Fabrication and testing of a thick film flow sensor

This work centers on the design of a Thick film Mass flow Sensor, its fabrication and testing. The principle of operation is based on the phenomenon of loss of heat from the heated resistors to the flowing mass. The design of the sensor fixture is such that the liquid or air flows over equally heated, similar and high TCR value thick film resistances of a Wheatstone bridge. The flow of liquid or air causes uneven cooling of the resistors. This uneven cooling results in an offset due to the imbalance in the bridge. The offset voltage is dependent on the temperature difference of the bridge thermistors which in turn varies in accordance with the mass flow rate. Water and 95% nitrogen are the masses used to test the sensor. Standard techniques of thick film technology are employed for the sensor fabrication. A traditional calibrated water clock setup with an adjustable flow rate valve is used to test the performance with water. A 95% nitrogen supply line from the microelectronics laboratory with a calibrated flow meter attached to the fixture is used to test the performance with gas. The final output graphs for the sensor against water and 95% nitrogen imply that the offset varies in accordance with the flow rate as all other parameters such as the power supply and temperature of the mass are kept constant. The sensor is found to be highly sensitive at lower flow rates and the sensitivity gradually decreases with the increase in flow rate.