Chapter 6. Fiber Optic Thermometer Ho Suk Ryou
Properties of Optical Fiber
Optical Fiber Composed of rod core surrounded by sheath Core: conducts electromagnetic wave Sheath: contains wave within the core
Refractive index How radiation propagates through the medium n 1 : Refractive index of the core material n 2 : Refractive index of the sheath material In optical fiber n 1 > n 2
Critical Angle
Critical Angle and Angle Cone
Optical Fiber materials Core material has to have low absorption coefficient at the transmitted wavelength. Wavelength range: from ultraviolet through visible up to infrared radiation
Optical Fiber material Most common: both core and sheath made of quartz glass At high temperature: Core materials made of quartz and sapphire
Classification of Optical Fiber General classification ( 분류 ) of optical fiber sensors is established by McGhee and Henderson (1989) General classification of optical fiber thermometer is established by Grattan (1995) and Wickersheim (1992)
Classification of Optical Fiber
Classification of Optical Fiber Extrinsic sensing Indirect use thermometer The light itself is modulated by some process outside optical transmission system
Classification of Optical Fiber Intrinsic sensing Direct use thermometer Optical fiber is used as sensor Light transmitting property of optical fiber is modulated by temperature
Extrinsic Sensing Thermometers GaAs sensor Thermochromic Fluorescent Black body sensor Fabry-Perot sensor Optical coupling thermometers
Thermometers with GaAs semiconductor sensors Based on dependence on absorption coefficient of semiconductor material upon its temperature and upon the frequency of incident radiation
Thermometers with GaAs semiconductor sensors
Thermometers with GaAs semiconductor sensors Resulting output signal depends upon the thermally dependent optical transmission of GaAs prism Measure the temperature below 50ºC
Thermochromic Thermometers Temperature dependence of reflection factor of liquid crystals Reflection factor depends on frequency of incident visible radiation Thermochromic crystal: change color with temperature
Thermochromic Thermometers Thermochromic Thermometers Block diagram Relative Absorption vs. Wavelength
Fluorescent Thermometers Uses fluorescent material ( 형광물질 ) excited by radiation of given wavelength H T Lam, Y Kostov, L Tolosa, S Falk and G Rao A high-resolution non-contact fluorescence-based temperature sensor for neonatal care Measurement Science and Technology 2012; 23(3)
Fluorescent Thermometers First generation Based on temperature dependence of monochromatic repartition of emitted radiation
Fluorescent Thermometers
Fluorescent Thermometers Second generation Based on decay-time concept of periodically excited probe
Fluorescent Thermometers
Fluorescent Thermometers The most popular fiber optic thermometers Applications: In medicine and biology (Chapter 21) In high voltage appliances Of rotating bodies (Chapter 9) Of microwave and dielectric heated bodies In chemical and physical research
Black Body Sensor Based on temperature dependence of spectral radiant intensity emitted by black body Platinum, rhodium or iridium are used for black body sensor cavity Temperature from 300 ~ 1900 ºC
Black Body Sensor
Black Body Sensor
Thermometers with Fabry-Perot sensors Use temperature dependent spectral reflection coefficient of thin mono-crystalline Si film Used in microwave drying and food processing Temperature range from 0 to 400 ºC
Thermometers with Fabry-Perot sensors
Optical coupling thermometers Based on extrinsic optical coupling of two light guides Optical coupling: interconnecting two devices to transfer optical signal using light waves
Optical coupling thermometers When liquid refractive index, n 2, is smaller than core, n 1, no coupling As temperature changes, when n 2 >n 1, initiate optical coupling which allows transmission of signal from light source to detector
Intrinsic Sensing Thermometers Raman scattering Change of refractive index
Raman Scattering Thermometers Raman scattering: inelastic scattering of photon Inelastic: scattered photons with different kinetic energy and wavelength Stokes : emitted energy is lower than absorbed (Long wavelength) Anti-stokes : emitted energy is higher than absorbed (Short wavelength) Rayleigh Scattering: elastic scattering of photon
Raman Scattering Thermometers Used in large surfaces and along long objects like pipe-lines Temperature up to 500ºC
Raman Scattering Thermometers
Refractive Index Thermometers Based on temperature dependence of refractive index of optical fibers Measure the average temperature along uniformly wound fiber or temperature distribution Temperature below 25ºC
Refractive Index Thermometers
Chapter 7 Quartz, Ultrasonic and Noise Thermometers and Distributed Parameter Sensors
Quartz Thermometers Convert temperature value directly to frequency output signal from quartz crystal oscillator Normally used in measuring temperature of sea and rivers, longlasting measurement
Ultrasonic Thermometers Based on the effect of temperature of medium upon velocity of some waves in the medium Use time lag (time difference) between two signals from transmitting transducer and receiving transducer to determine the temperature Application For temperatures over 2000 K In nuclear reactors In tanks or solid bodies
Ultrasonic Thermometers
Ultrasonic Thermometers Gas Measure the temperature of ionized gas and plasma (300~17000 K) Because of turbulence, the correction( 보정 ) has to be made Between 1600 and 3000, there is non-linear temperature dependence and to avoid this, replace the sensors of thorium doped polycrystalline tungsten by pure tungsten
Ultrasonic Thermometers Liquid
Ultrasonic Thermometers Solid Incent
Noise Thermometers Use sensing resistor whose output signal can be modeled as either a thermal noise voltage or current Widely used in meteorology or in industrial applications. Especially in extremely difficult conditions such as nuclear reactors
Noise Thermometers Nyquist-Johnson equation The voltage, however, is quite small Have to use amplifier
Noise Thermometers Look at Page 145
Noise Thermometers Substitution Method
Noise Thermometers Correlation method
Distributed Parameter Sensors Semiconductor sensor: Used for monitoring temperatures of long elements on larger surfaces The resistance between tube and central conductor decreases with increasing temperature
Distributed Parameter Sensors Semiconductor sensor: Due to non-linear relationship between resistance and temperature, overheating of even a small part results in a marked decrease of the resistance of the section which reveals a hot spot. For correct operation, the difference between normal working condition and alarm level has to be between 50 and 200 C
Distributed Parameter Sensors Continuous thermocouple sensor Composed of two thermocouple conductors embedded in special ceramic insulation of NTC characteristic and whole assembly is placed in a stainless steel protective sheath Heating the thermocouple at any given point results in decreases of the insulation resistance between the two conductors, and the thermoelectric force from this hot spot gives actual temperature. The accuracy of ±1 within the range of -29 to 900
Distributed Parameter Sensors Continuous thermocouple sensor Mostly used to prevent any abnormal operation of industrial equipment such as: Temperature changes above normal ambient Temperature exceeding pre-set absolute values Abnormal rate of temperature increase Also used for controlling the temperature in computer cabinet, power stations, warehouses and for activation of fire alarms Big advantage is that they do not need power supply
Chapter 8 Pyrometers Classification and Radiation Laws
Early Pyrometers The simplest and oldest non-contact ( 비접촉 ) way of estimating the temperature of a radiating body ( 복사체 ) is by observing its color.
Pyrometers Non-contact thermometers Measure the temperature of a body based upon its emitted thermal radiation
Pyrometers The most important radiation wavelengths is from 0.4 to 20 µm which belong to visible and infrared radiation ( 적외선 ) bands Possible to classify pyrometers according to their spectral response and operating method
Classification of Pyrometers
Manually Operated Human operator is the major part Human eye acts as a comparator 1. Compare between radiation from the source with a signal from reference unit 2. The operator activates the read-out instrument
Types of Manually Operated Disappearing filament pyrometers Based upon matching the luminance of the object and of the filament Two-color pyrometers (ratio pyrometers) Estimate the temperature from the ratio of the radiation intensity emitted by the object in two different spectral wavebands ( ex. 0.55 and 0.65 µm)
Types of Manually Operated
Automatic Pyrometers Have radiation detector and signal converter to automatically activate the measuring instrument
Types of Automatic Pyrometers Total radiation pyrometers Use thermal radiation detectors which are heated by incent radiation Photoelectric pyrometers Operate in chosen wavelength bands where the signal is generated by photons bombarding a photoelectric detector
Types of Automatic Pyrometers Two-wavelength pyrometers The emitted radiation intensity in two wavelength bands is compared by photoelectric detectors Multi-wavelength pyrometers The source radiation (concentrated in some wavelength bands) on photoelectric detectors Used for measuring the temperature of bodies with low emissivity
Absorption, Reflection and Transmission of Radiation Thermal radiation is a part of electro magnetic radiation By applying the principle of energy conservation shows that for every solid: α+ρ+ =1
Absorption, Reflection and Transmission of Radiation
Three specific cases α=1, ρ=0, =0 : Black body Absorbs all incident radiation α=0, ρ=1, =0 : White body Reflects all incident radiation α=0, ρ=0, =1 : Transparent body All of the incident radiation is completely transmitted
Radiation Laws The radiant intensity, W Heat flux per unit area
Radiation Laws Heat flux density Spectral radiant density
Radiation Laws Planck s law gives the radiant flux distribution of a black body as a function of the wavelength and of the body s temperature
Radiation Laws For a given wavelength λ 1, λ 2 If λt << c 2, Planck s equation can be replaced by simpler Wien s law
Radiation Laws The errors, which result from replacing Planck s law by Wien s law, are negligibly small
Radiation Laws
Radiation Laws Total spectral radiant intensity by black body Spectral emissivity : ratio of radiant intensity of non-black body to that of black body at the same temperature
Radiant Laws Total emissivity W : radiant intensity of any given body W o : radiant intensity of black body
Radiation Laws From Kirchhoff s law, spectral absorptivity, α λ, is equal to emissivity, ε λ for opaque bodies For a given wavelength λ 1, λ 2
Radiation Laws The Stefan-Boltzmann law The dependence of the total radiant intensity, W o, of a black body upon the temperature, T
Radiation Laws Simpler form
Radiation Laws For grey bodies
Total emissivity and spectral emissivity Knowledge of values of ε and ε λ at λ=0.65μm for different materials are used for reference values when measuring the temperature The emissivity of different materials depend heavily upon the surface state such as its homogeneity and temperature
Total emissivity and spectral emissivity
Total emissivity and spectral emissivity For specific spectral emissivity For the emissivity of non-conductors
Total emissivity and spectral emissivity Spectral emissivities of metals become lower at lower temperatures where the wavelengths are longer Non-metallic substances do not vary greatly with temperatures The appearance of non-metals in visible light cannot be the basis for predicting emissivities
Radiant heat exchange The heat exchange between two parallel surfaces
Radiant heat exchange 2 1 If one bodies of area A 1 is placed inside another one of area A 2 and with A 1 < A 2 When A 2 > 3A 1
Radiant heat exchange Lambert s directional law The radiant intensity of black body as a function of the radiation direction
Radiant heat exchange Radiant intensity, W o, is π times smaller than total radiant intensity If it is polished material, which φ is greater than π/4, there is large derivations from Lambert s law due to its dependence of emissivity on the observation angle
Radiant heat exchange Also applied to optical pyrometry Luminosity, I φ : radiant flux Density of luminosity
Radiant heat exchange Lambert s law for luminosity Combined form
Thank you