“The group of sensors known as fiber optic thermometers generally refers to those devices measuring higher temperatures wherein blackbody radiation physics are utilized.”1
Fiber optic thermometers or fiber optic temperature sensors are basically light pipes which are mainly employed for measurement of temperature in complex situations. They are solely constructed of thin glass fibers. They make use of the same type of fiber which is used in communication systems. There are basically two approaches by means of which one can easily measure temperature. They are:
Using Phase Interference: Fiber optic temperature measurement using this technique is shown in the figure below. In this, the material used for temperature sensing is applied directly on the cleaved portion of the optical fiber. A transparent temperature sensitive layer is also present there. Hence, the incoming light will be reflected back in two parts. By measuring the phase difference between two reflected beams of light, one can easily determine temperature.
Using Fiber Deformation: Another way is to employ a conventional temperature sensing mechanism like bimetallic strip as shown in figure for creating deformation of the optical fiber. This deformation or bending of the fiber must be adequate to produce a considerable difference.
Since the cost of fiber optic temperature sensors is very high, they are only utilized in situations where their use is undeniable and worth for money.
Optical fibers are employed for measurement of temperature owing to their following characteristics:
- They exhibit micro bending and interferometric effects.
- They undergo change in index of refraction and length of fiber.
- They demonstrate change in polarization and fiber diffraction grating effects.
- They experience Sagnac effect too in which light usually travels around a loop in different directions which can then be used to detect rotation.
Optical fibers are frequently employed for communications and industrial temperature measurements because of their ability to withstand harsh and intricate environmental conditions. Some of the major benefits offered by them are mentioned below:
- They are impervious to electromagnetic interferences generated from electric motors, transformers and welders etc.
- Also, they are not influenced by radio frequency interferences caused due to wireless communication technology and lightning action.
- They can be easily installed in areas which are difficult to reach or inspect.
- They can be applied for measurement of temperature in very small and specific areas.
- Optical fiber cables never conduct electricity hence they are considered appropriate for locations which are likely to explode.
- Optical fibers can be conveniently placed in existing conduit and cable trays or tied to beams, pipes or conduit. Hence, they are widely applied in areas requiring expansions or retrofits
- Moreover, they are capable enough to deal with ambient temperatures beyond 300°C. This temperature range can be increased further with air or water purging.
Optical fibers mainly find their use in following areas:
- They are widely employed for sensing temperature in hazardous and unsafe regions.
- Also, they are applied in situations where use of conventional temperature sensors is not feasible. For instance, temperature measurement of windings of high voltage oil cooled power transformer whose peak voltage is around 500 kV. In these conditions, use of wired sensors can prove dangerous to the health of the person handling measuring device. Besides, non-contact sensors cannot be employed since the windings are enclosed within the transformer and not traceable. Hence, the use of fiber optic temperature sensors turns out to be the obvious and only choice left to deal with these situations.
- They are very helpful for temperature measurements in case of basic metals and glass productions. Moreover, they can be employed in the preliminary hot forming methods for these materials.
- In power generation field, they are normally employed for temperature measurement in Boiler burner flames or tubes and vital turbine areas.
- They are also installed in fabricated metal plant areas, for example rolling lines in steel etc. due to their ability to put up with ruthless conditions present in these areas.
- They are also employed in the semiconductor industries for activities involving fusion, sputtering, and crystal growth.
- Advanced techniques have been developed for doping of fiber optics which would enable them to work directly as radiation emitters at hot spots. In this way they will operate as the sensing device and the media as well.
Other typical applications of fiber optic temperature sensors include:
- All kinds of furnaces
- Sintering processes
- Ovens and kilns
- Automated welding operations
- Processing actions in cement, refractory and chemical industries involving high temperatures
- Plastics processing, paper making and food processing activities involving low temperatures
- Brazing and annealing equipments producing heavy electrical disturbances
Fiber Optic Temperature Measuring System
The construction of a typical fiber optic probe is shown in the figure below. By stimulating a variety of sensing materials like phosphors, semiconductors or liquid crystals via fiber optic links, one can easily measure target temperatures which fall in the range of 100°C to 400°C. “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.”2
This type of arrangement will perform suitably for targets having non-glowing heated surfaces with temperature not more than 400°C.