A HIGHLY PRECISE AND LINEAR IC FOR HEAT PULSE BASED THERMAL BIDIRECTIONAL MASS FLOW SENSOR

Abstract

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. The design features high precision response received from analog integrated circuits. A computational fluid dynamic (CFD) model was designed for simulations with air and water Using COMSOL Multiphysics. Established mathematical models for the heat flow equations 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 to detect the bi-directional mass flow. Platinum sensors were used due to their linear characteristics within 0 ºC to 100 ºC range, and their high temperature coefficient(0.00385 Ω/Ω/ºC). Polyimide thinfilm heater was used as the heating 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. Three high precision instrumentation low power amplifiers with offset voltage ~2.5μV (50μV max) were used for bridge signal amplification and the difference circuit. The difference circuit was used to indicate the flow direction. A LM555 timer chip was utilized to provide the heat pulse period. Simulation and experimental measurements for heat pulses with different amplitude (temperature) were in good agreement. Also, the sensitivity of the flow sensor was observed to remain unaffected with the change in the duty cycle of the heat operation mode.