Fiber Optic Point Temperature Measurement Systems

Basic Fiber optic sensors are very effective, but expensive, approach to solving specific measurement problems. These include monitoring temperature profiles in both domestic and industrial microwave ovens, examining temperatures in power transformer oils, motor/generator windings, and similar areas where (primarily) the issue is the operation of a reasonably precise temperature probe within very high electromagnetic fields.

Principle of temperature measuring using optic fiber:

 

A metallic probe either distorts the electromagnetic field significantly (e.g., in microwave ovens) or is subjected to very high levels of interference, producing spurious readings. Other applications sectors exploit the small size or chemical passivity of the device, including operation within corrosive solvents or examination of extremely localized phenomena such as laser heating or in determining the selectivity of radiation and diathermy treatments.

The principles of the probe are quite simple and are shown in the figure. The rare earth phosphor is excited by an ultraviolet light source and the return spectrum is divided into “red” and “green” components, the intensity ratios of which are a simple single-valued function of phosphor temperature.

For precision measurement, the detectors and feed fiber require calibration and, especially for the detectors, the calibration is a function of ambient temperature. The instrument, which has now gone through several generations to improve upon the basic concept, is capable of accuracies of about ±0.1°C within subsecond integration times over a temperature range extending from approximately –50°C to ±200°C.

Quasi-distributed Optical Fiber Temperature Measurement Systems:

The stimulated Raman scatter (SRS) distributed temperature probe is the most well established of these and, in common with many of the point sensors. Within the Raman backscatter process (and also within the spontaneous Brillouin backscatter process), the amplitudes of the Stokes and anti-Stokes lines are related to the energy gap between these lines by a simple exp(–DE/kT) relationship.

Therefore, measuring this ratio immediately produces the temperature. This ratio is uniquely related to temperature and cannot be interfered with by the influence of other external parameters

 

 

The system block diagram is shown above. The currently available performance from such systems enables resolutions of around 1 K in integration times of the order of 1 min, with resolution lengths of one to a few meters over total interrogation lengths of kilometres.

This uses a simple step index fiber in which the refractive index of the core material has a different temperature
coefficient than that of the cladding material. The temperature coefficient is designed such that at a particular threshold temperature, the two indices become equal and thereafter that of the cladding exceeds that of the core. Within this section, light is no longer guided. Simple intensity transmittance measurement is then very sensitive to the occurrence of this threshold temperature at a particular point along the fiber.

If used with an optical time domain reflectometer, the position at which this first event occurs can be located. This system is now in use as a temperature alarm on liquefied natural gas storage tanks.

Advantages of optic fiber temperature measurement:

  • Immunity to electromagnetic interference within the sensor system and within the optical feed and return leads
  • The capacity for intrinsic distributed measurements
  • Chemical passivity within the sensor system itself and inherent immunity to corrosion
  • Small size, providing a physically, chemically, and electrically noninvasive measurement system
  • Mechanical ruggedness and flexibility: optical fibers are exceptionally strong and elastic — they can withstand strains of several percent.
  • High temperature capability — silica melts at over 1500°C
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