A fully printed flexible multidirectional thermal flow sensor

This work presents the development and evaluation of a fully printed multi-directional thermal flow sensor on PET substrate. The device consists of conductive Ag tracks and printed thermistors based on BaTiO3, activated carbon and a solvent based thermoset polymeric system. Each element presents similar temperature-electrical resistance behavior (in terms of normalized values), thus enabling utilization as sensing and active elements for thermal two-dimensional flow sensing. A custom experimental setup was used for evaluating the device under two modes of operation, namely constant current and constant resistance. It was observed that in the range of 0 to 25 standard liters per second (SLPM) of air flow, constant resistance provided better input dynamic range than constant current (2.75 and 0.12 mW respectively), while constant resistance mode exhibited a sensitivity of approximately two orders of magnitude greater than that of constant current mode. For constant resistance mode, sensitivities of 0.46 mW/SLPM for zero flow and 0.08 mW/SLPM for flow greater than 6 SLPM were extracted, while for constant current mode the corresponding sensitivity values were 0.018 mW/SLPM for zero flow and 0.0036 mW/SLPM for flow greater than 6 SLPM. The device was capable of successfully detecting the flow direction throughout the target flow range. The influence of the flow magnitude on detecting flow angle was found to be negligible. The proposed device can be mounted to various non-planar surfaces, after the necessary re-calibration. It can be mass produced with various printing technologies for applications as a flexible two-dimensional flow sensor with low fabrication cost.