Thermal Flow Sensor Research Articles

Fabrication and Characterization of Dual Coordinate Self Examined Thermal Flow Sensor Arrays Based on Longitudinal Heat Conduction

Thermal flow sensors have been widely used in wind energy utilization, dangerous chemicals detection, automobiles, intelligent robots and unmanned aerial vehicles (UAV). New applications put forward higher requirements for detecting parameters of a two-dimensional (2-D) gas-flow vector over a full range of 360°. A novel thermal flow sensor on Si/SiO2/Pt/AlN substrate is presented to optimize sensor parameters. Eight thermal elements patterned as equally divided sectors of a regular octagon compose the sensor probe. Dual coordinate systems are created based on the regular octagon distribution to realize the self examined function. The longitudinal heat conduction mode shortens the heat transfer distance from mm level to micron level. And the a-axis oriented AlN film improves the heat transfer efficiency due to its high thermal conductivity. Dual coordinate self examined method is proven through experiments for the first time. Measurement results show that the maximum measurement flow velocity is up to 37m/s, with an output voltage sensitivity of 28.1 mV/(m/s). Flow direction was measured over a full range of 360° with an accuracy of 0.5 m/s at low velocity and 5% at high velocity. We believe that the sensor presented in this work will pave the way for novel applications in calorimetric sensors and array sensors.

Reliability test of a micro heater for thermal type MEMS flow sensor

Reliability test of a micro heater for thermal type MEMS flow sensor

Investigation of the piezoresistive properties and temperature coefficient of resistance of epitaxial GaN layers for applications in MEMS and thermal flow sensors

The paper presents the results of an experimental determination of the piezoresistive gauge factor and temperature coefficient of resistance of a GaN films grown by molecular beam epitaxy on sapphire substrates. The measured values were used to calculate the design and characteristics of the GaN hot-wires anemometer for the MEMS gas flow sensor.

Highly Precise Multifunctional Thermal Management-Based Flexible Sensing Sheets.

Elaborate manipulation of heat transfer renders proper operation of diverse thermal-related technologies. However, accurate implementation of thermal-based or transduction sensing on a thin flexible film over unusual surfaces remains challenging. Herein, efficient thermal management realizes highly accurate flexible multifunctional sensor sheets using a low thermal conductive medium as a thermal barrier. An approximately 50-fold enhancement in the thermal sensing accuracy, which is nearly independent of the changes in the external surroundings, is achieved. Such rational control of heat convection and conduction allows to not only dynamically monitor air flow, but also sight the large-scale air flow distribution on curved surfaces using a flexible thermal flow sensor array. Additionally, accurate wearable skin temperature monitoring independent of the sudden surrounding variations is achieved. This work addresses the formidable challenge of untethered heat transfer induced imprecise thermal related sensing, which universally exists in skin-inspired Internet of Things (IoT) applications.

A Pulsed Thermal-Flow (PTF) Sensor Measures Velocity of Flow and Thermal Diffusivity

A prototype pulsed thermal-flow (PTF) sensor was designed, fabricated in micro-electro-mechanical system (MEMS) technology and tested on various gases. The objective of our development project was twofold: first, to realize a micro-sensor of low cost and ultra-low energy consumption that, secondly, accurately and rapidly measures gas flow velocities within a wide temperature range. The sensor of size 4.2 × 3.2 × 0.6 mm3 is of micro-beam type. It consists of one Joule heating element and several resistance thermometers at distinct distances from the heater. The heater is operated in a single short pulse mode of a fixed duration that can be chosen between 10 and 100 µs. Due to this ultra-short time of excitation, there is no natural convection during a run. For typical values of pulse width (30 µs) and heater power (6 mW), the energy per measurement is smaller than 0.2 µJ. Typically, the excess temperature of the heater is limited to 5 K. This relatively small overtemperature considerably reduces radiative heat transfers otherwise interfering with the process of measuring. The velocity of flow is determined by the time-of-flight technique of the imposed heat pulse as the tracer in a few milliseconds. The measurement uncertainty is preliminary assessed to be 1.5% within a range from 0 to 7 m/s. The working temperature may vary between − 100 and 300 °C. The sensor not only measures the velocity of flow but also the thermal diffusivity of the gas under test. The report provides a complete analytical model of the functional principle of the sensor, a numerical FEM analysis of the 2-dim. isotherms, the heat flow pattern of the active surface of the sensor and an empirical model to fit the temperature response of the thermometers. The latter one effectively supports the evaluation of the acquired data. Exemplary experimental results are presented for nitrogen, methane and the two gas binary mixtures 80CH4–20N2 and 80CH4–20H2. The results deviate from their analytically predicted values by at most ± 1%. To the best of our knowledge, the PTF-sensor operates at the lowest energy consumption known so far. This characteristic makes it the perfect flow sensor for battery-operated long-term flow velocity recording systems.

MEMS thermal gas flow sensor with self-test function

In this paper, the design, fabrication and characterization of a MEMS thermal gas flow sensor with self-test function are presented. The flow sensor is composed of a platinum heater and thermopiles, where the heater is served as a heat source and the thermopiles are used for voltage output. In order to improve the performance of the flow sensor, the heavily doped N/P-polysilicon is utilized to form the thermocouple and XeF2 front-side isotropic etching is adopted to realize thermal isolation of the device. At the same time, the effects of the chip position in the gas channel and heater voltage on device performance are also studied from both experiment and simulation. Moreover, a self-test method and corresponding test system based on the thermal flow sensor are proposed, which use an equivalent heater voltage to simulate the gas flow rate for monitoring the dynamic response of the flow sensor to the gas. This method is simple and effective compared with traditional methods for detecting the performance flow sensors. In addition, the basic output performance of the flow sensor using nitrogen as the test gas is characterized. The experimental results show that the flow sensor owns a relatively high sensitivity of 123.006 mV (m/s)−1 (no amplification), a low response time of about 250 ms and good accuracy of ±1.97%.

Photothermally generated bubble on fiber (BoF) for precise sensing and control of liquid flow along a microfluidic channel.

The recent development of liquid-phase chemical analyses, drug delivery, and flow cytometry requires precise sensing and control of the liquid flow in a microfluidic chip environment. The channel in microfluidic chips is getting narrower to cope with complex liquid controls on a single chip, where small-footprint sensors and actuators are in urgent demand for accurate flow management. In this study, a unique microscopic bubble-on-fiber (BoF) device that can be readily integrated to current microfluidic chips was proposed and demonstrated for in situ sensing and control of microfluidic flow rate. The single microbubble was optically generated on the gold-deposited facet of an optical fiber by the local heating due to optical absorption. The BoF is a microscopic Fabry-Perot cavity, which serves as a thermal flow sensor precisely detecting the flow-induced temperature changes in the optical frequency domain. Experimentally we achieved the minimum detectable flow rate of ~0.06 mm/s in a single microfluidic channel, which is equivalent to a volume flow rate of 22 nL/s, and a response time of ~6 s. We also demonstrated that the BoF functioned as a microfluidic valve to regulate the flow rate in a Y-shape microfluidic chip by optically varying the bubble diameter. In addition to advantages of highly integrated functionalities and microscopic form factor, the proposed BoF can obviate the usage of chemical tracer such as dyes and can provide a high sensitivity over repeated flow cycles in a highly consistent manner. The BoF is promising for the timely development of high-density lab-on-a-chip devices using its efficient liquid flow management capability.

An Instrument for the Determination of a Hydropneumograph in a Bubbling Spring.

In order to enable greater accuracy in the determination of the mass discharge of gas and water-gas ratios (WGR) in groundwater from springs, we have developed a field-deployable instrument using commercially available components to independently measure the gas and water mass flow rates in springs with bubbling mixed-phase flow. Collecting and measuring the free gas phase will allow for further compositional analysis that may be useful in improving gas-derived parameters such as recharge temperature and age, as well as quantification of methanogenesis and flux of crustal/mantle gasses. By installing a phase separator at the spring discharge, a thermal mass flow sensor is utilized to measure the gas flow rate (ebullition + flux) generated from a spring. The water flow rate is determined by a standard weir. Field performance of the device was tested on a spring discharging from the Arbuckle-Simpson aquifer near the town of Connerville in south-central Oklahoma, USA.

A novel single-short-pulse MEMS upstream thermal flow sensor for gases also measuring thermal conductivity and thermal diffusivity

A prototype thermal flow sensor for gases was designed, fabricated and tested on several distinct specimens. The MEMS sensor is of micro-beam type and consists of one Joule heating element and at least one resistance thermometer. The heater is operated in a single short pulse mode of a duration between 30 μs and 100 μs; its excess temperature is limited to 5 K. The velocity of flow is determined from the upstream thermometer. The uncertainty is preliminary assessed to be 1.5% within a range from 0 to 7.2m⋅s−1. The sensor not only measures the velocity of flow, but also the thermal conductivity and the thermal diffusivity of the gas under test. The knowledge of the latter two material properties can help to identify an unknown specimen. The report provides empirical working equations for the three measurands mentioned. Additionally, a precise fitting relation is given that effectively supports the evaluation of the acquired data.

Sensitivity Enhancement of Silicon-on-Insulator CMOS MEMS Thermal Hot-Film Flow Sensors by Minimizing Membrane Conductive Heat Losses.

Minimizing conductive heat losses in Micro-Electro-Mechanical-Systems (MEMS) thermal (hot-film) flow sensors is the key to minimize the sensors’ power consumption and maximize their sensitivity. Through a comprehensive review of literature on MEMS thermal (calorimetric, time of flight, hot-film/hot-film) flow sensors published during the last two decades, we establish that for curtailing conductive heat losses in the sensors, researchers have either used low thermal conductivity substrate materials or, as a more effective solution, created low thermal conductivity membranes under the heaters/hot-films. However, no systematic experimental study exists that investigates the effect of membrane shape, membrane size, heater/hot-film length and (size) to (hot-film length) Ratio (MHR) on sensors’ conductive heat losses. Therefore, in this paper we have provided experimental evidence of dependence of conductive heat losses in membrane based MEMS hot-film flow sensors on MHR by using eight MEMS hot-film flow sensors, fabricated in a 1 µm silicon-on-insulator (SOI) CMOS foundry, that are thermally isolated by square and circular membranes. Experimental results demonstrate that: (a) thermal resistance of both square and circular membrane hot-film sensors increases with increasing MHR, and (b) conduction losses in square membrane based hot-film flow sensors are lower than the sensors having circular membrane. The difference (or gain) in thermal resistance of square membrane hot-film flow sensors viz-a-viz the sensors on circular membrane, however, decreases with increasing MHR. At MHR = 2, this difference is 5.2%, which reduces to 3.0% and 2.6% at MHR = 3 and MHR = 4, respectively. The study establishes that for membrane based SOI CMOS MEMS hot-film sensors, the optimum MHR is 3.35 for square membranes and 3.30 for circular membranes, beyond which the gain in sensors’ thermal efficiency (thermal resistance) is not economical due to the associated sharp increase in the sensors’ (membrane) size, which makes sensors more expensive as well as fragile. This paper hence, provides a key guideline to MEMS researchers for designing the square and circular membranes-supported micro-machined thermal (hot-film) flow sensors that are thermally most-efficient, mechanically robust and economically viable.

MEMS Thermal Flow Sensors— An Accuracy Investigation

The geometric structure of a low-power thermal micro-hot film sensor has been investigated and optimized, using both computational and experimental methods. The response and accuracy of eight CMOS designs with different heater and membrane sizes were studied and found to vary considerably with geometry. It is found that reducing the heater length causes an improved electro-thermal efficiency and that a large reduction in accuracy was seen when reducing the membrane size. Our simulations suggest that this effect is due to higher temperature gradients causing localized stronger natural convective flows above the measuring resistor. However, the reduced accuracy disappears as flow rate increases due to a higher proportion of forced convection compared with natural convection. We believe that this paper will help in the design of a new generation of high accuracy MEMS thermal flow sensors for low-cost, low-power application.

Development and Characterization of Thermal Flow Sensors for Non-Invasive Measurements in HVAC Systems.

We investigated non-invasive flow rate measurements in heating, ventilation, and air conditioning (HVAC) systems utilizing thermal transduction instead of commonly used ultrasonic techniques. The proposed thermal flow transduction comprises two temperature sensors and a heater, all mounted non-invasively on the outer surface of metal-pipes and, therefore, not disturbing the fluid flow inside. One temperature sensor measures the heater temperature, whereas the other one, mounted upstream of the heater, follows the fluid temperature for reference. The temperature difference (i.e., the heater excess temperature) depends on the fluid flow velocity and can be used to derive the mean volume flow inside the pipe. Experimental characterizations were conducted using two sensor prototypes. Beside output characteristics, other main issues such as dynamic behavior and noise density were investigated in detail. Special attention was paid to error compensation allowing measurements within a large range of fluid temperatures. Measurement results confirm the feasibility of this approach, however with some constraints regarding response time.

Polyacrylonitrile‐carbon Nanotube‐polyacrylonitrile: A Versatile Robust Platform for Flexible Multifunctional Electronic Devices in Medical Applications

AbstractFlexible multifunctional electronic devices are of high interest for a wide range of applications including thermal therapy and respiratory devices in medical treatment, safety equipment, and structural health monitoring systems. This paper reports a scalable and efficient strategy of manufacturing a polyacrylonitrile‐carbon nanotube‐polyacrylonitrile (PAN‐CNT‐PAN) robust flexible platform for multifunctional electronic devices including flexible heaters, temperature sensors, and flexible thermal flow sensors. The key advantages of this platform include low cost, porosity, mechanical robustness, and electrical stability under mechanical bending, enabling the development of fast‐response flexible heaters with a response time of ≈1.5 s and relaxation time of ≈1.7 s. The temperature‐sensing functionality is also investigated with a range of temperature coefficient of resistances from −650 to −900 ppm K−1. A flexible hot‐film sensing concept is successfully demonstrated using PAN‐CNT‐PAN with a high sensitivity of 340 mV (m s−1)−1. The sensitivity enhancement of 50% W−1 is also observed with increasing supply power. The low cost, porosity, versatile, and robust properties of the proposed platform will enable the development of multifunctional electronic devices for numerous applications such as flexible thermal management, temperature stabilization in industrial processing, temperature sensing, and flexible/wearable devices for human healthcare applications.

A Thermal Flow Sensor Based on Printed Circuit Technology in Constant Temperature Mode for Various Fluids.

We present a thermal flow sensor designed for measuring air as well as water flow velocities in heating, ventilation, and air conditioning (HVAC) systems. The sensor is designed to integrate the flow along the entire diameter of the pipe also quantifying the volume flow rate of the streaming fluid where the calorimetric principle in constant temperature operation is utilized as a readout method. In the constant temperature mode, a controller keeps a specific excess temperature between sensing elements at a constant level resulting in a flow dependent heater voltage. To achieve cost-effective sensors, the fabrication of the transducer is fully based on printed circuit board technology allowing low-cost mass production with different form factors. In addition, 2D-FEM simulations were carried out in order to predict the sensor characteristic of envisaged setups. The simulation enables a fast and easy way to evaluate the sensor’s behaviour in different fluids. The results of the FEM simulations are compared to measurements in real environments, proving the credibility of the model.

Design and Evaluation of a Multidirectional Thermal Flow Sensor on Flexible Substrate

The development and the corresponding evaluation of a multidirectional thermal flow sensor are presented in this work. The sensor was fabricated on a flexible substrate, allowing for new applications, since it provides the possibility of installation in nonplanar surfaces such as pipelines. Furthermore, the sensing elements are not in direct contact with the fluid, which increases the device reliability, extends its application range, and allows the noncontact monitoring of fluids. This was achieved by utilizing the substrate as a protective layer between the sensing elements and the fluid under measurement. The operation principle is based on the determination of the flow-induced temperature profile variations. A dedicated experimental setup was designed and used for the device evaluation. Both flow velocity value and direction were successfully extracted, while the results were consistent with the predicted theoretical values. A single-layer back propagation neural network that correlates the sensors’ readouts to the angle of rotation was implemented, which leads to a mean absolute direction estimation error in the order of 2.7 degrees independent to the training procedure datasets.

Explosion-proof thermal type gas flow sensor structure analysis and sensor module development

<p>The present study studied structure analysis and sensor modules for explosion-proof thermal type flow sensors that can withstand the poor surroundings of vessel engines and power plants and can be installed in gas lines. The structure analysis evaluated the integrity of the structures of the explosion-proof thermal type flow sensors with finite element analysis, and with regard to the operation of the sensor modules, those sensor modules that can detect environments where vibrations occur or where there are flows of high pressure gases or fluids were researched and developed. The explosion-proof thermal type flow sensor module consists of a sensing part, an adjustment part, and an output part, and a noise prevention circuit was added to secure the stability of the measurement signals. The flow sensor developed with the present study is a sensor module capable of measuring flows in domestic gas engines and gas piping and it is presented as a reliable sensor module because it is strong against vibrations and has excellent explosion-proof performance.</p>

Motion Velocity, Acceleration and Energy Expenditure Estimation Using Micro Flow Sensor

Motion velocity, acceleration, and energy expenditure estimations are important in quantitative assessments for physical recovery and exercise-based interventions for post-stroke patients with partial losses of neurological functions. This paper proposes a novel wearable motion estimation device using a micro flow sensor, which realizes motion velocity, acceleration detection, and energy expenditure estimation for human limb motion. A homemade micro thermal flow sensor is used to detect motion velocity. Jerk-Kalman based algorithm is proposed to extract motion acceleration from the flow sensor outputs. Motion velocity and acceleration are used to estimate energy expenditure of limb motion. Calibration experiments and application cases are conducted to validate the effectiveness of the methodology. The micro flow sensor based motion estimation method is free of accumulated error, robust for dynamic motion measurement, and provides a promising auxiliary approach for evaluating energy expenditures of human limbs in rehabilitation training.

A hot-film air flow sensor for elevated temperatures.

We report a novel packaging and experimental technique for characterizing thermal flow sensors at high temperatures. This paper first reports the fabrication of 3C-SiC (silicon carbide) on a glass substrate via anodic bonding, followed by the investigation of thermoresistive and Joule heating effects in the 3C-SiC nano-thin film heater. The high thermal coefficient of resistance of approximately -20 720 ppm/K at ambient temperature and -9287 ppm/K at 200 °C suggests the potential use of silicon carbide for thermal sensing applications in harsh environments. During the Joule heating test, a high-temperature epoxy and a brass metal sheet were utilized to establish the electric conduction between the metal electrodes and SiC heater inside a temperature oven. In addition, the metal wires from the sensor to the external circuitry were protected by a fiberglass insulating sheath to avoid short circuit. The Joule heating test ensured the stability of mechanical and Ohmic contacts at elevated temperatures. Using a hot-wire anemometer as a reference flow sensor, calibration tests were performed at 25 °C, 35 °C, and 45 °C. Then, the SiC hot-film sensor was characterized for a range of low air flow velocity, indicating a sensitivity of 5 mm-1 s. The air flow was established by driving a metal propeller connected to a DC motor and controlled by a microcontroller. The materials, metallization, and interconnects used in our flow sensor were robust and survived temperatures of around 200 °C.

Development of MEMS-based thermal mass flow sensors for high sensitivity and wide flow rate range

In the natural gas trade, mass flow metering is preferred to volume flow metering. In order to improve the measurement performance related to sensitivity within wide flow rate range of the mass flow meter, a novel design of a MEMS-based thermal mass flow sensor is proposed. To determine of the correlation between the sensing performance and geometry of sensor components, in this case the heater and the thermopile, models with various designs were fabricated using MEMS technology. Flow characteristic tests of the fabricated models were conducted in the low pressure gas flow standard system of KRISS. The effect of the distance ratio between the heater and thermopiles on the measurement performance of the thermal flow sensor was examined according to the flow rate range. The results presented that the basic models with the different distance ratio show different sensitivity characteristics. The non-uniform model proposed in this study show high sensitivity in both low and high flow rate region compared with models with a certain distance ratio.

Highly sensitive 3C-SiC on glass based thermal flow sensor realized using MEMS technology

This paper presents a silicon carbide (SiC) based thermal flow sensor on a transparent and electrically insulating glass substrate via anodic bonding process. The paper elaborates on the fabrication steps of the thermal flow sensor. Three resistive heater size configurations of dimensions 100 μm × 100 μm, 300 μm × 300 μm, and 1000 μm × 1000 μm were fabricated. The thermoresistive properties of 3C-SiC on glass were investigated from ambient temperature to 443 K. The characterization of the SiC heater and temperature sensors revealed a high thermoresistive effect with a temperature coefficient of resistance (TCR) of approximately −20,716 ppm/K at ambient temperature(298 K) and −9367 ppm/K at 443 K respectively. The performance of the sensors was evaluated based on the sensitivity of the flow sensor. For a turbulent flow velocity of 7.4 m/s, the sensitivity of the sensor operating in the constant -voltage mode is 0.091 s/m with a power consumption of 133.50 mW for the 1000 μm × 1000 μm heater. Finally, a study on the flow direction was conducted to confirm the operation of 2-D direction independent hot-film flow sensor. Results indicated that the performance of the sensor remained the same when the flow direction was perpendicular to SiC heater and sensor respectively. However, the best sensitivity was achieved by passing air flow perpendicular to the sensing elements. The high TCR of the single crystalline 3C-SiC material, the relatively low power consumption on the order of milliwatts and the high sensitivity of our sensor demonstrates its potential use for high temperature flow sensing applications.