Heat Balance of Micro Hot-Film Sensor Elements Under the Different Gas Operation

The precise measurement of a mass flow rate of the gas is strongly required from the viewpoints of energy saving and the environmental problem in the manufacturing process. It is necessary to develop an accurate sensor for the mass flow rate with high sensitivity. The present micro hot-film sensor fabricated by utilizing MEMS technology is a thermal type gas flow sensor which is used for the measurement of the mass flow rate. The sensor which consists of the two heaters in a 1 μm-thick silicon-nitride membrane with cavity structure can detect the near-wall flow velocity proportional to the two heater electric power consumption. The static characteristics of the sensor have been poorly understood under the different gas operation instead of air. The purpose of the present paper is to clarify the heat balance of the sensor element corresponding to the heater as well as the static characteristics of the sensors by the experiment and the simulation, when the sensor is applied to the measurements for the different species of the gas. The air and helium are used as the working fluid. It is found that a unique correlation on the heater electric power is obtained regardless of the gas species used and the arrangement of the heater element. And, the simulation by SPICE provides with a similar tendency to the experiment about the arrangement of the heater element.