Multipoint Measurements Research Articles

Curvature envelope area based rapid identification method of bridge distributional element stiffness using microwave interference radar

The bearing capacity of a bridge mainly relies on its own stiffness, while, at present, it remains a challenge to identify the bridge stiffness rapidly. In this paper, a rapid and innovative identification method of bridge distributional element stiffness (BDES) is proposed based on microwave interference radar technology and curvature envelope area (CEA). The main contributions of the work are as follows: (1) Taking advantage of multi-point displacement synchronous measurement, the self-developed microwave inference radar makes possible the acquisition of rotation influence lines (RILs) through the measured multi-point displacement influence lines (DILs). (2) Based on the one-to-one accurate correspondence between CEA, moment envelope area (MEA) and stiffness (“point-to-point” matching), and using the essential relationships among CEA and rotation, the bridge stiffness can be derived by using the DILs and the MEA calculated from the calibrated moving load (“point-to-line” matching). (3) Due to the radar feature of synchronous observation over a large area and referring to the proposed stiffness identification method, the BDES of the monitored area can be further obtained (“line-to-surface” matching). The proposed bridge identification method was applied at a laboratory experiment to identify the BDES of a steel beam for both undamaged and damaged conditions, and the results support the feasibility of the proposed method in BDES identification.

A Wearable Flower-Shaped Sensor Based on Fiber Bragg Grating Technology for In-Vivo Plant Growth Monitoring

One of the biggest challenges facing the world agriculture is feeding a growing population in a sustainable way. Therefore, global food availability is under a severe strain exacerbated by climate changes and biological stresses. Moreover, the content of macro- and micronutrients obtained from in plant-food sources strongly depends on the plant development. An attractive strategy to increase agricultural productivity is the growth monitoring of plants and edible parts by using wearable systems. However, most of these tools measure dimensional changes uniaxially, while an accurate representation of the growth distribution requires multipoint strain measurement especially for plants that show an anisotropic development. Here, we present a stretchable multisensor wearable system highly adaptable to the curvilinear surface of leaves and fruits for multidirectional dimensional monitoring. The proposed sensor consists of six fiber Bragg gratings (FBGs) within a biomimetic flexible substrate with a flower design. FBGs with their miniaturized size, high metrological properties, and multiplexing capacities are well suited to this purpose. A finite-element model (FEM) guides the optimal design and sensors positioning within the flower-shaped matrix to exhibit an adequate strain sensitivity and negligible crosstalk effects among the six sensing elements with a reduced encumbrance. A metrological characterization is first performed followed by the application of the proposed system for in-vivo detection of dimensional changes of a leaf and fruit in both indoor and outdoor scenarios.

Multi-point temperature measurements in packed beds using phosphor thermometry and ray tracing simulations

Packed bed reactors are commonly found in the process industry, for example in flame-assisted calcination for cement production. Understanding the heat transfer inside the bed is essential for process control, product quality and energy efficiency. Here we propose a technique to determine the internal temperature distribution of packed beds based on a combination of lifetime-based phosphor thermometry, ray tracing simulations, and assimilation of temperature data using finite element heat transfer simulations. To establish and validate the technique, we considered a reproducible regular packing of 6 mm diameter aluminum spheres, with one of the spheres in the top layer being electrically heated. If a sphere inside the packing is coated with thermographic phosphors and excitation light is directed towards the packing, luminescence from the coated sphere exits the packed bed after multiple reflection and the sphere's temperature can be determined. Isothermal measurements showed that the temperature obtained by phosphor thermometry is independent of the luminescent sphere location. When imaging the luminescence on a camera, the luminescence distribution in recorded image depended, however, on the position of the sphere. Therefore, in setups with multiple phosphor-coated spheres, their signals can be separated using a least squares fit. We demonstrate the approach using a setup with three luminescent spheres and validated the temperature readings against thermocouple measurements. To obtain the spatial signatures for individual sphere positions required for the least squares fit, ray tracing simulations were used. These provide an efficient alternative to single sphere measurements that are only practical for regular spherical packed beds. Multi-point measurements were used as input to a finite element heat transfer simulations to determine parameters such as particle-to-particle air gap distance. With these, the full temperature distribution inside the bed could be assimilated from the measured values.

Analyses of Spatial Correlation and Coherence in ABL Flow with a Fleet of UAS

The spatial structures of turbulent flow in the atmospheric boundary layer (ABL) are complex and diverse. Multi-point spatial correlation measurements can help improve our understanding of these structures and their statistics. In this context, we investigate Taylor’s hypothesis and the statistics of spatial structures on the microscale. For the first time, simultaneous horizontally distributed wind measurements with a fleet of 20 quadrotor UAS (unmanned aerial systems) are realized. The measurements were taken at different heights and under different atmospheric conditions at the boundary layer field site in Falkenberg of the German Meteorological Service (DWD). A horizontal flight pattern has been specifically developed, consisting of measurements distributed along and lateral to the mean flow direction with separation distances of 5ldots 205 m. The validity of Taylor’s hypothesis is studied by examining the cross-correlations of longitudinally distributed UAS and comparing them with the autocorrelations of single UAS. To assess the similarity of flow structures on different scales, the lateral and longitudinal coherence of the streamwise velocity component is examined. Two modeling approaches for the decay of coherence are compared. The experimental results are in good agreement with the model approaches for neutral atmospheric conditions, whereas in stable and convective ABL, the exponential approaches are not unconditionally valid. The validation results and the agreement with the literature on coherence in the ABL underline the potential of the UAS fleet for the purpose of spatial turbulence measurements.

Investigation of Tip Leakage Vortex Breakdown in a High-Speed Multistage Axial Compressor

AbstractIn this article, an unsteady tip leakage flow instability is identified and investigated for an axial compressor at near-surge conditions. We describe the results of experimental verification of a new compressor developed by improving the blade geometry based on the criterion for the occurrence of this unsteady phenomenon. In a high-speed multistage axial flow compressor having a subsonic high stagger rotor blade, a surge test was carried out by changing the tip clearance. Under a condition of large tip clearance, a drastic decrease in the static pressure rise coefficient near the surge point was observed. At this operating condition, large, unsteady pressure fluctuation at the blade tip was confirmed, and the occurrence of tip leakage vortex breakdown was clarified by unsteady multipoint pressure measurement and detailed unsteady numerical simulations. Due to the blockage effect caused by vortex breakdown of the tip leakage, double leakage and axially reversed flow near the trailing edge were observed. It was found that the vortex breakdown region of the tip leakage vortex propagated in the circumferential direction and caused the rotating instability. In order to investigate the relationship among this unsteady flow phenomenon, tip clearance size, and flow pattern, unsteady calculation was conducted by changing the blade tip stagger and tip clearance size. A new concept of tip clearance of staggered pitch reference was proposed, which makes it possible to include the effect of blade loading on the clearance and clarifies that there exists a threshold at which vortex breakdown occurs/does not occur. On the basis of the aforementioned results, a high-speed multistage improved compressor was designed and manufactured to prevent tip leakage vortex breakdown. A clearance change test using active clearance control technology was conducted, and an increase in the static pressure rise coefficient near the surge point was confirmed for each clearance. The design concept of the improved blade, which suppressed the unsteady tip leakage flow instability, was tested and verified, and the effectiveness of the design guideline in actual gas turbines for power generation was confirmed.

Evaluation of muscle elasticity in patients with end-stage renal disease complicated with sarcopenia by real-time shear wave elastography multipoint measurement

Abstract Background To analyze the value of real-time shear wave elastography (SWE) multi-point measurement in the evaluation of muscle elasticity in patients with end-stage renal disease (ESRD) complicated with sarcopenia. Methods We enrolled 169 ESRD patients treated as the research objects from January 2019 to February 2022. According to whether they were complicated with sarcopenia, the patients were divided into sarcopenia group (n=63) and non-sarcopenia group (n=106). The Young’s modulus and shear wave velocity (SWV) of muscles in relaxed and contracted states were measured by SWE technology in the two groups. Results Logistic regression analysis showed that age and hs-CRP were independent risk factors for sarcopenia in ESRD patients (P<0.05), while BMI, muscle thickness, Young’s modulus in stretched state and SWV in stretched state were protective factors for sarcopenia in ESRD patients (P<0.05). BMI, muscle thickness, Young’s modulus in extended state, SWV in extended state and Young’s modulus in rest state were all negatively correlated with age and hs-CRP (P<0.05), while there was a significant positive correlation between age and hs-CRP (P<0.05). Independent influencing factors were used to construct the prediction model of nomogram. The consistency index (C-index) was 0.845 (95% CI: 0.830~0.857), and the AUC of ROC curve was 0.852 (95% CI: 0.836~0.871), which had good discrimination. Conclusion SWE could accurately evaluate the muscle elasticity of ESRD patients, so as to reflect the changes of muscle mass and stiffness of patients, and could provide the important imaging indicator for the prediction of sarcopenia.

3-D Positioning Method of Pipeline Robots Based on a Rotating Permanent Magnet Mechanical Antenna

At present, the 3-D positioning of pipeline robots is limited by complex structures, large device sizes, and difficulties in the miniaturization of the sensor array. In this article, a multipoint magnetic field measurement method (MFM) is proposed for the spatial positioning of pipeline robots. The method can effectively obtain the 3-D coordinate positioning of pipeline robots in a short range, without complex sensor arrays, and without knowing the direction of the pipeline in advance. Furthermore, to address the problem in which weak magnetic field value is very susceptible to background noise and ferromagnetic interference in the environment, a dislocation superposition method (DSM) is developed to process the measured signals. In the MFM, a rotating permanent magnet mechanical antenna is used as the transmitter of low-frequency magnetic signals due to its advantages of small size, low energy consumption, and high radiation efficiency. Moreover, a single three-axis fluxgate sensor is used to measure the magnetic signal at multiple points. Finally, to verify the superiority of the proposed MFM and DSM, experiments are carried out in a seabed engineering laboratory. The test results show that the average absolute error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${x}$ </tex-math></inline-formula> -axis positioning is 0.04 m, the average absolute error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${y}$ </tex-math></inline-formula> -axis positioning is 0.01 m, and the average absolute error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis positioning is 0.07 m.

Underwater Acoustic Positioning for Close Sources Using Time Division and Code Division Multiplexing

Underwater acoustic positioning system (UAPS) is used to know the positions of underwater robots and underwater structures. In ultra-short baseline (USBL) acoustic positioning systems, the three-dimensional position is determined by measuring the time difference of arrival (TDOA). In this paper, we investigate the acoustic positioning system targeting multiple sound sources and propose a simultaneous multi-point measurement method using time division and code division multiplexing (TD-CDM). TD-CDM provides higher position accuracy than code division multiplexing (CDM) and has a much shorter positioning time than time division and multiplexing (TDM). The effectiveness of TD-CDM has been proven by the results of the water tank experiment and simulation.

A portable front face fluorescent system for in situ detection of aluminum in flour foods

A simple and rapid method for in situ detection of aluminum in flour food was developed by using a self-made portable front face fluorescent system (PFFFS). The effects of pH, temperature, reaction time, protective agent and masking agent on the detection of Al3+ were investigated. The use of fluorescent probe protective agent, interfering ion masking agent, multi-point collection measurements and the working curves based on the analyte content in the real samples makes the present method have high accuracy, selectivity and reliability for in situ detection of Al3+ in flour foods. By comparison with the ICP-MS the accuracy and reliability of the present method were verified. The results showed that when 97 real samples were analyzed the Al3+ content values obtained by the present method and those obtained by ICP-MS method reached a highly significant correlation, with r ranging from 0.9747 to 0.9844. The self-made PFFFS combined with fluorescent probe does not require sample digestion, and can quickly detect Al3+ in flour food within 10 min. Therefore, the present method based on using FFFS has good practical application value for in-situ rapid detection of Al3+ in flour foods.

Investigation of dynamic mixedness characteristics of a transverse acoustically excited turbulent jet by high-repetition-rate acetone tracer planar laser-induced fluorescence technique

The dynamic mixedness characteristics of a bluff-body stabilized turbulent jet under transverse acoustic excitations were investigated using high-repetition-rate acetone planar laser-induced fluorescence (PLIF) at 7 kHz and multipoint scanning hot-wire measurements. Acetone mixedness imaging was applied for the turbulent jet to assess the interaction between the turbulent jet and the imposed transverse acoustic excitations at a driving frequency of 50 Hz. The high-repetition-rate acetone PLIF images showed that the acetone mixedness distribution swung left and right frequently under the transverse acoustic excitation, and the deflection angle could reach about 6°. The mixedness area of a turbulent jet flow could also be increased by 13.3% when excited by a transverse acoustic wave. Meanwhile, the sequence of instantaneous acetone PLIF images illustrated how the wrinkled edges were generated when acoustic excitations were imposed. The curvature of the acetone PLIF interface showed that the portion of large curvatures increased by 60% after applying an acoustic wave of 123 dB. Multipoint hot-wire measurements further stressed that the turbulent intensity at the transverse acoustic excitation of 123 dB increased to be about 1.3 times the natural turbulence. The proper orthogonal decomposition results showed that the large and small scales of the jet wrinkles both increased with the sound pressure level. RANS transient simulation also implied that distorted velocity streamlines could be achieved inside the turbulent jet due to the transverse acoustic excitation. They could further lead to inhomogeneous convection and increased mixing between the turbulent jet and the surrounding air.

Sound field reproduction based on Pressure Matching with transfer functions modeled by equivalent sources and image sources

Pressure matching (PM) method is an effective method to control a sound field using many loudspeakers. It is difficult to implement PM method because it requires multi-point room impulse response (RIR) measurements. Multi-point measurements are challenging because they require a large number of microphones and a narrow sampling interval when dealing with high frequencies. Therefore, to synthesize a sound field in a local region, we proposed a PM method using estimated RIRs from only a small number of measurements based on the sparse Equivalent Source Method (ESM). In the previous method, we modeled the transfer functions considering only the direct sound from the loudspeaker. However, when the sound field is synthesized in the actual environment, the reflected sounds must be considered. In this study, we propose a PM method using the estimated transfer functions by sparse ESM based on the image source method considering the direct sound and early reflected sounds. In the experiment, we evaluated the accuracies of synthesized sound field by the proposed method compared to the methods only considering the direct sound.

Spatial Interpolation of Early Room Impulse Responses Using Equivalent Source method based on Grouped Image Sources

The Room Impulse Response (RIR), which indicates the propagation characteristics of sound, is used in various acoustical applications. In particular, multi-point measurement of RIRs is important for applications that require spatial information, and measurement of these is difficult. Previously, spatial interpolation methods of RIRs based on physical models have been proposed. Especially, the sparse Equivalent Source Method (ESM) with Image Source Method (ISM) acheive the spatial interpolation of the early reflected sounds with higher accuracy at high frequency. However, this method has a problem that when the order of reflected sounds to be estimated increases, it becomes difficult to find the optimal solutions because the number of bases increases. In this study, to reduce the number of bases in each optimization problem, we proposed a sparse ESM with the image sources grouped by the arrival times of reflected sounds. The optimization problem for each group of image sources is individually solved. In the simulation experiment, it is revealed that the proposed method can estimate higher-order reflections with higher accuracy compared to the conventional method. In addition, we found that the estimated region with high accuracy became broader.

Liquid Metal‐Carbon Fiber Thermocouple Array for Detection of Human Thermal Parameters

AbstractContinuous and accurate measurement of human thermophysical properties has become a vital research field in non‐invasive physiological diagnosis. However, traditional electronic devices have been greatly restricted due to the disadvantages of large volume, heavy weight, and mechanical rigidity. In this work, a kind of fiber‐structured LMs thermocouples composed of LMs fibers and nickel‐coated carbon fibers to solve the dilemma of good thermoelectric performance and sufficient flexibility is synthesized. With high thermoelectric properties as well as flexibility and knittability, the fiber‐structured LMs thermocouples can be assembled into a thermocouple array for multipoint temperature measurement in living organisms. While the nickel‐coated carbon fiber as the electric heating component further endowed the heating function for the fiber‐structured LMs thermocouples array, thus a linear heat source‐pulse heating method is successfully developed for qualitative/quantitative thermophysical parameters measurement (including thermal conductivity, thermal diffusivity, and blood perfusion rate). Collectively, the fiber‐structured LMs thermocouple array has shown promising prospects in human physiological measurement and thermophysical properties measurement.

D Region Electron Density Derived From Sprite Observations

AbstractFour upward propagating streamers in two carrot sprites over northwest Texas were recorded at 100,000 frames per second at 07:46:35 UT, 2 June 2019, from the Langmuir Laboratory near Socorro, New Mexico. The streamers reached velocities of 50–80 × 106 m/s with accelerations up to 25 × 1010 m/s/s. The upward motion ended with the top of the streamers near 90 km altitude. At this time, the streamers reached maximum brightness. The streamer head then dissolved and the brightness decayed exponentially with time constants between 0.078 and 0.097 ms. We interpret the dissolution and decay to be the result of interaction with the bottom of the ionosphere. Assuming the decay to be dictated by electric field relaxation, the ambient D region electron density would be 107 to 108 m−3. The analysis suggests that sprite observations can provide multipoint measurements of the D region ionosphere impacted by powerful lightning capable of triggering sprites.

Ionospheric plasma structuring in relation to auroral particle precipitation

Auroral particle precipitation potentially plays the main role in ionospheric plasma structuring. The impact of auroral particle precipitation on plasma structuring is investigated using multi-point measurements from scintillation receivers and all-sky cameras from Longyearbyen, Ny-Ålesund, and Hornsund on Svalbard. This provides us with the unique possibility of studying the spatial and temporal dynamics of the aurora. Here we consider three case studies to investigate how plasma structuring is related to different auroral forms. We demonstrate that plasma structuring impacting the GNSS signals is largest at the edges of auroral forms. Here we studied two stable arcs, two dynamic auroral bands, and a spiral. Specifically for arcs, we find elevated phase scintillation index values at the poleward edge of the aurora. This is observed for auroral oxygen emissions (557.7 nm) at 150 km in the ionospheric E-region. This altitude is also used as the ionospheric piercing point for the GNSS signals as the observations remain the same regardless of different satellite elevations and azimuths. Further, there may be a time delay between the temporal evolution of aurora (e.g., commencement and fading of auroral activity) and observations of elevated phase scintillation index values. The time delay could be explained by the intense influx of particles, which increases the plasma density and causes recombination to carry on longer, which may lead to a persistence of structures – a “memory effect”. High values of phase scintillation index values can be observed even shortly after strong visible aurora and can then remain significant at low intensities of the aurora.

A New Benchmark for Vibration Displacement Detection of Rotor

Vision-based structural displacement monitoring of rotor has attracted extensive attention from scholars due to its non-contact, non-damage, and multi-point synchronous measurement advantages. However, there is a lack of one comprehensive dataset and benchmark for rotor detection since the problem of limited acquisition equipment or on-site environment in the industrial field. For that, a large benchmark for rotor’s vibration displacement detection is introduced in this paper. Firstly, a rotor vibration displacement detection network (RDNet) for rotating structures is proposed and state-of-the-art performance is achieved. Secondly, the evaluation criteria are designed to evaluate the rotor image detection accuracy using the mean Average Precision@0.5:0.95 (mAP@0.5:0.95), the Frames Per Second (FPS), and the Normalized Root Mean Squared Error (NRMSE) as performance indicators. Thirdly, a dataset namely Rotor V1.0 is built, which contains three frames (high, middle, and low) of 13,500 rotor images. It is the rotor dataset for vibration displacement monitoring of rotating structures. Lastly, the baseline performance resulted from traditional methods (i.e., match template and support vector machines) and deep learning methods (i.e., CenterNet, Faster Region-convolutional neural networks (Faster R-CNN), RetinaNet, Single Shot MultiBox Detector (SSD), and You only look once (YOLO)) are reported and analyzed. It is hoped that RDNet, evaluation criteria, and the novel Rotor V1.0 dataset can promote the implementation of visual vibration measurement tasks in industrial sites.

Temperature, Refractive Index, and Corrosion Simultaneous Monitoring Using Raman Anti-Stokes Reflectometric Optical Fiber Sensor

Optical sensors that can measure multiple parameters is a promising area of research. This can also be attributed to the fact that monitoring two or more measurands at the same time is crucial in embedded systems, especially when they are interconnected, as is the case of temperature and corrosion, or temperature and refractive index. In this work, an optical fiber sensor based on Raman scattering in single-mode optical fibers to simultaneously monitor temperature, refractive index and corrosion is proposed and experimentally demonstrated. This sensor consists of a distributed temperature detection system based on Raman scattering with additional sensor heads: for multipoint corrosion monitoring and multipoint refractive index variation measurement. The multiparameter sensor was evaluated for solutions with refractive indices in the range of 1.3370 – 1.3840 RIU (Refractive Index Unit), for temperatures between 30 °C and 100 °C, and for aluminum thin-film corrosion in acid solution. The experimental results show that the sensor presents sensitivities of 0.5 dB/10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> RIU to refractive index variations, of 0.43 dB/nm for corrosion, and of 0.243 dB/°C for temperature, with resolution of 2 °C. The sensor system can also provide the corrosion rate and operate in fiber links several kilometers long.

A spectIR-fluidic reactor for monitoring fast chemical reaction kinetics with on-chip attenuated total reflection Fourier transform infrared spectroscopy.

Microfluidics has emerged as a powerful technology with diverse applications in microbiology, medicine, chemistry, and physics. While its potential for controlling and studying chemical reactions is well recognized, the extraction and analysis of useful chemical information generated within microfluidic devices remain challenging. This is mainly due to the limited tools available for in situ measurements of chemical reactions. In this study, we present a proof-of-concept spectIR-fluidic reactor design that combines microfluidics with Fourier transform infrared (FTIR) spectroscopy for in situ kinetic studies of fast reactions. By integrating a multi-ridge silicon attenuated total reflection (ATR) wafer into the microfluidic device, we enable multi-point measurements for precise reaction time monitoring. As such, this work establishes a validated foundation for studying fast chemical reactions using on-chip ATR-FTIR spectroscopy in a microfluidic reactor environment, which enables simultaneous monitoring of reagents, intermediates, and products using a phosphate proton transfer reaction. The spectIR-fluidic reactor platform offers customizable designs, allowing for the investigation of reactions with various time scales, and has the potential to significantly advance studies exploring reaction mechanisms and optimization.

Multipoint displacement measurement based on low intracavity-loss FLRD method

In this paper, we propose a method of multipoint displacement measurement using fiber-loop ring-down (FLRD) technology. We demonstrated the feasibility of multipoint displacement measurement using three cascaded FLRDs. Single-mode optical fiber wrapped around the cylinder airbag surface (CA-SMF) is used as the displacement sensor. Due to the low insertion loss of CA-SMF and the high-power modulated signal output of OTDR, it is not necessary to add any device for compensating the pulse signal power in FLRD. Finally, we also consider the errors that arise in the engineering manufacturing process. We have experimentally demonstrated that the length and intrinsic losses of FLRD do not affect the sensing performance of displacement sensors in a reasonable range, which is extremely helpful for practical engineering applications.

Experimental assessment of an indirect method to measure the post-combustion flue gas flow rate in waste-to-energy plant based on multi-point measurements

In waste-to-energy plants, the determination of the flue gas flow rate in the post-combustion section is of the utmost importance, e.g., for the verification of the compliance to the minimum residence time requirements (tres>2s) or for the control of flue gas treatment reactant injection, but the harsh conditions (high temperature and content of pollutants) do not allow for a direct measurement. The present work reports an experimental assessment of an indirect approach to estimate the flue gas flow rate in the post-combustion section of a rotary kiln plant with reduced uncertainty. This method consists on the direct measurement of the flow rate at a “colder” section of the plant (the boiler outlet) combined to the simultaneous measurements of flue gas composition measurements upstream and downstream of the boiler. From these measurements it is then possible to determine the mass of false air and to retrieve the actual flue gas flow-rate in the post-combustion chamber. A massive experimental campaign has been conducted at a full-scale medical waste incinerator, in which flue gas flow rate was estimated at different waste loads and ambient conditions. The results show that the percentage of false air can be significant and simply neglecting it can lead to substantial under-performance of the plant. Issues related to the practical implementation of the methods are illustrated in detail and the possibility to extend the methodology towards an online determination of post-combustion flue gas flow rate is discussed.