of green ↑, red ↓
Measurement range: 1200 to 2000˚C
Error: ± 20 to 30 ˚C
Color
temperature
2. Two-Color Pyrometers
• Scale defining equation (For a black body and a grey body)
(W: Spectral radiant intensity,
(For grey body,
ελ1 : emissity)
ελ is constant.)
The ratio of spectral radiant intensities is a function of the temperature
Target temperature increases è the ratio of Wλ1/ Wλ2 decreases.
2. Two-Color Pyrometers
RED
GREEN
Reduction in the precision of temperature measurement
with increasing target temperature
The upper limit of the pyrometer: 2200˚C
The lower limit of the pyrometer: 700 ˚C
2. Two-Color Pyrometers
• Scale defining equation (For a non-grey body)
Non-black and non-grey bodies : selectively radiating bodies
è wavelength dependence of their spectral emissivity
è ελ1 ≠ ελ2
Manually operated two-color pyrometers are now being replaced
by automatic ones.
Automatic
Pyrometers
1. Optical Systems
• To reach a sufficiently high measurement precision…
è lenses, light-guides, or mirrors
• Lenses should be….
1)
2)
3)
4)
5)
6)
High transmission factor over a wide wavelength range
High mechanical strength
High working temperature
Good resistance to atmospheric and chemical influences
Good resistance to abrasion
Good resistance to rapid temperature variations
1. Optical Systems – lenses or mirrors
• In pyrometry, the upper cut-off wavelength of incident radiation is important.
è determines the lowest temperature which the pyrometer can measure.
• Quartz
- High mechanical and chemical resistance
- Withstand rapid temperature variations
• KRS-5
- Most commonly used material for lenses
of low temperature pyrometers
- starting from -50˚C
• Silicon
- Sometimes replaces KRS-5
- The Ardometer pyrometer(Siemens AG)
• Mirrors: the lowest measured temperatures, where no lenses may be applied .
• Metals with high reflection factor (e.g. Gold)
1. Optical Systems – light guides
• When the objects are too small
• When pyrometer would be endangered by excessive temperatures
èLight guides replace lenses.
• Absorption along the rod, imperfect reflection from the rod walls and
reflection losses è some of the transmitted energy is lost.
• the angle of incidence > the critical angle
• Materials: Artificial sapphire(Al2O3) or quartz(SiO2)
• Solid rod or flexible stranded fibreoptic cable of thin fibres
2. Radiation Detectors – thermal radiation detector
• Used in <total radiation pyrometers>
• Detector is heated by incident radiation.
• Detector should be….
1)
2)
3)
4)
5)
6)
7)
8)
High sensitivity (output signal / incident radiation power)
Time stable properties
High resistance to shocks and vibrations
Low thermal inertia
Output signal independent of the pyrometer position
High output signal-to-noise ratio
High emissivity
Sensitivity independent of wavelength
• Thermopile:
most commonly used, the reference junctions at the pyrometer housing temperature
• Thermistor and metal bolometers:
used in AC bridge circuits è easy amplification of the output signals
• Pyroelectric detectors:
low temperature radiation pyrometers, high sensitivity but complicated construction of pyrometer
2. Radiation Detectors – thermal radiation detector
Thermopile
Thermistor
Pyroelectric
detector
2. Radiation Detectors – photoelectric detector
• Used in <two/multi-wavelength pyrometers>
• Photoconductors:
incident radiation à captured incident photons à photoelectrons à current
• Photodiodes:
conductivity s proportional to the intensity of the radiation
• Photovoltaic cells:
generated voltage is a logarithmic function of the incident radiation
• Vacuum photocells:
incident radiation à emission of electrons from a metallic photocathode in a vacuum glass
• Photovoltaic cells:
generated voltage is a logarithmic function of the incident radiation
2. Radiation Detectors – photoelectric detector
Photoconductors
Vacuum photocells
Photodiodes
Photovoltaic cells
Photomultipliers
2. Radiation Detectors – photoelectric detector
• Properties of detectors
PbS, CdS : Photoconductors
Si, Ge: Photodiodes
InSb: Photovoltaic cells
3. Total Radiation Pyrometers
• The temperature of a body is determined by the thermal radiation,
which it emits over a large range of wavelengths.
• This radiation is concentrated onto a thermal radiation detector
3. Total Radiation Pyrometers
• Scale defining equation (For a black body)
(Ap: Plate area)
(K2: Heat transfer coefficient by convection and conduction)
when the plate is in the thermal steady-state…
3. Total Radiation Pyrometers
• Scale defining equation (For a non-black body)
simplify
• Equation
• Diagram
3. Total Radiation Pyrometers
• Influence of housing temperature
to make the readings independent of the housing temperature,
the difference between Tp and TH should be equal.
• Influence of target distance
the whole field of view should be filled by the target area
• Extension of measurement range
with grey filter è weakening the radiant flux coming from the object
(Tt: reading of pyrometer with grey filter,
T’t: measured temperature,
τ1: filter transmission factor)
4. Photoelectric Pyrometers
• Measurement of rapidly changing temperatures
Total radiation pyrometers: 1 ms ~ 15 ms
Photoelectric pyrometers: 1 ~ 2 μs
Photoelectric
Pyrometer
with
Direct radiant flux
Photoelectric
Pyrometer
with
Modulated radiant flux
4. Photoelectric Pyrometers
• Scale defining equation (For a black body)
the output signal of the photoelectric radiation detectors is proportional
to the number of photons (N)
(Warnke, 1972)
(in a narrow temperature range)
(IT: output current, B: constant, T: black body temperature,
n: 5~12)
4. Photoelectric Pyrometers
• Scale defining equation (For a non-black body)
(Reynolds, 1961)
Band emissivity is never precisely known.
in practical, (Worthing, 1941)
5. Two-Wavelength Pyrometers
• Automatic two-color pyrometers
• The eye of observer is replaced by a photoelectric detector.
Rotating disk
Mirror
Light guide
6. Multi-Wavelength Pyrometers
• Used to measure the temperature of non-grey bodies of low emissivity
• Good precision of the method in the temperature range up to about 1700˚C
• Splitting the incoming radiation : light guide systems, filters, and prisms
Thank you.
Any questions?