lecture 14

Pyrometer

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 radiationWaves can be characterized by

their intensities and wavelengths◦The hotter the object:

the shorter the wavelength the more emitted light

Wien's law:

cmKT 2896.0max

Planck's law

1

21)(

2

5

kT

hc

e

hcF

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 effectEmissivity of the cavity increases

and approaches unityAccording to Stefan-Boltzmann’s

law, the ideal emitter’s photon flux from area a is

In practice:

40 Ta

0 r

Cavity effectFor a single reflection, effective

emissivity is

Every reflection increases the emissivity by a factor (1-ε)

bbr )1(0

Cavity effect

Classification of Pyrometer

Disappearing Filament Pyrometers

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

Disappearing Filament Pyrometers

Disappearing Filament Pyrometers Red Filter

Comparison occurs at one wavelength .The effective wavelength, λe =0.65μm λe is nearly constant at all measured temperatures. (1300Kà3600K, Δ: .003μm)Comparison occurs at one color subjective estimation of color cannot influence the measurement results.

Reasons for the application of a red filterComparison takes place only at one

wavelength è eliminating the influence of subjective color estimation by different observers

At λe = 0.65μm, the lowest possible temperature can be measured.

At λe = 0.65μm, pyrometer sensitivity is higher than for the total radiation.

Easy to produce good filters of λe = 0.65μm which are stable in time

The smallest color changes as a function of wavelength are observed.

Disappearing Filament Pyrometers Grey Filter

Two color Pyrometer

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 the percentage of green ↑, red ↓

Automatic pyrometers

Lenses should be…. High transmission factor over a wide

wavelength rangeHigh mechanical strengthHigh working temperatureGood resistance to atmospheric and

chemical influencesGood resistance to abrasionGood resistance to rapid

temperature variation

Light Guides When the objects are too small When pyrometer would be

endangered by excessive temperatures

Light 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

DetectorHigh sensitivity (output signal / incident

radiation power)Time stable propertiesHigh resistance to shocks and vibrationsLow thermal inertiaOutput signal independent of the

pyrometer positionHigh output signal-to-noise ratioHigh emissivitySensitivity independent of wavelength

DetectorsThermopile: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

DetectorsPhotoconductors:

◦ incident radiation à captured incident photons à photoelectrons à current

Photodiodes:◦conductivity s proportional to the intensity of

the radiationPhotovoltaic 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

Sensitivity