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 analyzer (DMA) for high resolution measurements based on field model calculations which include fluid flow, electric field, and convective/diffusive transport. To avoid the predicted flow recirculation for the current inlet design at an aerosol-to-sheath flow ratio of 0.05, the slit width is reduced and 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 particle concentration as a function of the flow ratio, and the historical observation that, while performing tandem differential mobility analyzer measurements (TDMA), the voltage applied on the second DMA for the peak particle concentration is higher than that for the first. Measurements demonstrate that 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 cm³ s^-1 (10 and 20 l min^-1). The challenge of performing measurements at these low flow ratios will be discussed including flow calibration, flow matching, and pressure monitoring. The new inlet is applied to the measurement of the National Institute of Standards and Technology 0.1 μm Standard Reference Material 1963, and it is shown that the DMA can accurately measure the standard deviation of this narrowly distributed aerosol (σ/D_p = 0.02).