MULTIPARAMETER DETECTION IN FLUID-FLOWS

A micromechanically fabricated thermal flow sensor is presented. The s ensor incorporates multiple, 100 mu m spaced, resistive sensing elemen ts on a glass substrate in a silicon flow channel. This sensor works o n the principle of a travelling heat pulse through the fluid. The resp onse to this heat pulse at different positions upstream and downstream from a heater is used to determine not only flow velocities but also fluid properties. Theoretical analysis of the sensor response shows th at the sensor is more able to discriminate between flow velocities and fluid properties when certain combinations of sensing element signals are used. It is shown that the sensor can also measure mass flow as l ong as the 'time of flight' of a heat pulse can be measured at equal d istances from the heater upstream and downstream. Combination of the ' time of flight' at two different positions downstream can be used to d etermine the diffusivity of the fluid. The sensor can be made sensitiv e to flow velocity by taking the heat pulse response at two different locations downstream at an instant in time when the signal amplitudes are equal. The 'time of flight' measured at one position downstream is only accurate when the velocities are high enough since the diffusive effect can be neglected. The ability of an artificial neural network to learn to discriminate between the flow velocity and fluid propertie s is analyzed.