Temperature Measurement Techniques

• Module 4 discusses temperature measurement techniques, infrared thermography, thermo-reflectance thermography, interferometry and electronic speckle pattern interferometry.

Temperature Measurement Importance

• Temperature is a critical parameter in many processes requiring monitoring or control.
• These processes range from simple applications like monitoring water temperature to complex ones like laser welding.
• Temperature measurements are also vital for monitoring exhaust gases and fluids in various industrial and support applications.
• Numerous temperature measurement probes are available, each varying in accuracy and suitability for specific applications.

Temperature Measurement Techniques: Thermocouples

• The Seebeck effect refers to a voltage difference produced by a temperature difference between two dissimilar electrical conductors or semiconductors.
• Heating one of the conductors or semiconductors causes heated electrons to flow to the cooler one.
• The generated voltage is proportional to the temperature difference between the "hot" and "cold" junctions.
• Temperature monitoring can indicate material condition and process quality, potentially leading to energy conservation.
• The voltage difference is directly proportional to the temperature difference between the two junctions (Th, Tc).

Types and Selection of Thermocouples

• Type K (Chromel-Alumel), Type J (Iron-Constantan), and Type T (Copper-Constantan) thermocouples utilize different metal combinations for specific temperature ranges.
• Sensor selection should consider sensor characteristics, costs, and available instrumentation.
• Thermocouples are inexpensive, rugged, and can measure temperatures over wide ranges, but they are a contact type.
• Infrared sensors measure temperatures higher than other devices without direct contact.
• Infrared sensors are less accurate and sensitive to surface emissivity.

Emissivity

• Emissivity refers to a material's ability to emit infrared energy.
• It is an optical property of matter.
• Each material possesses a unique emissivity that varies with temperature and infrared wavelength.
• Emissivity varies across the electromagnetic spectrum.
• Emissivity values range from 0 to 1, representing a perfect mirror surface (0) and a blackbody (1).
• A material with an emissivity of 0.95 absorbs and emits 95% of infrared radiation while reflecting 5%.
• Factors influencing emissivity include material type, surface condition, temperature, and radiation wavelength.
• Metals generally exhibit lower emissivities than nonmetals, while polished surfaces have lower emissivities than rough or oxidized surfaces.
• Emissivity knowledge is vital for accurate non-contact temperature measurement and heat transfer calculations.
• Radiation thermometers detect thermal radiation emitted by a surface.
• Reflectance is the amount of light reflected from a material's surface.
• Transmittance is a material's capacity to transmit thermal energy to an infrared camera.
• Estimating or determining emissivity is necessary for temperature measurement with an infrared imager.

Thermal Imaging Techniques

• Infrared Thermography (IRT) provides temperature profiling of a surface or point.
• Every object emits infrared energy and the intensity of this radiation depends on temperature.
• Infrared waves are invisible, with wavelengths longer than visible light, specifically between 0.8 and 1000 microns.

Infrared Thermography (IRT)

• IRT acquires and processes thermal information using non-contact measurement devices.
• Objects above absolute zero (0K) emit infrared radiation, which infrared devices measure, since it is invisible to the human eye.
• Visible images represent reflected light, infrared images represent the source and can be observed with an infrared camera, even without light.
• Infrared cameras convert acquired images into visible ones by assigning colors to infrared energy levels, creating a false-color image called a thermogram.
• Spot infrared thermometers measure temperature at a single spot.
• Infrared scanning systems scan broader areas using a rotating mirror and are used in industrial conveyor systems.
• Infrared thermal imaging cameras measure temperatures across many points to create a thermographic image or thermogram.

Applications & Theory Behind IR Thermography

• Thermography, based on black body radiation, allows observation with or without light because infrared radiation is emitted by all objects above absolute zero.
• Increased temperature raises radiation emissions, allowing thermography to show temperature variations.
• Thermal cameras make warm objects easily visible against cooler backgrounds, useful for military and surveillance.
• Thermography monitors physiological changes for clinical and veterinary use, such as swine flu detection during the 2009 pandemic.
• The intensity of infrared radiation is mainly a function of temperature.
• Infrared thermography is used in medical and mechanical applications, for detecting heat loss and electrical issues.
• With accuracy, non-contact nature, and two-dimensional characters, IR thermography is becoming more popular.
• Infrared emissivity varies with surface coating, roughness, and other surface parameters.

Practical Applications of Thermography

• In chemical industries, thermography has many uses.
• High-speed thermography can be utilized in additive manufacturing.
• IR can detect electrical connections that are bad in a mechanical system because hot connections cause an increase in electrical resistance.
• Semiconductors generate heat during operation that flows through the component. Temperatures at junctions can be measured to evaluate heat dissipation.

Corrosion and Defect Detection via IR

• IR detects material thinning in thin structures by identifying different thermal masses that absorb/radiate heat at varying rates.
• Monitoring the surface after heating allows thickness map creation, detecting normally invisible corrosion on structure back sides.
• IR reduces costs by detecting internal aircraft damage without disassembly.
• IR detects flaws by monitoring heat flow from a solid's surface, revealing voids or inclusions.
• Sound material conducts heat well, but defects retain heat longer.

IR Camera Uses

• The IR cameras can show cracks by identifying friction and ultrasonic vibrations.
• Infrared thermography can be used for inspections in the aerospace industry on aircrafts.
• Unexpected shutdowns from equipment and applications for testing can be avoided by infrared testing.
• IR identifies loose connections, failing transformers and lubrication issues, detecting problems before failures occur due to temperature increases above 50°C.
• Thermal pointers/scanners are typical thermography instrument.
• Thermal pointers read temperature at a dimensional point, and scanners map thermal profile of an area.
• Basic IR systems include an IR energy detector and monitor.
• Thermographers apply high emissivity materials for accurate measurements.
• Scanners with optomechanical devices convert IR energy to electrical signals.
• Monitor displays process/present signals as temperature levels or video for thermal profiles.
• Infrared cameras assign colors to visualize temperature differences.

IR Camera Factors, and Testing Techniques

• Factors to consider when performing IR testing with an IR Camera include the distance between the component and the IR camera as well as the output clarity, resolutions and range.
• IR Testing has many techniques: Passive, Active, Flash, Vibrothermography and Lock-in Thermography.
• Passive thermography measures the target object's natural radiation without external heat stimulation.
• It is useful in dark or enclosed spaces as it doesn't need lights or special equipment.
• Passive thermography is used for industrial condition monitoring, building efficiency studies, and monitoring processes.
• The computation of temperature from infrared images relies on the camera calibration and the emissivity of the object radiating energy.
• A calibration setup is required to take accurate measurements.

Active Thermography

• In active thermography, the specimen is subjected to external thermal stimulation, observing temperature decay with time.
• Heat propagation relies on material thermal properties, but also subsurface anomalies producing varying surface temperatures.
• This measures radiation emitted as the thermal response to external excitation.
• Flash thermography, known as pulse thermography, is a commonly used form of active thermography.
• A light pulse, often from a xenon flash tube, momentarily heats a surface, and resulting temperature is observed with a thermal camera.

Advantages and Disadvantages of IR Thermography

• IR is a non-contact technique that is fast, safe, reliable, and presents visual/digital data.
• Infrared thermography poses no radiation hazards, enabling prolonged use and has software back-up for processing/analysis, needing little training.
• Though costly, the instrument is difficult to interpret and reliant on working conditions like temperature and airflow.
• It has two-dimensional thermal images that can give a comparison between different objects.
• It can be viewed in real time, which is suitable if prolonged and repeated.
• IR is non-invasive.
• IR thermography is used in medicine, alternative diagnostic tool, building inspection, field and maintenance.

Thermo Reflectance Thermometry and Device Reliability

• Thermoreflectance thermometry measures temperature changes, allowing non-contact & high-resolution thermal imaging and characterization.
• Managing heat in semiconductor and optoelectronic devices is a barrier to reliability.
• Device complexity increases the need/challenges when understanding thermal behavior & thermal defects.
• A full understanding of static and dynamic thermal behavior is essential for ensuring best tradeoffs between performance and device reliability.
• Very small temperature hot spots cause functional anomalies in a circuit that has a tight design margin.

Understanding Temperature Distribution

• Knowing average temperatures isn't enough and a clear understanding of temperature distributions relies on local hotspot detection and nanosecond/picosecond resolution.
• Thermal imaging provides sub-micron spatial resolution and temporal resolution in the picosecond range.
• Changes in surface reflectivity depend on surface temperature.
• Thermoreflectance measures temperature changes through surface reflectivity monitoring.
• Thermo-reflectance depends on wave-length, material, surface characteristics, and the angle of incidence and temperature.
• The thermo-reflectance coefficient(CTR) defines a change in surface reflectivity due to a surface temperature.

Thermo-Optic Effect

• In the thermoreflectance method, a sample's temperature is modulated with pulsed/frequency-modulated laser beam and a second laser beam monitors the sample surface, reflecting and responding to temperature.
• Pump beam pulse energy is optically absorbed at the sample surface that creates a local temperature increase, altering optical constants, leading to reflectivity change.
• To ensure absorption, the sample surface is covered with a metal film (transducer).
• Transducer reflectivity change ΔR/R due to a temperature change ΔT is linear