Towards a gas independent thermal flow meter

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.