Thermal Flow Sensor Research Articles

Investigations into packaging technology for membrane-based thermal flow sensors

Abstract. A new packaging method to mount a membrane-based thermal flow sensor, flush with the surface, is presented. Therefore, a specific design for the housing is shown, which is also adaptable to other conditions. It has been experimentally shown that it is important to mount the sensor flush with the surface. In addition, the experimental results are discussed. If a membrane-based thermal flow sensor is not mounted flush with the surface, vortices can occur (depending on velocity and fluid properties) or the reduction in the channel cross section plus a decrease in sensitivity have to be taken into account.

Gas Concentration and Flow Speed Measurement by Thermal Excitation using one Single Chip

In this contribution, a novel approach is presented to measure simultaneously gas concentration and flow speed of binary gas mixtures. This multi parameter detection is achieved by a thermal flow sensor using two different time independent excitation modes. Gas concentration is obtained by power mode; afterwards flow speed is measured by temperature excitation. The combination of these two principles improves the accuracy and sensitivity. One possible application of this measurement technique might be in biogas plant in order to control the fermentation process and calculate the gas’ fuel value.

An experimental setup for traceable measurement and calibration of liquid flow rates down to 5 nl/min.

This work presents the improvements of an experimental setup for measuring ultra-low flow rates down to 5 nl/min. The system uses a telecentric CCD imaging system mounted on a high-precision, computer-controlled linear stage to track a moving liquid meniscus inside a glass capillary. Compared to the original setup, the lowest attainable expanded uncertainty at any flow rate has been reduced from 5.4% to 2%. In addition, the conformity with specification of three commercial micro-fluidic devices was evaluated using the new setup: one syringe pump, one implantable infusion pump and one thermal flow sensor. The flow sensor and the implantable infusion pump met the compliance criteria (coverage probability 95%). The syringe pump however, failed to meet the specifications at 5 nl/min and 10 nl/min. No assessment could be made at higher flow rates.

Characteristics of a micro-mechanical thermal flow sensor based on a two hot wires principle with constant temperature operation in a small channel

A thermal mass flow sensor with high dynamic flow range in silicon bulk micromachining membrane technology is presented. The response behavior of this sensor based on a two hot wire principle is described. This sensor configuration uses two hot wires mounted closely spaced one behind the other within a thin membrane, kept at the same temperature. The power to compensate cooling of and the interaction between the two hot wires are used to calculate mass flows and directionality of flows of gaseous and liquid fluids in a small channel. The response shows both anemometric and calorimetric behavior, depending on fluid, channel height and flow range. The hot wires are heated relative to temperature sensors located also on an isolated membrane on the same chip. Choosing an appropriate evaluation procedure an independence of the signal from the ambient temperature is achieved. This configuration will be discussed in view of the published results of single heater flow sensors, being of calorimetric or anemometric type. A calculation method for the determination of linear and power law range of the response is given.

Microfabrication of a variable range and multi-directionally sensitive thermal flow sensor

This investigation reports the design, fabrication and characterization of a variable range and multi-directionally sensitive micromachined thermal flow sensor based on amorphous germanium thermistors. For measuring various flow ranges and flow directions with a single device, 12 thermistors are placed on an 2mm×2mm supporting square shaped dielectric membrane made with silicon nitride. The thermistors are located at different optimized distances from a central micro-heater that is made with a bilayer of Ti/Pt and can be heated to 200°C with about 580mW applied power. The thermistors are laid as six pairs on the aforementioned substrate with three pairs located along the X-axis and another three pairs located along the Y-axis. The device was characterized in the presence of air flow velocities up to 20m/s and at constant heating power modes of 500mW and 250mW in order to measure the achievable flow speed and direction. Characterization results were compared with simulation results and the errors in flow speed and angle measurement have been discussed.

A Method for Gas Identification in Thermal Dispersion Mass Flow Meters

A novel measurement method for identifying the type of gas in a thermal dispersion mass flow meter is presented. The physical background of the gas-identification method is discussed by employing a simple, one-dimensional, mathematical model of a thermal flow sensor. For a practical realization of the gas-identification method, the thermal dispersion mass flow meter has to contain two thermal flow sensors with different constructional or operational parameters. A thermal dispersion mass flow meter containing two thermal flow sensors with circular and square cross-sections was developed and calibrated for five different gases in order to experimentally validate the gas-identification method. If the measurement characteristics for the improper gas are employed, the mass flow readings of the thermal flow sensors will generally differ. The absolute value of the relative difference in the mass flow readings can be used as an objective function for identifying the type of gas from a defined set of gases with known compositions. In addition, the normalized error is proposed as an objective function to consider the dispersion that could be reasonably attributed to the difference in the mass flow readings.

Thermal Flow-Sensor Drift Reduction by Thermopile Voltage Cancellation via Power Feedback Control

The research question that is addressed in this paper relates to the performance limitations of thermal flow sensors due to miniaturization. Sensor elements in current microflow sensors are mostly made by metal thin films. The problem is that thin-films reproduce poorly and that practically all material properties are subject to drift. This drift and poor reproducibility translates directly into the accuracy of thermal microflow sensors. This paper presents a thermal flow sensor consisting of freely suspended silicon-rich silicon-nitride microchannels with an integrated thermopile in combination with up and downstream Al heater resistors. The drift-free zero offset of a thermopile at uniform temperature is exploited in a feedback loop controlling the dissipated powers in Al heater resistors, reducing inevitable influences of resistance drift, and mismatch of thin-film metal resistors. The control system attempts to cancel the flow-induced temperature imbalance across the thermopile by controlling a power difference between both heater resistors, thereby giving a measure of the flow rate nearly independent of material drift.

Thermal-wave balancing flow sensor with low-drift power feedback

A control system using a low-drift power-feedback signal was implemented applying thermal waves, giving a sensor output independent of resistance drift and thermo-electric offset voltages on interface wires. Kelvin-contact sensing and power control is used on heater resistors, thereby inhibiting the influence of heater resistance drift. The thermal waves are detected with a sensing resistor using a lock-in amplifier and are mutually cancel­led by a thermal-wave balancing controller. Offset due to thermal gradient across the chip and resistor drift are eliminated by the lock-in amplifier and power controller, and therefore do not influence the sensor output signal. A microchannel thermal-wave balancing flow sensor with integrated Al resistors has successfully been fabricated. The thermal flow sensor is capable of measuring water flow rates with nl ⋅ min−1 precision, up to about 500 nl ⋅ min−1 full scale. Measurement results are in good agreement with a dynamic model of the flow sensor. Drift measurements show the sensor output signal to be compensated for resistance drift and thermal gradient across the chip.

Silicon Thermal Flow Sensor Based on Ceramic Substrate

A silicon thermal flow-sensor system based on ceramic substrate is described in this paper. The sensor chip is mounted on a ceramic substrate with a metal heat sink. The simulation by ANSYS software and the experiment are done for determining suitable package size. The results from the ANSYS simulation and the standard flow test in the flow tunnel show that when the radius of the ceramic is 10mm and the heat conduction is35Wm-1K-1, the performance of the sensor is much better. The heat sink will make the flow-sensor more stable. The wind speed up to 35m/s and the precision of 0.5m/s are acquired in this experiment.

Thermal Flow Sensor based on Printed Circuit Board Technology for Ventilation and Air Conditioning Systems

A thermal flow sensor optimized for ventilation, and air conditioning systems is presented in this work. The design of the sensor is exclusively based on printed circuit board technology that allow for, cost-efficient fabrication, easy handling, and simple retrofitting and replacement. The measurement utilizes a calorimetric principle with thermore- sistive heat transfer. The behavior and performance of the sensor concept has been studied by means of finite element simulations and verified by experimental results. The experiments focus on flow measurements with a Lock-In am- plifier technique to evaluate the operation range and sensitivity.

Thermal Flow Measurements by a Flexible Sensor, Implemented on the External Surface of the Flow Channel

A thermal gas flow sensor was developed and evaluated. The presented implementation requires only low-cost manufacturing techniques and readily available components, while maintaining a high level of detection range and sensitivity. Heater and sensing elements were integrated on a flexible substrate and the device was formed by bending the substrate so that the active elements were placed on the external surface of the formed channel, therefore zero flow interference is achieved and a wide variety of fluids can be measured without compromising the sensor integrity. Evaluation was made using air flow rates in the range of 0-65SLPM utilizing electrical measurements and IR imaging techniques simultaneously.

High Precision Bidirectional Heat Pulsed Flow Sensor

In this work we have designed and simulated a thermal bi-directional integrated circuit mass flow sensor. The approach used here was an extension to the gas flow model given by Mayer and Lechner [1]. The design features high precision response received from analog integrated circuits. An established mathematical model for the heat flow equation including CFD parameters were used within COMSOL simulation (COMSOL Multiphysics, Sweden). Heat pulses of 55°C for a period of nearly 120 seconds and 50% duty cycles were applied as thermal sources to the flowstream. The boundary conditions of the heat equations at the solid (heating element) fluid interface were set up in the software for the thermal response. The hardware design included one heating element and two sensing elements were used to detect the bi-directional mass flow. Platinum sensors were used due to their linear characteristics within 0°C to 100°C range, and their high temperature coefficient (0.00385 Ω/°C). Polyimide thin film heater was used as the element due to its high throughput and good thermal efficiency. Two bridge circuits were used to sense the temperature distribution in the vicinity of the sensing elements.

A dynamic thermal flow sensor for simultaneous measurement of thermal conductivity and flow velocity of gases

Through finite element modeling and experiments, we describe individual influences of gas flow velocity (v) and thermal conductivity (λ) on temperature amplitude and phase measurements from a dynamic MEMS thermal flow sensor that employs AC heating at 200Hz. We further describe the relationships among the temperature phase, time-of-flight, and time-of-diffusion proposing a new boundary layer definition based on the existing flow and thermal boundary layer theories. The sensor has five primary thermistors made of amorphous germanium for high sensitivity. Four of these are symmetrically distributed around four central heater elements on a thermally isolating diaphragm where the fifth is in the center. Simulations and experimental results show that phase shifts (time lags) of temperature between thermistors primarily depend on λ, and negligibly on v below a velocity limit. Simulation results also suggest that the influences of density (ρ) and specific heat capacity (cP) on these phase shifts are relatively small or even negligible. Thus, λ can be accurately determined independent of the flow velocity for gases of similar ρ·cP product, up to 1m/s under the set flow boundary conditions. Hence, simultaneous determination of thermal conductivity and flow velocity is expected to be feasible with this sensor under these circumstances, which, in turn, allows medium-independent flow sensing for a such selected set of gases. Experimental results show that the measurement resolution and maximum inaccuracy of λ within the prescribed flow conditions are approximately equal to 2.445% and 3.18+4.20% (nonlinearity+velocity dependence) of the actual thermal conductivity, respectively.

Out-of-plane MEMS-based mechanical airflow sensor co-integrated in SOI CMOS technology

This paper demonstrates the monolithic integration of an airflow sensor based on out-of-plane movable cantilevers with a CMOS integrated circuit providing small footprint and low-power sensing. Airflow is sensed by mechanical deflection of cantilevers without static power consumption contrary to classical thermal flow sensors based on micro-heater. The interfacing circuit is a CMOS ring oscillator (RO), fabricated on the same chip with silicon-on-insulator (SOI) technology, offering unique properties such as harsh environment resistance and lower power consumption. Moreover, the additional post-processing steps required by the built-in of out-of-plane cantilevers are minimum thanks to the use of standard CMOS materials and dry etching of the sacrificial layer.The developed microsystem demonstrates 10% variation of the RO frequency for an airflow varying from 0m/s to 120m/s, with a static power consumption of the order of 1μW.

Automatic compensation of pressure effects on smart flow sensors in the analog and digital domain

Two different approaches for the automatic compensation of pressure effects on thermal flow sensors are investigated. One approach operates in the analog domain and it is based on a closed-loop circuit that uses a pressure dependent signal to keep the sensor output constant. The digital approach operates in an open loop fashion and is capable of producing also a pressure reading. The effectiveness of the proposed methods has been verified by means of a smart flow sensor integrating on the same chip the sensing structures and a configurable electronic interface performing signal reading and non idealities compensation. The chip has been designed with a commercial CMOS process and fabricated by means of a post-processing technique. The experimental results performed in nitrogen confirm that both methods are capable of reducing the sensitivity of the flow sensor output signal to pressure variations.

Three-Dimensional Flexible Thermal Sensor for Intravascular Flow Monitoring

A novel design and assembly technology is developed for a three-dimensional (3-D) flexible thermal flow sensor based on convective heat transfer to reduce detection error caused by position variation of a sensor inside the flow of narrow and curved geometries, such as coronary artery. The 3-D sensor has three independent sensing elements equally distributed around the catheter tube. This arrangement introduces three independent information channels, and cross-comparisons are used to provide accurate flow measurement. The resistance of the sensing elements is measured at ~ 1-1.2 kΩ with the temperature coefficient of resistance at 0.086%/°C. Using a constant-current circuit, the three sensing elements are heated to ~ 10°C above ambient temperature. Flow testing is implemented in a pipe channel at two positions: on the wall and along the center line. Experimental results from these two positions are discussed and computational fluid dynamic simulation based on Newtonian fluid properties is implemented, showing comparable results within an acceptable range of experimental to simulation errors. Therefore, we demonstrate the capability of 3-D thermal flow sensor for detecting the position of the catheter in the flow channel, thereby providing an accurate flow measurement.

Flex-to-Rigid (F2R): A Generic Platform for the Fabrication and Assembly of Flexible Sensors for Minimally Invasive Instruments

The flex-to-rigid (F2R) platform allows for the fabrication of miniature and flexible sensor assemblies for minimally invasive medical instruments such as catheters and guidewires. Devices fabricated on silicon wafers are transferred onto polyimide and partially rendered flexible by means of a two-step backside silicon deep reactive ion etching. The flexibility allows for the devices to be folded into, or wrapped around the tip of the instruments. The fabrication of the whole assembly is based on standard integrated circuit manufacturing technologies so that it is possible to scale the devices down to the dimension of the smallest guidewires. Several demonstrations are presented to show the versatility of the F2R technology, and a polyimide-based flexible thermal flow sensor for application at the tip of a 360- μm diameter guidewire is introduced.

Sampling Flow Channel Design in Automobile Engine Intake Manifold

As a result of turbulence and wide flow velocity range, flow velocity in engine intake manifold cannot be tested directly by using a thermal flow sensor. In this study, a sampling flow channel is designed according to the testing conditions and range of the thermal flow sensor. The flow velocity in the whole intake manifold was simulated by the finite element method software program. was found proportional to , and slope K was constant. ranged from 0 m/s to 4.8 m/s at the suitable outlet position of the sampling flow channel, which could be tested for the thermal flow sensor. The mass flow intake manifold was obtained by testing the mean flow velocity in the test section of the sampling flow channel .

Minimum Detectable Air Velocity by Thermal Flow Sensors

Miniaturized thermal flow sensors have opened the doors for a large variety of new applications due to their small size, high sensitivity and low power consumption. Theoretically, very small detection limits of air velocity of some micrometers per second are achievable. However, the superimposed free convection is the main obstacle which prevents reaching these expected limits. Furthermore, experimental investigations are an additional challenge since it is difficult to generate very low flows. In this paper, we introduce a physical method, capable of generating very low flow values in the mixed convection region. Additionally, we present the sensor characteristic curves at the zero flow case and in the mixed convection region. Results show that the estimated minimum detectable air velocity by the presented method is 0.8 mm/s. The equivalent air velocity to the noise level of the sensor at the zero flow case is about 0.13 mm/s.

A robust thermal microstructure for mass flow rate measurement in steady and unsteady flows

A silicon micro-machined thermal gas flow sensor operating in anemometric mode has been designed, fabricated and investigated for continuous and pulsatile flows. The sensor is specifically designed to achieve high sensitivity, fast response time and high robustness. It is composed of four metallic resistors interconnected to form a Wheatstone bridge. Two of them act simultaneously as the heating and sensing elements and the two others are used as a temperature reference. The heating element consists of a metallic wire of platinum Pt (2 µm width, 2 mm length) maintained on each lateral side by periodic silicon oxide SiO2 micro-bridges. Finite element simulations show that this structure achieves a fast thermal response time of 200 µs in constant current operating mode and a coefficient of temperature rise close to 25 °C/120 µW based on bulk electrical resistivity and when the Pt wire and SiO2 thicknesses are close to 100 nm and 500 nm, respectively. This design allows the fabrication of a robust thermal flow sensor with heating elements as long as possible, which enables accurate measurements with high signal to noise ratio. The sensor is then characterised experimentally; its electrical and thermal properties are obtained in the absence of fluid flow. These results confirm the effectiveness of the thermal insulation as predicted by the simulations. In a second step, the fluidic characterizations are reported and discussed for both continuous and pulsatile flows. In continuous mode, the sensor response was studied for gas flow rate ranging from 0 L min−1 to 10 L min−1. In pulsatile mode, the sensor is integrated inside a channel of a micro-valve actuated at 200 Hz. The measurements are compared with those obtained by a classical commercial hot wire.