Distributed temperature sensing

Optical fiber temperature measurement (English Distributed Temperature Sensing DTS ) refers to the use of opto- electronic devices for measuring the temperature, wherein glass fibers are used as linear sensors.

Basics

Fiber optic systems are not only suitable for the transmission of information but also as a distributed sensing sensors. Physical parameters such as temperature or pressure and tensile forces acting on the optical fiber and change the properties of the light pipes in the fiber locally. As a result of the attenuation of light in the silica glass fibers by scattering of the location of an external physical effect can be detected, so that the optical waveguide can be used as a linear sensor.

For temperature measurement with optical waveguides made ​​of quartz glass is especially the so-called Raman effect is useful. The light in the fiber scatters in microscopically small density fluctuations, which are smaller than the wavelength. In backscatter can be found in addition to the elastic scattering portion ( Rayleigh scattering ) to the same wavelength as the incident light, also additional components for the other wavelengths that are coupled to the molecular vibration, and thus to the local temperature ( Raman scattering).

Measurement methods

The fiber optic temperature measurement system based on a fiber- optical Raman backscattering. The actual temperature sensor is a heat - and radiation-sensitive fiber-optic cable ( fiber optic cables). With the help of an evaluation device (optical Raman reflectometer ), the temperature values ​​can be determined in the fiber of the fiber optic cable in a spatially resolved. FO have low attenuation ( typically 0.2 to 1.5 dB / km in the near-infrared region ). The minimum possible damping of glass fibers, is limited by the Rayleigh scattering of light caused by the amorphous structure of the glass fiber. In addition to the Rayleigh scattering results from the effects of heat from glass fiber material, a further light scattering, the so-called Raman scattering. Temperature changes induce lattice vibrations in the molecular structure of the silica glass. Light falls on the thermally excited molecular vibrations, there is an interaction between the particles of light (photons) and the electrons of the molecule. This results in the optical fiber the temperature-dependent light scattering ( Raman scattering ), which is shifted relative to the incoming light spectrally, by the amount of the resonant frequency of the grating oscillation. The shift to lower energy is referred to as back Stokes band to higher energy as the anti -Stokes band.

Raman scattering has compared to the Rayleigh scattering a very small, negligible scattered component in many applications and can not be measured by the conventional OTDR technique. The intensity of the Raman scattering is dependent on temperature, wherein the anti- Stokes band has a higher temperature dependence than the Stokes band. The temperature at any location of the optical fiber is obtained from the ratio of the intensities of Anti- Stokes and Stokes light, and the propagation time to the location and back to the detector. A peculiarity of this Raman technique is the direct temperature measurement with a kelvin scale. By the use of an optical Raman backscattering process, the temperature along the fiber can be measured as a function of place and time. The best-known backscattering method is the OTDR method (OTDR: Optical Time Domain Reflectometry). It operates according to a pulse - echo method, from the time difference between transmission and detection of the light pulses scattered level and Streuort be determined. In comparison with the Rayleigh scattered light is Raman scattered light measurement before a smaller by a factor of 1000 backscatter signal. A spatially distributed Raman temperature sensor using OTDR technique is therefore only possible with powerful pulse laser sources (eg solid-state laser) and fast signal averaging techniques. The measurement time is calculated from the transit time of the light pulse along the fiber and the return time of the Raman scattering toward the detector, eg 100 microseconds for a 10 km long optical fiber. Due to the low strength of the Raman scattering many pulse measurements are typically over a period of time ( eg 10 seconds ) are averaged to obtain a desired signal -to-noise ratio.

Developed by the company LIOS Technology GmbH OFDR Raman temperature sensor ( OFDR, Optical Frequency Domain Reflectometry ) is not operating as the OTDR technique in the time domain, but in frequency domain. The OFDR method gives an indication of the local temperature gradient, if the detected over the entire measuring time backscatter signal as a function of frequency, and thus measured complex ( complex transfer function ), and then Fourier transformed is. The main advantages of the technique are the OFDR quasi continuous wave operation of the laser and the detection of the narrow-band optical backscatter signal, thereby providing a significantly higher signal -to-noise ratio than with the pulse technique is achieved. This technical advantage allows the use of inexpensive semiconductor laser diodes and the use of less expensive electronic modules for signal averaging. This contrasts with ( complex measurement of magnitude and phase ) and a complex by the FFT computation signal processing with higher linearity requirements of the electronic assemblies technically difficult measurement of the Raman scattered light.

The further developed by AP Sensing GmbH for fiber optic temperature measurement " Correlation Code " method uses fast Ein-/Ausschaltfolgen the light source, so that instead of single pulses digital code trains of finite length ( eg, 128 bits) with suitable properties, eg are sent to the measurement fiber, which corresponds to a modification of the OTDR technique. Golay codes. The recorded scattering signal has to, similar to the OFDR technology are translated by a transformation, such as cross-correlation in the spatial profile. The advantage of the code - correlation method is that the light source requires less peak power, so that, for example, long-lived semiconductor lasers can be used in the telecommunications industry. Simultaneously, the duration of light emission in the fiber is limited, so that a weak scattering signal from great distances is not overlapped by a strong scatter signal from short distances, which the shot noise is reduced and thus improves the signal -to-noise ratio.

System Design

The schematic structure of the fiber optic temperature measurement system consists of an amplifier with a frequency generator, laser source, optical module, receiver and microprocessor unit and a fiber optic cable (quartz fiber) as a line-shaped temperature sensor. According to the OFDR method, the laser is modulated sinusoidally within a measurement interval in the intensity and in the chirped frequency. The frequency deviation is a direct measure of the spatial resolution of the reflectometer. The frequency-modulated laser light is coupled into the optical waveguide. At any location along the fiber is formed Raman scattered light that radiates in all directions in space. A portion of the Raman - scattered light in the reverse direction reaches the analyzer. The backscattered light is spectrally filtered and converted into the measuring channels by means of photo detectors into electric signals, amplified and processed by electronic means. The microprocessor performs the computation of the Fourier transform. As an intermediate result we obtain the Raman backscattering curves as a function of cable length. The amplitudes of the curves are proportional to the backscatter intensity of the Raman scattering. From the ratio of the backscatter curves, the fiber temperature results along the fiber optic cable. The technical specifications of the Raman temperature sensing system can be optimized application- oriented by setting the device parameters (range, spatial resolution, temperature accuracy, measuring time, etc.). The fiber optic cable can also be adjusted by variations in the structure of the application. The thermal resistance of the fiber coating limits the maximum temperature range of fiber optic cable. Standard fibers for information transmission are provided with an acrylic type or UV - cured coating and designed for a temperature range up to about 80 ° C. For example, with polyimide coating of the optical fiber, this can be used up to 400 ° C.

Applications

Typical applications for linear fiber optic temperature sensors are safety-critical applications such as fire alarm

  • In road, rail or service tunnel. The passive fiber optic sensors offer many advantages over conventional fire detection technologies, such as the monitoring of the dynamics of a fire over a temperature range up to 1000 ° C has been shown
  • In deposits, aircraft hangars, floating roof tanks
  • Radioactively contaminated intermediate deposits
  • In conveyor systems, eg for early detection of smoldering fires

DTS markets systems in other industrial applications, such as

  • Monitoring of high-voltage underground cables up to 220 kV with real-time calculation of the capacity
  • Combination with systems for thermal forecasting of power cables (Real Time Thermal Rating, RTTR )
  • Thermal monitoring of power cables and overhead lines for the optimization of the operating conditions ( overhead line monitoring )
  • Increasing the efficiency of oil and gas drilling
  • Ensure safe operation conditions of industrial induction furnaces
  • Monitoring the density of LNG containers on ships and loading terminals
  • Detection of leaks in dams and dikes
  • Temperature monitoring of large chemical processes
  • Detection of leaks in pipelines

Specifications and Characteristics

Characteristics of the optical fiber sensor

  • Passive stretch and neutral, no influence of the temperature field
  • Small volume with light weight, flexible and simple to install
  • Installation even at later no longer accessible places
  • Immunity to electromagnetic interference
  • No potential carryover, ground loops, etc.
  • Can be used in hazardous areas
  • Combination with stainless steel tube: high mechanical protection, for use under high pressure
  • Different cladding options, eg with halogen-free, flame-retardant materials, no corrosion problems

Features of fiber-optic measuring method

  • Direct temperature measurement in Kelvin scale
  • Locally distributed temperature measurement related to a distance, area or volume
  • Possibility of redundant design
  • Computer-aided analysis and visualization (parameterization of zones, thresholds, reporting and alarm functions) and data communication
  • Evaluation of the temporal and local temperature change
  • Low maintenance costs: system -related self-test

Typical measuring parameters of fiber optic temperature measurement systems

(variable according to application )

  • Range of measurement: Variable, typically up to 30 km
  • Spatial resolution: Variable, typically 50 cm to 4 m
  • Temperature resolution: Variable, typically / - 0.1 K to 2 K
  • Fiber types: Monomode-/Singlemode-Faser 9/125 and multimode fiber GI 50/125 or GI 62,5 / 125
  • Fiber switch: Options to 24 channels per device
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