Relevant bibliographies by topics / Bridge measuring circuit

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Academic literature on the topic 'Bridge measuring circuit'

Author: Grafiati
Published: 5 June 2025
Last updated: 16 July 2025

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Journal articles on the topic "Bridge measuring circuit"

1

Vasylkivskyi, Ihor, Vasyl Fedynets, and Yaroslav Yusyk. "Thermometric bridge circuits for measuring thermophysical properties." Energy engineering and control systems 6, no. 2 (2020): 127–36. http://dx.doi.org/10.23939/jeecs2020.02.127.

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The article presents the designs of a number of devices for measuring the thermal conductivity of solids developed using the new methodological approaches proposed by the authors, which enable measurements in a wide range of thermal conductivity values with better accuracy. The proposed approaches rely on the principle of invariance, which consists in ensuring the compensation of the effect of various non-informative parameters on the measurement result. For calculating the developed thermometric bridge circuits (balanced, unbalanced and partially balanced), there was applied the theory of thermal circuits based on the similarity between heat transfer and electricity transfer. The design of thermometric devices based on thermometric bridge circuits makes it possible to raise significantly the accuracy of measuring thermophysical properties of materials due to the reduced errors stemming from the effect of non-informative parameters on the measurement result. This, in turn, allowed the extended measurement range for the thermal conductivity, increased reliability and reduced cost of the devices owing to the simplified measuring circuit.
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MIKHAL, Aleksander A., Dmitro V. MELESHCHUK, and Zygmunt Lech WARSZA. "APPLICATION OF THE HYBRID BALANCED RATIOMETRIC MEASUREMENT METHOD IN THE HIGH-PRECISION AC THERMOMETRY BRIDGES." Physics for Economy 3, no. 1 (2019): 29–40. http://dx.doi.org/10.7862/rf.2019.pfe.3.

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The paper article describes the circuit of an automatic AC bridge for high precision temperature measurements using a standard platinum SPRT sensor. An original method for measuring the impedance parameters of SPRT sensor which allows to carry out the measurement process without loss of accuracy, is described in detail. This measurement method is proposed to name as hybrid method. It is a combination of a balanced method for rough compensation of the circuit and a ratiometric method which accurately measures the ratio of two values of the imbalance signal, before and after its known change. With this method, the measuring circuit also does not require a circuit to compensate for the reactive component of the SPRT sensor impedance. The inductive voltage divider with lower number of digits is needed only for the coarse compensation. This circuit is simpler and at lower cost of the hardware resources allows to achieve the same accuracy as the most accurate thermometric bridges with fully balanced circuits.
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Melnyk, V. G., P. I. Borshchov, O. D. Vasylenko, and I. O. Brahynets. "OPTIMIZATION OF BALANCING IN A BRIDGE MEASURING CIRCUIT WITH A DIFFERENTIAL CONDUCTOMETRIC SENSOR." Tekhnichna Elektrodynamika 2022, no. 4 (2022): 78–88. http://dx.doi.org/10.15407/techned2022.04.078.

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The article is devoted to solving the problem of the occurrence of an additive error in determining local changes in the electrical conductivity of electrolyte solutions under conditions of changes in the background electrical conductivity of the measurement medium, which often occurs in biosensor and other systems with a differential pair of conductometric transducers, if their electrical parameters are not identical. The goal is to provide a deep suppression of the influence of background changes with significant differences in both reactance and active resistance in the transducers of a pair of sensor. The essence of the issue, the causes and mechanism of this type of error, as well as the methods and means of its reduction, developed earlier, are briefly considered. A diagram and description of the structure of a differential conductometric channel of a biosensor system based on an AC bridge, an algorithm for its balancing operations by controlling the module and phase of the test voltage, as well as a vector diagram of currents and voltages in the bridge circuit during this process. The balancing of the bridge has been was modeled analytically, bringing it to a quasi-equilibrium state, in which changes in the background electrical conductivity do not change its output signal. Additional operations for balancing the bridge are determined to achieve such a state with significant differences in both capacitances and active resistances in the impedances of a pair of conductometric transducers of a differential sensor. The results of experimental studies of the suppression of the influence of changes in the background electrical conductivity of a solution in a differential conductometric channel with using its computer model and experimental sample of a conductometric instrument with an electrical equivalent of a differential sensor are presented. A comparison of the results obtained and the corresponding data for balancing bridge circuits by previously developed methods is given. References 16, figures 3, tables 3.
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Semenets, D. "STATIC CONVERSION FUNCTIONS OF THREE-ELECTRODE DIFFERENTIAL CAPACITIVE SENSORS." Slovak international scientific journal, no. 72 (June 8, 2023): 32–36. https://doi.org/10.5281/zenodo.8016968.

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The paper considers the designs of three-electrode differential capacitive sensors for use in displacement transducers. For the three simplest designs, the transformation functions are determined when sensors are included in the RC circuit of the maximum type and the Wien-Robinson measuring bridge circuit. The conversion functions are defined at a certain power frequency of the measuring circuit and an information output signal in the form of voltage.
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Okanuma, S., O. Ichinokura, and K. Murakami. "A dc Power-Measuring Circuit Using a Bridge-Connected Magnetic Circuit." Journal of the Magnetics Society of Japan 19, no. 2 (1995): 565–68. http://dx.doi.org/10.3379/jmsjmag.19.565.

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Wang, Yong Qiang, and Wei Hong Li. "A Method of Calculation on Balance Resistance of Measuring Bridge in Strain Test." Applied Mechanics and Materials 494-495 (February 2014): 837–40. http://dx.doi.org/10.4028/www.scientific.net/amm.494-495.837.

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Considering the scattering difference in resistance of the strain gauge which will cause the bridge unbalance, the designers of strain measurement instrument have to add bridge-balance circuit into the design of measuring bridge in order to eliminate the problem of a DC component output caused by the unbalance. Thus, in the design of the bridge-balance circuit, the selection of balance resistances is of great significance. In this paper, through calculation analysis of the balance bridge, we obtained a couple of ideal values of balance resistances and tested them by experiments. In this condition, the experimental results have shown that the circuit has a great ability of regulation equilibrium. In brief, this paper provides the designers an effective method of choosing the balance resistances.
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Zihajehzadeh, Sh, M. Maroufi, M. Shamshirsaz, and A. H. Rezaie. "Self-sensing and quality factor control circuits for piezoelectric millimeter-sized resonant cantilevers." Journal of Intelligent Material Systems and Structures 22, no. 17 (2011): 2079–89. http://dx.doi.org/10.1177/1045389x11424215.

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For ever-increasing applications of resonant piezoelectric-excited millimeter-sized cantilever (PEMC) such as biosensors, viscosity, and density sensors, the need for design and implementation of a portable circuit for measuring the resonant frequency shift and/or the variation in the quality factor of PEMC becomes crucial. In this article, active and passive self-sensing bridge circuits are designed, fabricated, and implemented for a PEMC. The performances of these circuits are examined for the resonant frequency and quality factor measurements for vibration of PEMC in two different environments. For this purpose, a parameter-tuning procedure for the passive bridge based on experimental identification of Van Dyke model parameters is proposed and applied for the vibration of PEMC in air and 98% glycerol solution. Also, a compensation method for potential instability of active bridge circuit is proposed and developed experimentally for the vibration of PEMC in these environments. To increase the quality factor, the fabricated passive bridge is used in a designed control circuit, which is based on positive feedback signal proportional to the vibration velocity. The experiments show that with the proposed and implemented control circuit, the quality factor will increase by about 80% in air and 25% in glycerol.
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Ishinbaev, N. A., A. N. Krasnov, M. Yu Prakhova, and Yu V. Novikova. "Analysis of the Downhole Measurement System’s Pressure and Temperature Measuring Channel Calibration Errors." Journal of Physics: Conference Series 2096, no. 1 (2021): 012066. http://dx.doi.org/10.1088/1742-6596/2096/1/012066.

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Abstract Various measurements in wells are quite challenging due to the specific measurement conditions. There are some additional requirements for measurement systems, in particular, space restrictions. Therefore, measuring several parameters with a single sensor is rather important. The paper discusses a measurement system that allows measuring temperature and pressure with a single sensor – an SOS-based strain gauge pressure transducer with a bridge or half-bridge circuit. In this case, pressure and temperature measuring channels are calibrated individually, which creates another error component. The numerical simulation of calibration described herein shows that regardless of the sensor circuit, the voltage uncertainty band of both measuring channels is characterized by a reduced error of 0.03 % with a confidence probability P = 0.9.
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Hotra, Les, Oksana Boyko, Ihor Helzhynskyy, Hryhorii Barylo, and Tetyana Kolach. "Methods for the Linearisation of the Transfer Function of Thermoresistive Transducers." Security of Infocommunication Systems and Internet of Things 2, no. 2 (2024): 02012. https://doi.org/10.31861/sisiot2024.2.02012.

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The article describes digital and analogue methods for linearising the conversion function of thermoresistive transducers, with a detailed analysis of analogue methods. Analogue methods for linearising bridge circuits and measurement circuits based on passing a reference current through a resistance temperature detector (RTD) are considered. Linearisation of bridge circuits is based on the formation of the compensating supply voltage of the bridge circuit, which depends on the change in the measured temperature. When using the measurement method based on passing a reference current through an RTD, nonlinearity compensation is achieved by changing the conversion coefficient of the measuring signal or passing an additional current through the RTD, which linearly depends on the value of the RTD’s voltage change (measured temperature value). When passing an additional compensation current through the RTD, the nonlinearity error is not grater than 0.1°C in the range of 0…800°C, and the schematic diagram of the measuring transducer contains a minimum number of elements, which allows to increase its reliability. In general, the choice of a linearisation method depends on the requirements for accuracy, operation rate and resource limitations of the measuring system.
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Melnik, V. G., P. I. Borshchov, S. V. Dzyadevych, and O. Y. Saiapina. "METHOD OF MEASUREMENT OF LOCAL CHANGES IN ELECTRIC CONDUCTIVITY OF SOLUTIONS IN DIFFERENTIAL CONDUCTOMETRIC BIOSENSOR SYSTEMS." Praci Institutu elektrodinamiki Nacionalanoi akademii nauk Ukraini, no. 61 (May 25, 2022): 62–67. http://dx.doi.org/10.15407/publishing2022.61.062.

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A method for measuring local changes in the electrical conductivity of solutions using differential conductometric sensors is considered. The method makes it possible to significantly reduce the measurement error due to changes in the background electrical conductivity of the measuring medium when applying the test substance with non-identical parameters of equivalent electrical circuits of the sensor components. The bridge measuring circuit is brought to a quasi-balanced state, while the voltages in the sections of the working solution of the working and reference transducers of the sensor coincide in phase with each other, and their values are proportional to the conductivity of these sections. At the same time, changes in the background electrical conductivity of the solution do not lead to a change in the imbalance signal of the bridge circuit, which allows measuring the informative local change in the electrical conductivity of the working transducer with high sensitivity. The method can be used for highly sensitive and accurate quantification of solutions of electrically conductive substances in technological processes in the energy, chemical, and food industries, in the latest technologies, in particular in biosensor analyzers. The research results of the experimental sample of the conductometric analyzer are given. Ref. 9, table.
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Book chapters on the topic "Bridge measuring circuit"

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Tuz, Yulian, Bogdan Kokotenko, and Yuriy Samartsev. "Metrological Support of Measurement Channels with Bridge Circuits." In Advanced Information-Measuring Technologies and Systems I. Springer Nature Switzerland, 2023. http://dx.doi.org/10.1007/978-3-031-40718-5_1.

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You, Kok Yeow, Chia Yew Lee, Nadera Najib AL Areqi, Kim Yee Lee, Ee Meng Cheng, and Yeng Seng Lee. "Microwave Complex-Ratio-Measuring Circuits." In Advances in Computer and Electrical Engineering. IGI Global, 2020. http://dx.doi.org/10.4018/978-1-7998-0117-7.ch003.

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This chapter reviews the microwave complex ratio measuring (MCRM) circuits which are used for complex reflection coefficient measurement. This MCRM circuit is relatively simple and cost-effective. There are various structures for the MCRM circuit, such as multi-probe transmission line circuits, five-port ring circuits, six-port hybrid coupler-based circuits, switched-reflector circuits, dual-generator circuits, and Wheatstone bridge-based circuits. Each structure of the circuits has its own advantages and disadvantages. In this chapter, the MCRM circuit calibration process has been described in detail.
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"Cable between the strain gage bridge circuit and measuring instrument." In Technology and Practical Use of Strain Gages With Particular Consideration of Stress Analysis Using Strain Gages. Wilhelm Ernst & Sohn, 2017. http://dx.doi.org/10.1002/9783433606667.ch6.

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Graf, Rudolf F. "Bridge Circuits." In Measuring Circuits. Elsevier, 1997. https://doi.org/10.1016/b978-0-08-051134-4.50007-7.

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"Bridge Measuring Circuits." In Essential Formulae for Electronic and Electrical Engineers. Bloomsbury Academic, 1993. http://dx.doi.org/10.5040/9781350393837.0019.

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Conference papers on the topic "Bridge measuring circuit"

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Dunn, Ryan C., Guy D. Davis, and Robert A. Ross. "Corrosion Monitoring of Steel Reinforced Concrete Structures Using Embedded Instrumentation." In CORROSION 2010. NACE International, 2010. https://doi.org/10.5006/c2010-10173.

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Abstract A reinforced concrete corrosivity monitor (RCCM) is an embeddable non-destructive evaluation (NDE) corrosion-monitoring instrument. It is capable of measuring several parameters important to long term corrosion monitoring including linear polarization resistance (LPR), open circuit potential (OCP), resistivity, temperature and a potential related to chloride ion concentration ([Cl-]). Each RCCM unit is a digital peripheral connected on an embedded local area network. The instruments communicate with each other and an external data logger using the SDI-12 industry standard protocol. The RCCM has many applications in the construction and maintenance of commercial and civil structures. These structures can include high rise buildings, parking garages, bridges, dams, spillways, flood control channels, piers, pylons and erosion control structures. During construction, engineers, builders and supervisors can monitor parameters such as chloride concentration, resistivity and temperature. These parameters can identify errors at an early stage of construction. One error that may be detectable is the use of sea water or contaminated water during mixing of the concrete ([Cl-]). The moisture content and temperature of the structure can be monitored during the curing process to ensure maximum strength of the concrete. Once construction is complete, the instrument can be used to conduct long term monitoring of corrosion conditions over time.
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Okanuma, S., P. Gigih, O. Ichinokura, and K. Murakam. "A Spice Simulation Of The Power Measuring Circuit Using Bridge-connected Magnetic Circuit." In 1993 Digests of International Magnetics Conference. IEEE, 1993. http://dx.doi.org/10.1109/intmag.1993.642322.

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Rego Segundo, Alan Kardek, Erica Silva Pinto, Paulo Marcos de Barros Monteiro, and Jose Helvecio Martins. "Sensor for measuring electrical parameters of soil based on auto-balancing bridge circuit." In 2017 IEEE SENSORS. IEEE, 2017. http://dx.doi.org/10.1109/icsens.2017.8234001.

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Zhao, Lin, Linlin Xia, and Qi Nie. "Co-Rich Amorphous Wire Study for GMI Micro Geomagnetic Sensors." In 2007 First International Conference on Integration and Commercialization of Micro and Nanosystems. ASMEDC, 2007. http://dx.doi.org/10.1115/mnc2007-21527.

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In this paper, a kind of micro geomagnetic sensor is researched due to the giant magneto-impedance (GMI) effect of Co-rich amorphous wires. It is proposed a multivibrator bridge should be adopted as the structure module, which can realize the circuit’s auto excitation. In sequence, the measuring principle of the sensor is intensively analyzed by establishing the mathematic model, which illustrates this sensor is a linear one. In addition, a signal processing system that includes differential circuit, low pass filter, amplifier circuit, CPUSCM, A/D conversion circuit and LED are introduced. The scheme shows that the sensitivity of this sensor can reach lager than 28mv /Oe by choosing appropriate parameters of the modules, and the calculated field error closes to 1.8%. This sensor is proved to be feasible for measuring weak signal of geomagnetic field, and it can greatly benefit further development of the geomagnetism-aided navigation.
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Avedisian, C. T., W. S. Osborne, and F. D. McLeod. "On Measuring Bubble Nucleation Temperature of Water/Methanol Mixtures Using Ink-Jet Printer Technology." In ASME 2001 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/imece2001/htd-24195.

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Abstract The bubble nucleation temperature of water/methanol mixtures is measured using the fast transient process of thermal ink-jet printer technology. The heater element is placed in a dynamic bridge circuit coupled with an instrumentation amplifier to measure the change in resistance of the heater as a programmed voltage is applied across the bridge. An inflection point in the evolution of resistance signals bubble nucleation. A separate calibration relates resistance to average heater temperature. The results show that the nucleation temperature increases as the power input to the bridge increases for a given concentration. The heating rates are extremely high, in some cases reaching over a quarter billion degrees per second. As methanol concentration increases, the nucleation temperature decreases. It was difficult to measure the nucleation temperature at high methanol dilution because the difference between liquid and vapor thermal conductivity decreases as methanol concentration increases for a given temperature. The nucleation temperatures are successfully correlated with a generalized corresponding states theory.
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Dabiri, Arman. "Micro Double Pressure Sensor for Measuring the Applying Forces in Balloon Angioplasty." In ASME 2013 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2013. http://dx.doi.org/10.1115/imece2013-65705.

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This paper describes a new catheter based on double pressure sensor for measuring reaction forces of cardiovascular vessel walls in balloon angioplasty. This medical device is based on Wheatstone bridge and a passive transformer module. It assists cardiologists to measure reaction forces exiting between the catheter and the vascular wall. Reaction forces on the catheter can be grouped into two types: 1) reaction forces on the catheter head and 2) reaction forces between the balloon and the vascular wall. Its new proposed transducer module aids doctors decrease cardiology steps leading to the reduction of patients’ pains from inputting consecutive catheters into their bodies. Moreover, its special circuit design reduces needing wires for power supplying of the sensors, and simplifies the fabrication processes. Finally, mechanical behaviors of the sensor have been simulated in SolidWorks and its electrical circuit is modeled in Simulink\electrical. Also, Fabrication processes are projected in the final step of designing.
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Johnson, D. Kody, Md Fazle Rabbi, Debakanta Mishra, and Radim Bruzek. "Quantification of Vertical and Lateral Loads Using Strain Gauges: Eliminating the Wheatstone Bridge." In 2019 Joint Rail Conference. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/jrc2019-1271.

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Real-time measurement of vertical wheel loads applied to the rail is commonly carried out using strain gauges. One standard approach involves measurement of shear strains at the rail neutral axis, and use of the differential shear concept. Strain gauges are typically mounted on the rail neutral axis between two adjacent ties (over the crib section). A set of four strain measurements (two each, pointed at 45 degrees up and down from the horizontal) are carried out at each end of the crib section, and the measured strains are used to calculate the shear strain magnitudes; this shear strain is in turn used to calculate the applied load. In practice, the four individual strain measurements on each end of the crib (on either face of the rail) are arranged in a single Wheatstone bridge circuit. The purpose for using this common strain measurement configuration lies in the circuits’ ability to eliminate crosstalk, or strain unrelated to the load being measured, e.g. bending strain, or strain due to lateral loading, etc. This paper will propose a new measurement approach whereby eliminating this Wheatstone bridge configuration and measuring eight independent strain signals will enable direct quantification of the vertical as well as lateral load magnitudes. Instead of having to install additional strain gauges on the rail base to measure the lateral loads, the same strain gauges mounted on the rail neutral axis can be used to measure both vertical as well as lateral loads. This proposed technique will simplify the process of vertical and horizontal wheel load detection, and may increase the applicability of these circuits to detect loads in curved sections of track as well as near special track work.
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Chen, X., T. Butler, and J. P. Brill. "Capacitance Probes for Measurement of Liquid Film Thickness." In ASME 1996 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 1996. http://dx.doi.org/10.1115/imece1996-0112.

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Abstract Liquid film thickness measurement is very important in multiphase science and technology. A capacitance probe is presented in this paper, including its basic principle, geometrical design, electronic circuit, and experimental tests in measuring film thickness during gas-liquid two-phase flow. This method is based on the difference between the dielectric constants of the gas and liquid phases. A ratio-arm bridge circuit is used to convert the capacitance of the fluids between twin parallel plates into voltage. The voltage output from the capacitance probe exhibits an excellent linear relationship with the liquid film thickness. Experiments have shown that the parallel-probe capacitance method is very suitable to gas-hydrocarbon liquid system.
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Zheng, X. Y., S. Z. Li, M. Chen, and K. L. Wang. "Giant Reduction in Lateral Thermal Conductivity of Thin Nitride/Silicon/Oxide Membrane Measured With a Suspended Micro Structure." In ASME 1996 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 1996. http://dx.doi.org/10.1115/imece1996-1341.

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Abstract A suspended micro structure, a thermal bridge, was developed for measurement of lateral thermal conductivity of membrane materials. Integration of several polysilicon thermal elements (heaters and temperature sensors) with a membrane in a thermal bridge makes direct and accurate measurement of lateral thermal conductivity possible. A multilayer membrane composed of a layer of silicon nitride, a layer of monocrystalline silicon and a layer of silicon oxide was tested. Two measuring methods, a small signal differential method and an equivalent circuit method, were employed for evaluation of the thermal conductivity. The two-dimensional thermal conductivity of the membrane was found to be 0.34 × 10−6 W / K. This yields a lateral thermal conductivity of less than 2.2 W / m K for the silicon layer sandwiched in the membrane. In contrast, the thermal conductivity for bulk silicon is 148 W / m K. The accuracy of the result is also discussed.
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Terashima, Osamu, Kazuhiro Onishi, Yasuhiko Sakai, and Kouji Nagata. "Improvement of Constant Temperature Anemometer and Measurement of Energy Spectra in a Plane Turbulent Jet." In ASME 2012 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/imece2012-86509.

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A constant temperature anemometer (CTA) is a useful instrument for measuring the velocity fluctuations in turbulent flow. However, in our calibration test, the actual frequency response of a typical CTA was no more than 5 kHz under normal laboratory conditions: for example, the diameter of the hot wire is 5 μm and the free stream velocity is 20 m/s. Therefore, in some cases, a typical CTA is not enough to measure accurately turbulent velocity fluctuations for fine scale structures. In this paper, we present a rearranged CTA circuit to obtain a faster frequency response so that in turn fine-scale structures can be more accurately investigated. A typical CTA circuit consists of a Wheatstone bridge and a feed back circuit. To improve the frequency response, the ratio of the electrical resistance of the Wheatstone bridge is set to 1 and two operational amplifiers with a gain-band width product of 100 MHz and a slew rate of 20 V/μs are used in the feedback circuit. An experiment to estimate the frequency response of the rearranged CTA circuit is performed with a free stream velocity of 20 m/s and using hot wires of diameter 5 μm and 3 μm. Experimental results show that the roll-off frequency of the rearranged CTA circuit is improved from 5 kHz to 20 kHz for the 5 μm hot wire and from 6 kHz to 40 kHz for the 3 μm hot wire. Velocity measurements are made using the rearranged CTA circuit in a plane turbulent jet where the value of the Taylor microscale λ is 3.2 mm and the Taylor-scale Reynolds number Reλ is 440. Measurements shows that the power spectrum obeys the reliable numerical profile derived by a LDIA (Lagrangian Direct-Interaction Approximation) theory until more than 0.20 of the non-dimensional wave number κ1η, which is a wider range in comparison with the results obtained when using a typical CTA circuit. Here, κ1 is the axial wave number and η is the Kolmogorov microscale. Further, velocity measurements are performed taken using the rearranged CTA circuit with a square jet where the value of λ is 6.3 mm and Reλ is 1,720. Measurements shows that the power spectrum obeys the numerical profile by the LDIA theory in the range 0.04 < κ1η < 0.20, which is a much wider range than the results obtained when using a typical CTA circuit (0.04 < κ1η < 0.08). These results indicate that the rearranged CTA circuit can be used to investigate fine-scale structures in turbulent flows more accurately.
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Reports on the topic "Bridge measuring circuit"

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Bruce and Fiore. L51629 Users Manual-Field Validation of the Low-Frequency Eddy Current Instrument-Software Listings. Pipeline Research Council International, Inc. (PRCI), 1990. http://dx.doi.org/10.55274/r0010602.

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When an eddy current probe is placed near a conductive material, the alternating magnetic field from the probe causes electrical currents to flow in the material. These currents have associated with them their own magnetic field, which opposes the original magnetic field from the coil. The result is that the impedance of the probe is greatly reduced by the presence of the conducting material. If the conductor is also magnetic, as is normal steel, the situation is similar though slightly more complicated. Here, the impedance of the probe may be either increased or decreased depending on the permeability of the material and the frequency of the alternating field. Anything that affects the flow of current in the conductive material will also affect the impedance of the eddy current probe. For example, the electrical currents cannot flow through a crack but must flow around it. The alteration in the currents also changes the magnetic field produced by the currents and, consequently, the impedance of the probe. Normally, the impedance change caused by a defect is much smaller than that caused by the presence of the material in the first place, and measuring this small change requires a bridge circuit much like the balanced bridge used with strain gauges. The balanced bridge allows one to amplify the small changes in impedance caused by defects in the presence of the much larger change caused by the presence of the conductive and magnetic pipeline steel. The LFEC instrument is constructed using an� IBM-AT compatible portable computer. Inside the PAC-386 are two plug-in circuit cards that turn the PAC-386 into an eddy current instrument. The first, also commercially available, is a Spectrum DSP56000 digital signal processing card, while the second is a specially-built interface card for the eddy current probe. The PAC-386 is a standard MS-DOS machine and will operate most MS-DOS software. In the discussion below, it is assumed that the user is familiar with the MSDOS operating system.
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