Design and testing of an aerosol sheath inlet for high resolution measurements with a DMA

A modified aerosol/sheath inlet was designed for a differential mobility an alyzer (DMA) for high resolution measurements based on field model calculat ions which include fluid flow, electric field, and convective/diffusive tra nsport. To avoid the predicted flow recirculation for the current inlet des ign at an aerosol-to-sheath flow ratio of 0.05, the slit width is reduced a nd aerodynamically shaped so that the sheath velocity and aerosol velocity more nearly match. Numerical results are presented comparing the fluid flow of the old and new inlet. Problems associated with the old inlet include: flow unsteadiness at a flow ratio of 0.025, voltage shift at the peak parti cle concentration as a function of the flow ratio, and the historical obser vation that, while performing tandem differential mobility analyzer measure ments (TDMA), the voltage applied on the second DMA for the peak particle c oncentration is higher than that for the first. Measurements demonstrate th at all these problems are reduced or eliminated with the new inlet design. The TDMA measurements include flow ratios of 0.1, 0.05, 0.025 and 0.0125 at sheath flows of 166 and 333 cm(3) s(-1) (10 and 20 l min(-1)). The challen ge of performing measurements at these low flow ratios will be discussed in cluding flow calibration, flow matching, and pressure monitoring. The new i nlet is applied to the measurement of the National Institute of Standards a nd Technology 0.1 mu m Standard Reference Material 1963, and it is shown th at the DMA can accurately measure the standard deviation of this narrowly d istributed aerosol (sigma/D-p = 0.02). (C) 1999 Published by Elsevier Scien ce Ltd. All rights reserved.