Garth Naar - Introduction to Fiber Optics Temperature Measurement

Fiber optics are essentially light pipes. Garth Naar says the group of sensors known as fiber optic thermometers generally refer to those devices measuring higher temperatures wherein blackbody radiation physics are utilized.

Lower temperature targets--say from -100°C to 400°C--can be measured by activating various sensing materials such as phosphors, semiconductors or liquid crystals with fiber optic links offering the environmental and remoteness advantages.

Advantages of Using Fiber Optics for Temperature Measurements

Whether used for communications or infrared temperature measurement, fiber optics offer some inherent advantages for measurements in industrial and/or harsh environments:

- Unaffected by electromagnetic interference (EMI) from large motors, transformers, welders and the like;

- Unaffected by radio frequency interference (RFI) from wireless communications and lightning activity;

- Can be positioned in hard-to-reach or view places;

- Can be focused to measure small or precise locations;

- Does not or will not carry electrical current (ideal for explosive hazard locations);

- Fiber cables can be run in existing conduit, cable trays or be strapped onto beams, pipes or conduit (easily installed for expansions or retrofits);

- Certain cables can handle ambient temperatures to over 300°C--higher with air or water purging.

Monitoring SystemMonitoring Systems

Non-contact infrared thermal monitoring systems that represent a unique technological approach for monitoring and controlling process temperatures. These units combine fiber optics or line-of-sight optics with advanced electronic technology into a system that continuously monitors infrared radiation (a function of temperature) in real time and without physically contacting the target material. The result is a highly reliable system offering outstanding accuracy and repeatability with high response speed.

Fiber Optics Applications

Fiber optic thermometers have proven invaluable in measuring temperatures in basic metals and glass productions as well as in the initial hot forming processes for such materials. Boiler burner flames and tube temperatures as well as critical turbine areas are typical applications in power generation operations. Rolling lines in steel and other fabricated metal plants also pose harsh conditions which are well handled by fiber optics.

Typical applications include furnaces of all sorts, sintering operations, ovens and kilns. Automated welding, brazing and annealing equipment often generate large electrical fields which can disturb conventional sensors.

High temperature processing operations in cement, refractory and chemical industries often use fiber optic temperature sensing. At somewhat lesser temperatures, plastics processing, paper making and food processing operations are making more use of the technology. Fiber optics are also used in fusion, sputtering, and crystal growth processes in the semiconductor industry.

Beyond direct radiant energy collection or two-color methods, fiber optic glasses can be doped to serve directly as radiation emitters at hot spots so that the fiber optics serve as both the sensor and the media. Westinghouse has developed such an approach for distributed temperature monitoring in nuclear reactors. A similar approach can be used for fire detection around turbines or jet engines. Internal "hot spot" reflecting circuitry has been incorporated to determine the location of the hot area.

Garth Naar said an activated temperature measuring system involves a sensing head containing a luminescing phosphor attached at the tip of an optical fiber. A pulsed light source from the instrument package excites the phosphor to luminescence and the decay rate of the luminescence is dependent on the temperature. These methods work well for non-glowing, but hot surfaces below about 400°C.