Optical- Radiation Pyrometry Flashcards
How does radiation pyrometer work?
Optical system focuses emitted radiation onto detector. Optics only transmit part of the spectrum and detector only sensitive to certain wavelengths. But system response gives measure of energy emitted and allows determination of the temperature if properly calibrated.
How does energy per second over an area from a point source vary with radius of circle it is spread over?
This is irradiance and decreases by inverse square law with radius of the circle the energy is spread over.
Why does it not matter how far the source is away from the pyrometer?
Area viewed rises with r^2 so the r^2s cancel if the object fills the field of view
Definition of emissivity
Symbol e. Ratio of energy radiated from a material’s surface to that from a blackbody at the same temperature and wavelength and under the same viewing conditions. Dimensionless number between 0 and 1.
What does emissivity depend on?
Nature of the surface and its temperature. Varies with wavelength and angle but often use average value.
Finding size of emissivity error
Tmeas^4=eTact^4
Tmeas=e^(1/4)Tact
Can find percentage error
How can emissivity vary during a process?
Example is slow oxidation of a surface
Describe two detectors
Semiconductor devices operate at characteristic wavelength ranges and can convert optical energy into electrical energy. Thermopiles consisting of thermocouple stacks for broad band measurement.
What should be considered when choosing detector?
Atmospheric transmission windows.
E.g available IR atmospheric windows defined by OH and CO2 absorptions
Why can emissivity errors vary?
Total energy varies with T^4 but different parts of spectrum can vary as higher power of T if shorter wavelength. nth root of e gets closer to 1 as n increases so emissivity error may be smaller than calculated using T^4. Works opposite for longer wavelengths.
Systems where no corrections are necessary
Viewing inside a hot furnace. Viewing bottom of long measuring tube at the furnace temperature. Measuring a surface T via a small hole in a gold coated reflecting hemispherical cup using a radiation pyrometer
Advantages of IR thermometry
Remote sensing. Wide T range of operation. Useful in hazardous environments. Device itself is at low temperature reducing ageing. Unaffected by stray electric fields and vibrations. Can use on moving objects. Blackbody sources can be used for calibration
Problems with IR thermometry
Need emissivity corrections. Affected by background reflected radiation. Affected by absorption of transmitted radiation so erroneous answer when looking through (hot) glass or absorbing gases or viewing partially opaque object. Object must fill field of view. Obscuration of field of view by dust on lens, absorbing species in air at certain λ.
What does thermal equilibrium inside a constant temperature enclosure mean?
Radiation has characteristics of a blackbody, because replacing a section of wall by a perfect blackbody changes nothing.
Describe how blackbody radiation works in a constant T enclosure
Blackbody radiation incident on inside surface of enclosure. Radiation from the surface is equal to incident BB radiation and made of reflected radiation and emitted radiation. What is being absorbed by body is same as that emitted by the surface.
