A conduction-convection design for liquid flow sensing

Abstract A modified hot-wire fluid-flow sensor design is described in which the convection of heat takes place at a distant point from the temperature sensor itself. The distant arrangement is also applied to the cold point of the flow meter. Thermal contact between the sensors themselves and the distant points is ensured by metal leads. This novel approach is implemented by using a low-cost packaging solution consisting of a lead frame and low-thermal-conductivity plastic epoxy encapsulation. The sensors are bipolar junction common-collector transistor arrays, the hot-point sensor being surrounded by an NiCr resistor. The system is completed by a sigma-delta thermoelectric converter which ensures the feedback loop and provides a digital readout. This device has been fabricated and tested and shows a sensing range of 0.2–91 min −1 water flow.