Development of a Self-Calibrating MEMS Pressure Sensor Using a Liquid-to-Vapor Phase Change
A growing industry demand for smart pressure sensors that can be quickly calibrated to compensate for sensor drift, nonlinearity effects, and hysteresis without the need for expensive equipment has led to the development of a self-calibrating pressure sensor. Pressure sensor inaccuracies are often resolved with sensor calibration, which typically requires the use of laboratory equipment that can produce a known, standard pressure to actuate the sensor. The developed MEMS-based, self-calibrating pressure sensor is a piezoresistive-type sensor with a sensing element made from a silicon on insulator (SOI) wafer using deep reactive-ion etching to create a hollow reference cavity. Using a micro-heater to heat the small, air-filled reference cavity of the sensing element, a standard pressure is generated to actuate the sensor's pressure-sensitive membrane, creating a self-calibration effect. Previous work focused on modeling and improving the thermal performance of the sensor identified potential solutions to extend the sensor's calibration and operating range without increasing the micro-heater's power consumption. This report focuses on using a water liquid-to-vapor phase change inside the sensor's reference cavity to increase the sensor's effective range and response time without increasing power demands. A combination of Ansys Fluent CFD modeling and benchtop experiments were used to guide the development of the two-phase, self-calibrating pressure sensor. A two-phase benchtop testing rig was built to demonstrate the anticipated effects of a liquid-to-vapor phase change in a closed domain and to provide experimental data to anchor CFD models. Due to the complexity of modeling a phase-change within a closed domain with Ansys Fluent R21.1, the CFD modeling was performed in two stages. First, the two-phase benchtop rig was modeled, and validated using benchtop test data to verify the Volume of Fluid multiphase model setup in Ansys Fluent. Then, a 2D Ansys Fluent model of the self-calibrating pressure sensor's reference cavity using the validated multiphase model was made, demonstrating the potential temperature, pressure, and density gradients inside the reference cavity at steady state. Using the guidance from the benchtop testing and CFD modeling, a prototype two-phase, self-calibrating pressure sensor was fabricated with a water volume fraction of at least 0.1 in the reference cavity. Testing the prototype two-phase sensor showed that the addition of a water liquid-to-vapor phase change inside the sensor's reference cavity can nearly triple the sensor's effective range of operation and self-calibration without increasing the power consumption of the cavity micro-heater.