Fiber-Optic Multipoint Sensor System with Low Drift for the Long-Term Monitoring of High-Temperature Distributions in Chemical Reactors

Abstract

A low-drift fiber-optic sensor system, consisting of 24 regenerated fiber Bragg gratings (RFBG), equally distributed over a length of 2.3 m, is presented here. The sensor system can monitor spatially extended temperature profiles with a time resolution of 1 Hz at temperatures of up to 500 • C. The system is intended to be used in chemical reactors for both the control of the production ramp-up, where a fast time response is needed, as well as for production surveillance, where low sensor drifts over several years are required. The fiber-optic sensor system was installed in a pilot test reactor and was exposed to a constant temperature profile, with temperatures in the range of 150-500 • C for more than two years. During this period, the temperature profile was measured every three to five months and the fiber-optic temperature data were compared with data from a three-point thermocouple array and a calibrated single-point thermocouple. A very good agreement between all temperature measurements was found. The drift rates of the 24 RFBG sensor elements were determined by comparing the Bragg wavelengths at a precisely defined reference temperature near room temperature before and after the two-year deployment. They were found to be in the range of 0.0 K/a to 2.3 K/a, with an average value of 1.0 K/a. These low drift rates were achieved by a dedicated temperature treatment of the RFBGs during fabrication. Here, the demonstrated robustness, accuracy, and low drift characteristics show the potential of fiber-optic sensors for future industrial applications. gratings strongly decay at high temperatures [1,2]. Amongst others, type II FBGs inscribed with femtosecond (fs) lasers [3][4][5] and regenerated FBGs (RFBGs) [6][7][8] have been reported to be suitable for temperatures up to 1200 • C. Most often, type II FBGs are inscribed with high-intensity femtosecond laser beams by phase masks (type II-PM) [3,9] or by a point-by-point (type II-PbP) [10] technique, and their main features are a high grating reflectivity, inherent temperature stability, and high initial tensile strength. The disadvantages of type II FBGs arise from their polarization sensitivity [10,11] and strong cladding mode coupling [5,10], which can limit their multiplexing capabilities. Type II-PbP gratings show significant wavelength drift when exposed to high temperatures [12,13]. For type II-PM FBGs, a stabilization of the wavelength drift after an annealing procedure of 100 h at 1000 • C has been reported [14]. Due to temperature treatments, a thermal-induced glass corrosion and thus a reduction of the tensile strength has to be taken into account.Regenerated fiber Bragg gratings (RFBGs) emerge from type I gratings in H 2 -loaded fibers after an annealing process at temperatures in the range of 800 • C to 1100 • C [7,15]. Their uniform spectral line shapes and the lack of cladding mode coupling make RFBGs perfectly suited for multiplexing applications. When compared to type II FBGs, RFBGs typically exhibit lower values of grating reflectivity. However, ...

Fiber Bragg Grating Wavelength Drift in Long-Term High Temperature Annealing

“…RFBGs were used to monitor the temperature of a melt during metal processing [ 19 ], the temperature of a turbo engine exhaust [ 20 ], during structural fire testing [ 21 ] and for measurement of the temperature distribution inside the substrate during a MCVD process [ 22 ]. An array of six RBFGs was used to thermally map the exhaust of a gas turbine [ 23 ] and 24 RFBGs incorporated into six fibers were used to monitor the temperature profile of a catalytic fixed-bed tubular reactor for more than two years [ 24 ].…”

High-Temperature Profile Monitoring in Gas Turbine Exhaust-Gas Diffusors with Six-Point Fiber-Optic Sensor Array

“…The individual positions of the peak wavelengths depend on the temperatures of the respective fiber segments. Using a wavelength-temperature calibration function [10,11,14], the local temperatures can be calculated from the corresponding wavelength shifts. It has been shown that RFBG sensor elements, which were fabricated in the same type of optical fiber and which were produced with the same parameters, share the same calibration function [14].…”

“…It has been shown that RFBG sensor elements, which were fabricated in the same type of optical fiber and which were produced with the same parameters, share the same calibration function [14]. By applying this calibration procedure, a temperature uncertainty of 4.5 K has been estimated in a temperature range up to 500 • C [11]. In previous investigations with three-point RFBG temperature sensors in a gas turbine of the same kind, RFBG-based temperature data were compared with thermocouple data, and the comparison showed deviations of less than 3 K [10], which was in good accordance with the estimated temperature uncertainty.…”

“…RFBGs can be used at temperatures up to 1200 • C [7]. Applications of multipoint sensing based on high-temperature FBGs have been reported for combustor systems [8], nuclear reactors [9], chemical reactors [10,11], and gas turbines [10,12,13]. In gas turbines, temperature measurements in the hot exhaust gas are used to control and protect the turbine.…”

Fiber Bragg Sensors Embedded in Cast Aluminum Parts: Axial Strain and Temperature Response

“…Fiber optic sensors are getting more and more popular due to their unique advantages [ 16 ], which are e.g., a small size (typical diameter of 125 ) [ 17 ], a wavelength multiplexing ability (multiple sensing elements in a single fiber) [ 17 , 18 ], their immunity to electromagnetic fields (e.g., monitor electrical power generators) [ 19 , 20 , 21 ], and their chemical resistance (suitable for harsh and high-temperature environments) [ 22 ]. Depending on the application (distributed or local sensing), there are different fiber optic sensor techniques available [ 23 , 24 ].…”