Thermal Gas Flow Sensor Research Articles

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%.

MEMS-Based Smart Gas Metering for Internet of Things

Utilities have traditionally employed or contracted meter readers to collect natural gas usage data, which is expensive and time consuming, and thus necessitates the need of smart natural gas metering. Existing gas metering systems mainly focus on measuring the amount of gas flowing through an microelectro mechanical system (MEMS) thermal gas flow sensor and simply ignore the detailed gas composition. From computational fluid mechanics simulations, however, we discover that gases with different compositions will cause different effects on the reading of an MEMS sensor. Based on a thorough analysis of the working principle of MEMS thermal gas flow sensor, we propose an innovative mechanism to compensate the errors caused by different types of natural gases on the sensor’s reading. The proposed solution first measures the physical property of metered gas to derive the composition correction coefficient that will then be used to correct the meter’s reading errors, considering the relation between the calorific value and physical property of natural gases. In this way, the proposed solution realizes a real-time multicomposition gas metering via thermal gas flow sensors. We implement and evaluate the proposed gas metering technique in various Internet of Things systems, including industrial flow metering, gas metering in smart home, and gas metering in low-power wide-area networks. Experiment results verify the innovative design and confirm that the proposed solution features high sensitivity, high precision, and high range ratio.

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.

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.

Evaluation of a gas flow sensor implemented on organic substrate

AbstractIn this paper we present results of the evaluation of a thermal gas flow sensor device implemented on a printed circuit board (PCB) substrate by a combination of PCB and MEMs technologies. The device consists of 3 parallel Pt strip resistors in direct electrical contact to patterned copper tracks of the PCB. For planarization purposes, the Pt strips reside on a thick polymer layer (SU8). The central resistor plays the role of a heater and the two other resistors, situated symmetrically on both sides of the heater, are temperature dependent sensing elements. The three main advantages of this specific sensor are: the low thermal conductivity of the substrate materials (FR4 and SU8, with thermal conductivity ∼0.2 W(m·K)) which permits high sensitivity at low power, its robustness compared to suspended membrane sensors and its simple fabrication process. The sensor can operate in both hot wire and calorimetric modes. The sensor was mounted inside a 1.5m long tube with 25 mm2 cross sectional area and was evaluated in nitrogen flow up to 100 SLPM, which corresponds to Reynolds numbers up to 25.000. The sensor signal in both the calorimetric and the hot‐wire modes is a monotonous function of the flow rate. A very high sensitivity is obtained in the calorimetric mode at very low flow rates (<0.05SLPM), while the sensor signal in the hot‐wire mode appears to be a linear function of the flow rate throughout the entire measurement region. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A novel microfabrication technology on organic substrates – application to a thermal flow sensor

A new technology that allows the formation of thermal sensors on organic substrates by combining the standard PCB technology with the well established microelectronic techniques, is proposed. The obtained structures consist of low thermal conductivity material, therefore the heat dissipation to the substrate is minimized, which result to the enhancement of the device sensitivity and the improvement of the corresponding response time. The proposed technology exhibits a series of advantageous characteristics such as significant cost reduction, elimination of both wire-die bonding and die cutting, direct integration with electronics and potential expansion on flexible substrates. Furthermore, the final structure provides a planar surface, which allows for further lithographic steps to take place, but is also a major advantage for specific type of applications such as non-invasive flow measurements. In the context of the proposed technology, a thermal gas flow sensor was fabricated and tested in a specially designed experimental set-up. The sensor consisted of three thin Pt strips directly connected to the copper tracks of the organic substrate. The middle Pt resistor act as a heater while the other two serve as temperature sensing elements.

Fabrication and testing of an integrated thermal flow sensor employing thermal isolation by a porous silicon membrane over an air cavity

In this work, an integrated thermal gas flow sensor was fabricated and evaluated using a new thermal isolation technique based on a porous silicon membrane over an air cavity in silicon. The fabrication process of a thermal gas flow sensor employing this type of micro-hotplate is described together with the theoretical estimation of the thermal distribution around the heater of the sensor. Experimental results of the thermal isolation achieved are shown. A flow evaluation of the sensor is presented and discussed. All the results obtained are compared with the corresponding ones for an identical sensor using a micro-hotplate made of porous silicon membrane but without the air cavity underneath. The improvement achieved by the air cavity is evaluated.

Characterization of a silicon thermal gas-flow sensor with porous silicon thermal isolation

In this paper, the results of full characterization of a micromachined-silicon thermal gas flow sensor will be presented. The sensor is composed of two series of thermocouples on the right and left side of a polysilicon resistor, used as heater. The resistor and the hot contacts of the thermocouples lie on a thick porous silicon layer, which assures local thermal isolation, while the thermopile cold contacts lie on bulk silicon. Gas flow is parallel to the surface of the sensor and perpendicular to the resistor, which is heated at constant temperature. The power of the heater is stabilized by an external circuit, which provides a feedback current to compensate changes in the resistance of the heater under flow. Characterization of the sensor both under static conditions and under flow of different gases will be presented. The sensor shows high sensitivity [of the order of 175 /spl times/ 10/sup -3/ mV/(m/s)/sup 1/2/ per thermocouple] and very rapid response, below 1 ms, which makes it appropriate for use both under laminar and under turbulent flows.

Thermal simulation of a micromachined thermopile-based thin-film gas flow sensor

FEM calculations of a thermal gas flow sensor based on thermopiles made of thin films of Bi 0.87Sb 0.13 as n-type material and Sb as p-type material are presented. The results of thermal simulations concerning the response of the signal voltage to variations of the flow velocity at constant gas temperature as well as to variations of the gas temperature at constant flow velocity are compared with experimental data. The good agreement between calculated and experimental results confirms the proposed two-dimensional thermal model. The design as a classical delta-T-type flow sensor did not result in a complete suppression of the influence of gas temperature variations to the sensor response.

Low power consumption thermal gas-flow sensor based on thermopiles of highly effective thermoelectric materials

A thermal gas-flow sensor based on thermopiles made of thin films of Bi 0.87Sb 0.13 for the n-type legs and Sb for the p-type legs with a high thermoelectric figure of merit is presented. The micromachined sensor chip is mounted within a package, which is kept at a constant temperature of 22°C. Experimental data and the results of finite-element analysis calculations concerning the response of the signal voltage to variations of the flow velocity at constant gas temperature as well as to variations of the gas temperature at constant flow velocity are given. A high sensitivity of 5.9 V W −1 m −1 s at low flows is achieved, which makes the sensor suitable for battery-powered devices where a low power consumption is desirable.

A CMOS integrated silicon gas-flow sensor with pulse-modulated output

An on-chip integrated thermal gas-flow sensor with pulse-modulated output where the pulse-width ratio is related to the flow velocity has been developed. The sensor was fabricated in a standard CMOS process, except for the shaping of the sensor, which was accomplished using an EDP anisotropic etchant. Polyimide was used as thermal insulator between the heated and non-heated parts of the sensor. An internal gas-temperature compensation was built in. The gas flow sensitivity is 3% m/s in the 2 – 30 m/s range. The design of the sensor also makes it suitable for use in a two-wire arrangement.