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The Fundamental Principle of Thermometry Using Fluorescent Materials


Almost all materials exhibit fluorescence under appropriate conditions. Fluorescence can be succinctly defined as the emission of light when a material is exposed to electromagnetic radiation. Following the initial excitation, this emission can persist for a duration of time. This period of time is the product of numerous interactions taking place at the atomic level and the quantity of absorbed energy. Both excitation and emission intensity exhibit exponential changes over time. This dual time-dependent behavior is a unique property that can be utilized to indicate the state of fluorescent material molecules.

Scientists have discovered various types of fluorescent materials that can be doped with specific elements to make their behavior highly dependent on certain physical characteristics, which hold practical significance for sensing applications. For instance, it was found that the fluorescent properties of certain crystal matrices can be used to measure temperature, pressure, humidity, oxygen, and carbon dioxide. All these physical properties can be gauged by accurately determining the exponential time constants of unique fluorescent materials. A series of cost-effective fiber optic temperature sensors have been developed that utilize these principles. A notable advantage of fiber optic sensors over competing sensing technologies is their inherent immunity to electromagnetic noise and interference, as they do not have metal conductors serving as antennas transmitting current and voltage. This renders fluorescent fiber optic sensors particularly suitable for application in high-voltage power transmission, microwave, and plasma environments. Additionally, fiber optic thermometry technology allows for the use of inexpensive large-core polymer plastic fibers in applications below 150°C. These plastic fibers are highly robust and durable, and have been widely used in the automotive, industrial, and telecommunications sectors.

Fiber Optic Temperature Sensor Solutions

Fiber optic temperature sensors consist of one or more fiber optic probes connected to electronic devices known as temperature transmitters (also called signal conditioners). Multi-channel fiber optic temperature sensors have been developed for applications such as power transmission and distribution. Different from previous fiber optic temperature transmitters, which were bulky and expensive laboratory devices, fiber optic signal conditioners are similar in appearance and installation to thermocouples or RTD (Resistance Temperature Detector) transmitters. They are DIN rail-mounted and include standard 4-20mA analog outputs, as well as industrial RS-485 serial bus MODBUS for daisy chain device communication. The optical probes of fluorescent fiber optic temperature sensors also share a similar appearance and feel to standard thermocouples and RTDs. The cost of fiber optic temperature sensors is comparable to commercial RTDs and transmitter combinations.

The accuracy and stability of fiber optic sensors surpass traditional thermocouples and can approach PRT (Platinum Resistance Thermometers) in calibration applications. Solutions are available with long-term stability of ±1°C, as well as products with absolute accuracy of ±0.1°C.

Transformer Winding Hot Spot Temperature Monitoring

Fiber optic temperature sensors, completely unaffected by EMI/RFI and high-voltage environments, are the ideal choice for monitoring transformer winding hot spots. Precise, real-time, intelligent grid temperature monitoring solutions have been developed for power and distribution equipment.

Fiber optic temperature sensors have proven to be an excellent solution for high-voltage dry-type transformer temperature measurement and are now considered the preferred method for transformer monitoring. The benefits that fiber optic sensors bring to Transmission and Distribution (T&D) companies are economically significant. By monitoring the temperature of each transformer winding hot spot, utility companies can operate dry-type transformers at peak capacity without stretching into overload conditions that could significantly shorten the transformer’s lifespan. This efficiency in transmission throughput and lifespan can save significant funds annually, making direct winding temperature capability a necessity.

Cost-effective fiber optic temperature sensing technology makes intelligent grid transformer temperature monitoring more compelling. Fiber optic temperature probes are designed with high dielectric strength materials, such as PTFE and polyimide-coated quartz fibers, so they can withstand long-term immersion in transformer oil and kerosene desorption during the manufacturing process. Fiber optic sensors using special optical temperature transmitters transmit signals to probes directly installed at transformer winding hot spot locations. The fiber optic temperature transmitter is mounted in an external control cabinet, and the temperature output is input into real-time monitoring software. With optical temperature sensors installed, operators can monitor the load in real-time,

Switchgear Temperature Monitoring

Economical fiber optic temperature sensors provide continuous, real-time monitoring of switchgear temperatures at critical contact points, enabling the rapid detection of overloads and faults. Temperature transmitters provide analog output and RS-485 Modbus communication, which can be easily integrated with existing PLCs (Programmable Logic Controllers) and master monitoring software. Optical temperature sensors provide years of accurate sensing, ensuring safe and efficient switchgear operation.

Power switchgear manufacturers around the world are using fiber optic sensors for smart grid temperature monitoring of critical media and high-voltage switch equipment. These sensors provide real-time temperature data, allowing operators to maximize load efficiency and balance thermal stresses that could lead to catastrophic failures. Over time, switchgear contact points, busbars, and critical connection points develop slowly corroding hot spots, resulting in increased resistance. If left unchecked, even a small increase in resistance can quickly become uncontrolled, as higher resistance produces hotter conductors, which in turn generate higher resistance. Therefore, T&D companies often specify the requirement for continuous switchgear temperature monitoring to optimize maintenance schedules and extend equipment life.

However, one challenge is finding cost-effective technology for high-voltage sensing applications. Various RF (Radio Frequency) wireless and IR (Infrared) thermometers have been used, but each has its shortcomings. RF transmitter/receiver sensors suffer from inherent noise and interference present in high-voltage environments and may lose signal or display temperature spikes during switch operation, which can lead to false alarms. Moreover, since these sensors use electronic components, their temperature range is typically limited to below 120°C for long-term use. Remote infrared temperature sensors are similar, as they require shielding of wires and special installation points, and precise spatial alignment with the surface of the object being sensed. Infrared thermometers are known to report temperature changes due to dust accumulation and emissivity changes caused by minor surface corrosion, especially on bright metal surfaces like copper busbars. The reported temperature can be distorted by infrared energy reflected from surrounding objects, and sudden changes in ambient temperature can also cause measurement errors.

Fiber optic temperature sensors do not encounter any of the technical challenges associated with wireless and infrared thermometers. Fiber optic sensors can be directly routed to critical switchgear monitoring points. Optical temperature sensors are rigidly connected to hot spot locations and are completely unaffected by electromagnetic interference and noise bursts caused by high-voltage switches. Fiber optic sensors are robust and durable, can be manufactured in various lengths, and work like traditional thermocouples. Most importantly, each optical temperature sensor transmitter can monitor three phases, providing analog output and digital RS-485 Modbus RTU communication. Fiber optic temperature probes are highly suitable for smart grid switchgear temperature monitoring.

Generator Winding Temperature Monitoring

Now, online fiber optic temperature monitoring is common for both low-voltage and high-voltage generator equipment. Fiber optic sensors provide an economical and efficient solution for real-time temperature monitoring, allowing equipment to operate at optimal performance and extend its lifespan.

Fiber optic temperature sensors are now commonly installed in large motor and generator equipment to provide real-time monitoring and thermal protection of critical stator windings and bearings. The safe operating temperature of rotating machine windings is limited by the amount of heat that the insulation material can withstand before it ultimately degrades. This temperature and the rate of degradation vary with different categories of insulation materials. Insulation degradation at a given temperature is roughly proportional to the length of time the temperature exceeds a critical threshold. Until recently, RTDs (Resistance Temperature Detectors) were typically embedded in windings to provide continuous monitoring, though inaccuracies caused by EMI/RFI interference were present. Now, economical fiber optic temperature sensors can be installed where high voltage and alternating electromagnetic fields pose problems for traditional RTD winding sensors. Fiber optic sensors can be inserted between the windings of motors and generators for continuous temperature measurements to protect insulation and extend maintenance schedules. By installing optical temperature sensors, operators can monitor the load in real-time and maximize energy and economic efficiency. Better energy efficiency is advantageous for businesses and beneficial for the environment.

MRI Temperature Monitoring

Various life sciences and patient monitoring applications require high-precision fiber optic temperature sensing. Single and multi-channel fiber optic temperature probes are offered for MRI (Magnetic Resonance Imaging), NMR (Nuclear Magnetic Resonance Imaging), and RF (Radio Frequency) environments, including low-cost disposable temperature probes with fast response and excellent accuracy.

Various life science applications rely on fiber optic temperature sensors for high-precision sensing in environments that are unfavorable for standard thermometers and RTDs (Resistance Temperature Detectors). One use of fiber optic sensors is in MRI, NMRI, and MRT environments for patient monitoring, where extremely high magnetic fields combined with pulsed RF (Radio Frequency) energy prohibit the use of metal sensors. OSENSA’s fiber optic sensors are made of non-metallic materials and are highly suitable for monitoring patient body temperature to ensure that the specific absorption rate of tissue does not exceed destructive levels. Fiber optic temperature sensors can also be used to monitor the low temperatures of superconducting magnets cooling.

A variety of high-precision fiber optic temperature sensors in different sizes and materials are available, highly suitable for MRI and CT (X-ray Computed Tomography) studies. Fiber optic probes are made of X-ray transparent materials and non-magnetic connectors, fully compatible in MRI and CT scanning rooms. In addition, fast response, ultra-small diameter fiber optic probes are available, designed to meet the requirements of many demanding applications.

Semiconductor Chuck Temperature and Process Control

High-precision fiber optic temperature sensors are engineered to meet the stringent requirements of semiconductor process control applications. Custom OEM solutions that offer rapid response for dielectric and conductor etching applications are provided for both contact and non-contact optical temperature sensing. Fiber optic sensors are embedded within multiple zones of electrostatic chucks to deliver maximal control and thermal uniformity.

Many semiconductor wafer processing applications rely on fiber optic temperature sensors for precise process control in high RF (Radio Frequency) and plasma environments. In typical processing applications, silicon wafers are placed on an electrostatic chuck that is rapidly heated and cooled within a plasma environment. Fiber optic temperature sensors are embedded at the base of the electrostatic chuck, offering high precision and rapid response for stringent process control. Each electrostatic chuck is divided into multiple zones, necessitating multiple fiber optic temperature sensors to maximize temperature uniformity across the wafer surface. Semiconductor applications that typically employ this type of setup are dielectric and conductor etching processes.

For enhanced plasma chemical vapor deposition (EPCVD) processes, showerhead reactors are employed to disperse reaction gases throughout the processing chamber while applying strong RF power. In these applications, fiber optic temperature sensors are used to control the showerhead temperature and monitor sidewall temperatures to minimize deposition on the chamber surfaces. Fiber optic temperature sensors are developed to support various cutting-edge semiconductor processes. The sensors feature a non-contact geometric structure, with sensing materials embedded at the base of the electrostatic chuck, while the fiber optic is positioned remotely. This method maximizes response times and eliminates stem conduction losses. It also simplifies the replacement and refurbishment of electronic collets.

Microwave and Inductive Heating Control

Multi-channel fiber optic temperature sensors provide an economical and convenient solution for temperature monitoring in industrial microwave processes, including microwave-assisted chemistry, microwave sterilization, and microwave sintering. Optical temperature probes for microwave environments are constructed from materials that offer maximum chemical and biological compatibility, or from robust stainless steel and high-temperature ceramics.

Fiber optic temperature sensors are inherently immune to microwave radiation and high-frequency electromagnetic fields. Microwave environments using fiber optic temperature sensors include industrial microwave ovens for food processing and drying, microwave kilns for glass melting, and for drying paper, textiles, or timber. Other applications include microwave sintering of ceramics and dental instruments, microwave sterilization, and microwave pest control.

Developed fiber optic temperature probes can be customized for a variety of industrial microwave oven and kiln applications. The technology also allows for short-range non-contact optical temperature measurements and can measure temperatures exceeding very high thresholds.

Induction heaters and furnaces use high-power alternating electromagnetic fields to rapidly heat conductive objects. An example is the use of barrel and in-mold induction heating for injection molding equipment. These heaters operate at frequencies ranging from 5 to 100kHz and can consume power from 10 to 40kW. Industrial-grade fiber optic temperature sensors are particularly suited to these applications due to their robustness against strong electromagnetic energy, high reliability, and rapid response times. Multi-channel temperature transmitters (signal conditioners) with 4-20mA analog outputs can be easily integrated into injection molding process control equipment.

Research and Education Fiber Optic Temperature Sensors

Research and development activities that require precise fiber optic temperature sensing solutions. Software and fiber optic probes can be customized and calibrated for various laboratory and testing applications. Fiber optic temperature sensors are often selected for research applications due to their immunity to strong electromagnetic fields, nuclear, and X-ray radiation. Due to their flexibility and ease of use, fiber optic sensors are the ideal choice for research applications. Simply install the software and connect the USB cable to begin optical monitoring of temperature. Application support engineers will provide friendly assistance, and for more demanding applications, engineering consulting services can be offered.

Fiber optic temperature sensors are also highly suitable for student laboratory work and provide unique educational activities for teachers of physics, chemistry, biology, electronics, and instrumentation. Advanced software facilitates simple temperature logging and calibration at a low cost.


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