Fiber optic temperature measurement

With fiber-optic temperature measurement (engl. DTS for distributed temperature sensing ) refers to the use of opto-electronic devices for measurement of temperature, with glass fibers as linear sensors are used.

Basics

Fiber optic systems are not only suitable for the transmission of information, but also as spatially distributed measuring sensors. Physical measured variables such as temperature or pressure and tensile forces can act on the glass fiber and change the properties of the light guides in the fiber locally. As a result of the attenuation of the light in the quartz glass fibers through scattering, the location of an external physical effect can be determined so that the optical waveguide can be used as a linear sensor.

The so-called Raman effect is particularly suitable for temperature measurement with optical fibers made of quartz glass . The light in the glass fiber scatters on microscopic density fluctuations that are smaller than the wavelength. In addition to the elastic scattering component ( Rayleigh scattering ) at the same wavelength as the incident light, the backscattering also contains additional components at other wavelengths that are coupled with the molecular oscillation and thus with the local temperature (Raman scattering).

Measurement method

Most fiber optic temperature measurement systems are based on fiber optic Raman backscattering methods. The actual temperature sensor is a heat and radiation sensitive fiber optic cable (fiber optic cable). With the help of an evaluation device (optical Raman reflectometer), the temperature values in the fiber optic cable can be determined with spatial resolution. LWL have low attenuation (typically 0.2 to 1.5 dB / km in the near infrared range). The minimum attainable attenuation of glass fibers is limited by the Rayleigh scattering of the light, which is caused by the amorphous structure of the glass fiber. In addition to Rayleigh scattering, the effects of heat in the glass fiber material cause further light scattering, the so-called Raman scattering. Changes in temperature induce lattice vibrations in the molecular structure of the quartz glass. If light falls on these thermally excited molecular vibrations, there is an interaction between the light particles (photons) and the electrons of the molecule. The temperature-dependent light scattering (Raman scattering) occurs in the fiber optic cable, which is spectrally shifted by the amount of the resonance frequency of the lattice oscillation compared to the incident light. The shift towards lower energy is called the Stokes band, and towards higher energy the anti-Stokes band.

Time Domain Reflectometer (OTDR)

Compared to Rayleigh scattering, Raman scattering has a very small, negligible proportion of scattering in many applications and cannot be measured with classic OTDR technology. The intensity of the Raman scattering is temperature-dependent, with the anti-Stokes band having a higher temperature dependence than the Stokes band. The temperature at any location on the optical waveguide results from the ratio of the intensities of anti-Stokes and Stokes light and the transit time to the location and back to the detector. A special feature of this Raman technology is the direct temperature measurement with a Kelvin scale. Using a Raman optical backscattering method, the temperature along the fiber can be measured as a function of location and time. The best-known backscatter method is the OTDR method (OTDR: Optical Time Domain Reflectometry). It works according to a pulse-echo method, from the difference in transit time between emission and detection of the light pulses, the scatter level and the scatter location are determined. Compared to Rayleigh scattered light, the Raman scattered light measurement has a backscatter signal that is 1000 times smaller. A locally distributed Raman temperature sensor with OTDR technology can therefore only be implemented with powerful pulse laser sources (e.g. solid-state lasers) and fast signal averaging techniques. The measurement time results from the transit time of the light pulse along the fiber and the return time of the Raman scattering to the detector, e.g. B. 100 microseconds for a 10 km long fiber. Due to the low strength of the Raman scattering, many pulse measurements are usually averaged over a period of time (e.g. 10 s) in order to obtain a desired signal-to-noise ratio.

The "Code Correlation" method, which has been further developed by the company AP Sensing GmbH for fiber-optic temperature measurement, works with rapid on / off sequences of the light source so that instead of individual pulses, digital code trains of finite length (e.g. 128 bits) with suitable properties, e.g. B. Golay codes , are sent into the measurement fiber, which corresponds to a modification of OTDR technology. The recorded scatter signal has to be transformed, similar to OFDR technology, e.g. B. cross correlation , can be converted into the location profile. The advantage of the code correlation method is that the light source needs less peak power, so that e.g. B. long-life semiconductor lasers from the telecommunications industry can be used. At the same time, the duration of the light emission into the fiber is limited, so that weak scattered signals from great distances are not superimposed by strong scattered signals from short distances, which reduces shot noise and thus improves the signal-to-noise ratio .

Frequency domain reflectometer (OFDR)

The OFDR Raman temperature sensor ( OFDR , Optical Frequency Domain Reflectometry) does not work in the time domain like OTDR technology, but in the frequency domain. The OFDR method provides information about the local temperature profile if the backscatter signal detected during the entire measurement time is measured as a function of frequency and is therefore complex (complex transfer function) and then Fourier transformed . The main advantages of OFDR technology are the quasi continuous wave operation of the laser and the narrow-band detection of the optical backscatter signal, which results in a significantly higher signal-to-noise ratio than with pulse technology. This technical advantage enables the use of inexpensive semiconductor laser diodes and the use of cheaper electronic assemblies for signal averaging. On the other hand, there is the technically difficult measurement of the Raman scattered light (complex measurement according to amount and phase) and signal processing, which is complex due to the FFT calculation, with higher linearity requirements of the electronic components.

A limiting factor of the spatial resolution that can be achieved with the Raman-based OFDR method is the very low intensity of the Raman component of the backscattered light. In an alternative embodiment of the OFDR method, instead of the Raman signal, the Rayleigh signal of the scattered light is evaluated, which is approximately three to four orders of magnitude larger than the Raman signal. With the Rayleigh-based OFDR, a spatial resolution of 1 mm, a temperature resolution of 0.1 ° C and a measuring rate of 5 Hz can be achieved. Another advantage of this method arises from the fact that even very low-scatter fibers still produce enough Rayleigh scattering that many standard commercial glass fibers can be used.

The fiber optic temperature measurement process developed by Optocon AG is based on the effect of the dependence of the strip edge of a semiconductor on the temperature. A gallium arsenide crystal is used as an indicator for temperature changes. A light source is used to guide light through a glass fiber to a gallium arsenide crystal at the end of the glass fiber. Here the light hits the crystal. There is partial absorption and reflection back into the fiber. The light reflected back passes through the glass fiber into the evaluation unit, in which a spectrometer determines the spectrum and thus the position of the strip edge, from which the temperature is calculated using an algorithm. As an alternative to the gallium arsenide crystal, nano-scaled gallium arsenide powder (e.g. fiber-optic nano temperature sensor ) can also be used.

System structure

The schematic structure of the fiber optic temperature measurement system consists of an evaluation device with frequency generator, laser source, optical module, receiver and microprocessor unit as well as a fiber optic cable (quartz glass fiber) as a linear temperature sensor. According to the OFDR method, the laser is sinusoidally modulated in intensity within a measuring time interval and chirped in frequency. The frequency deviation is a direct measure of the spatial resolution of the reflectometer. The frequency-modulated laser light is coupled into the fiber optic cable. Raman scattered light is generated at every location along the fiber, which radiates in all spatial directions. Part of the Raman scattered light reaches the evaluation device in the reverse direction. The backscattered light is spectrally filtered and converted into electrical signals in the measuring channels by means of photo detectors, amplified and electronically processed. The microprocessor performs the calculation of the Fourier transform. As an intermediate result, the Raman backscatter curves are obtained as a function of the cable length. The amplitudes of the backscatter curves are proportional to the intensity of the respective Raman scattering. The fiber temperature along the fiber optic cable results from the ratio of the backscatter curves. The technical specifications of the Raman temperature measurement system can be optimized in an application-oriented manner by setting the device parameters (range, spatial resolution, temperature accuracy, measurement time, etc.). The fiber optic cable can also be adapted to the respective application by varying the structure. The thermal strength of the fiberglass coating limits the maximum temperature range of the fiber optic cable. Standard fibers for information transmission are provided with an acrylic type or UV-hardened coating and are designed for a temperature range of up to approx. 80 ° C. At z. B. Polyimide coatings of the glass fiber, these can be used up to a maximum of 400 ° C.

Areas of application

Typical applications for linear fiber optic temperature sensors are safety-relevant applications such as B. the fire alarm

In road, rail or service tunnels. The passive fiber optic sensors offer many advantages compared to traditional fire detection technologies, e.g. B. the monitoring of the dynamics of a fire over a temperature range up to 1000 ° C was shown
in storage facilities, aircraft hangars, floating roof tanks
in radioactive interim storage facilities
in belt conveyor systems, e.g. B. for the early detection of smoldering fires

DTS systems marketed in other industrial application areas, e.g. B.

Monitoring of heavy current 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 energy cables and overhead lines to optimize the operating conditions ( overhead line monitoring )
Increasing the efficiency of oil and gas wells
Ensuring safe operating conditions of industrial induction melting furnaces
Monitoring the tightness of liquefied natural gas containers on ships and loading terminals
Detection of leaks on dams and dikes
Temperature monitoring of large chemical processes
Detection of leaks on pipelines
Temperature monitoring in generators and transformers

Specifications and properties

Features of the fiber optic sensor

passive and route-neutral, no influence on the temperature field
small volume with low weight, flexible and easy to lay
Installation also in places that are no longer accessible later
Insensitivity to electromagnetic interference
no potential transfers, earth loops etc.
Can be used in systems at risk of explosion
Combination with stainless steel tubes: high mechanical protection, can be used under high pressure
different sheathing options, e.g. B. with halogen-free, flame-retardant materials, no corrosion problems

Performance characteristics of the fiber optic measurement process

direct temperature measurement in Kelvin scale
locally distributed temperature measurement based on a distance, area or volume
Possibility of redundant construction
Computer-aided analysis and visualization (parameterization of zones, threshold values, message and alarm functions) and data communication
Evaluation of the temporal and local temperature change
low maintenance costs: system-related self-test

Typical measurement parameters of fiber optic temperature measurement systems

(variable according to application)

Measurement range: Variable, typically up to 30 km
Spatial resolution: variable, typically 50 cm to 4 m
Temperature resolution: variable, typ. ± 0.1 K to 2 K
Fiber types: monomode or singlemode fiber 9/125 and multimode fiber GI 50/125 or GI 62.5 / 125
Fiber switch: Options up to 24 channels per device

Web links

Information series of the VDI / VDE Technologiezentrum Informationstechnik GmbH - microsystem technology information exchange (PDF file; 214 kB)
Publication at international conference on automatic fire detection AUBE04; University of Duisburg; LIOS Technology GmbH; engl. (PDF file; 393 kB)
Application examples for fiber optic temperature measurement - Article on elektroniknet.de

Individual evidence

↑ Real-time long range complementary correlation optical time domain reflectometer.

↑ Method and apparatus for performing optical time domain reflectometry. ( Memento of the original from October 5, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Patent specification @1@ 2

↑ OPTICAL REFLECTOMETRY ANALYSIS BASED ON FIRST ORDER AND SECOND ORDER SCATTER SIGNALS. , Patent specification

↑ polytec.com ( Memento of the original from September 24, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 24 kB) @1@ 2