Towards a gas independent thermal flow meter
Shirin Azadi Kenari is a PhD student in the Department of Integrated Devices and Systems. Promotors are prof.dr.ir. J.C. Lötters and dr.ir. R.J. Wiegerink from the Faculty of Electrical Engineering, Mathematics and Computer Science.
Flow sensors have been used in diverse applications ranging from medical equipment and environmental monitoring to industrial automation and energy production. They play a crucial role in precisely measuring fluid flow rates, with various techniques such as thermal, differential-pressure, cantilever-based, ultrasonic, and Coriolis mass flow sensors. Among these, thermal flow sensors stand out for their fast response time, low cost, simple fabrication, and straightforward measuring principle.
The working principle of thermal flow sensors is based on heat transfer through conduction and convection from a heated element to the surroundings. The simplest way to make a thermal flow sensor is using an electrically heated wire in which, when a fluid flows over the heated element, the temperature drops off so that the resistance of the wire changes as a function of the fluid's velocity.
Different typologies for thermal flow sensors, such as anemometry, calorimetry, and times-of-flight, and sensing transduction principles, like thermoresistive, thermoelectric, thermoelectronic, and frequency analog, can be used to achieve the desired results. Despite their advantages, thermal flow meters depend on fluid properties like thermal conductivity and volumetric heat capacity, requiring calibration for different fluids. Therefore, having medium-independent thermal flow sensors is becoming a very attractive solution. To address this, researchers have proposed methods such as utilizing different measurement principles and excitation techniques, including DC and AC excitations, multi-parameter sensors, and velocity-independent sensor designs to extract fluid properties.
This research aims to develop a micromachined medium-independent thermal flow sensor capable of accurately measuring flow rates independent of fluid composition using different excitation techniques and a velocity-independent fluid property sensor design.
The proposed sensor comprises a thermal flow meter and a fluid property meter. The latter consists of a wire suspended on a V-groove cavity, oriented perpendicular to the flow direction inside a tube. Due to the stationary flow conditions within the cavity and around the sensor, the wire's output signal is solely influenced by the fluid's properties. As gas enters the tube, the temperature of the electrically heated wire decreases because of heat loss from the wire to the silicon V-groove wall through the gas in the cavity. The amount of heat loss is determined by the thermal properties of the gas.
Two measurement techniques, DC and AC excitation, are employed to extract these thermal properties—namely, thermal conductivity and volumetric heat capacity. Under DC excitation, the heat loss in the V-groove cavity, and consequently the voltage drop over the wire, is affected primarily by the gas's thermal conductivity. In contrast, when the wire is fed by an AC current, the heat loss depends on both the gas's thermal conductivity and volumetric heat capacity.
The second sensing element of the proposed chip is the thermal flow meter, which consists of three pairs of calorimetric thermal flow sensors. Each pair is integrated into a separate Wheatstone bridge circuit, with each bridge comprising two variable resistors and two fixed resistors. These pairs are placed at different locations within the tube, ranging from the center to near the wall, to measure a wide range of flow rates.
The variable resistors in each Wheatstone bridge are suspended wires parallel to each other and perpendicular to the flow direction. These wires are heated by a constant voltage applied to the Wheatstone bridge, and the resulting bridge voltage is measured to determine the flow rate. Additionally, because there are two suspended wires, the flow direction can also be obtained; the sign of the bridge voltage changes based on the flow direction.
The flow rate calculated from the bridge voltage, which varies depending on the fluid's properties, can be simultaneously corrected using the fluid properties obtained from the gas property meter.
The main advantage of this sensor is that the gas property sensor can determine the gas properties in actual flow conditions when the fluid flows inside the tube enabling a real-time medium-independent flow rate measurement.
