The electrohydrodynamic micro flow meter

A method for the measurement of fluid flow and velocity was developed. It is based on the structure of a micromachined electrohydrodynamic (EHD) injection pump. This technique is especially suited for small flow rates from 100 ml/min down to the sub- mu l/min range, which is difficult to measure by known methods. The method presented is based on the measurement of the ion transit time between two grids. Upon application of a step voltage between the grids a cusp is observed in the associated transient space-charge-limited current. The cusp signifies the arrival of the first charge carriers at the opposite electrode and hence the ion transit time. The transit time increases or decreases depending on the flow rate and direction and is therefore a direct measure of the fluid flow rate. This behavior was inferred from 1-D calculations and subsequently confirmed by experiments. Volumetric flow rates down to 8 mu l/min were measured with a 2.5*2.5 mm/sup 2/ grid area.<>