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Factors influencing the behaviour of FBG sensors for temperature measurements
Abstract
Investigating Fiber Bragg Gratings (FBG) behavior in varied fluid environments for temperature measurements. Analysis includes secondary effects and liquid metal influence on peak spectrum.
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Turbulent superstructures in horizontally extended three-dimensional Rayleigh–Bénard convection flows are investigated in controlled laboratory experiments in water at Prandtl number . A Rayleigh–Bénard cell with square cross-section, aspect ratio , side length l and height h is used. Three different Rayleigh numbers in the range are considered. The cell is accessible optically, such that thermochromic liquid crystals can be seeded as tracer particles to monitor simultaneously temperature and velocity fields in a large section of the horizontal mid-plane for long time periods of up to 6 h, corresponding to approximately convective free-fall time units. The joint application of stereoscopic particle image velocimetry and thermometry opens the possibility to assess the local convective heat flux fields in the bulk of the convection cell and thus to analyse the characteristic large-scale transport patterns in the flow. A direct comparison with existing direct numerical simulation data in the same parameter range of Pr , and reveals the same superstructure patterns and global turbulent heat transfer scaling . Slight quantitative differences can be traced back to violations of the isothermal boundary condition at the extended water-cooled glass plate at the top. The characteristic scales of the patterns fall into the same size range, but are systematically larger. It is confirmed experimentally that the superstructure patterns are an important backbone of the heat transfer. The present experiments enable, furthermore, the study of the gradual evolution of the large-scale patterns in time, which is challenging in simulations of large-aspect-ratio turbulent convection.
Because of their small size, passive nature, immunity to electromagnetic interference, and capability to directly measure physical parameters such as temperature and strain, fiber Bragg grating sensors have developed beyond a laboratory curiosity and are becoming a mainstream sensing technology. Recently, high temperature stable gratings based on regeneration techniques and femtosecond infrared laser processing have shown promise for use in extreme environments such as high temperature, pressure or ionizing radiation. Such gratings are ideally suited for energy production applications where there is a requirement for advanced energy system instrumentation and controls that are operable in harsh environments. This paper will present a review of some of the more recent developments.
Real-time thermal sensing on flexible substrates could enable a plethora of new applications. However, achieving fast, sub-millisecond response times even in a single sensor is difficult, due to the thermal mass of the sensor and encapsulation. Here, we fabricate flexible monolayer molybdenum disulfide (MoS2) temperature sensors and arrays, which can detect temperature changes within a few microseconds, over 100× faster than flexible thin-film metal sensors. Thermal simulations indicate the sensors' response time is only limited by the MoS2 interfaces and encapsulation. The sensors also have high temperature coefficient of resistance, ∼1-2%/K and stable operation upon cycling and long-term measurement when they are encapsulated with alumina. These results, together with their biocompatibility, make these devices excellent candidates for biomedical sensor arrays and many other Internet of Things applications.
Most critical process temperatures in nuclear power plants are measured using resistance temperature detectors (RTDs) and thermocouples. In addition to excellent reliability and accident survivability, nuclear safety-related RTDs are expected to have good calibration and fast dynamic response time, as these characteristics are important to plant safety and economy. In plants where RTDs are installed in thermowells in the primary coolant pipes, response-time requirements have a range of 4.0–8.0s versus the direct-immersion RTDs installed in bypass loops which have a required response range of 1.0–3.0s. The variety of problems that can affect the accuracy and response time of RTDs is extensive: dynamic response problems, failure of extension leads, low-insulation resistance, premature failure, wrong calibration tables, loose or bad connections, large EMF effects, open elements, thinning of the platinum wire, lead-wire imbalance, seeping of chemicals from the connection head into the thermowell, cracking of the thermowell, and erroneous indication. The causes of core-exit thermocouples failure can take the form of large calibration shifts, erratic and noisy output, saturated output, accidental reverse connections, and response-time degradation. Several effective methods for detecting RTD and thermocouple performance failure while the plant is operating are available. To detect accuracy problems, the cross-calibration technique is effective for both RTDs and core-exit thermocouples. It involves recording the readings of redundant online RTDs, averaging these readings, and calculating the deviation of each RTD from the average, less any outliers. To detect response time degradation online, the loop current step response (LCSR) test is the most accurate method. However, the noise analysis technique remains the most popular for detecting response time degradation in core-exit thermocouples.