The Trials and Tribulations of Scanning Electrical Mobility Measurements

FRED BRECHTEL, Andy Corless, Xerxes Lopez-Yglesias, Brechtel Mfg. Inc.

     Abstract Number: 600
     Working Group: Exhibitor and Instrument Application Showcase

Abstract
Rapid voltage scanning in the differential mobility analyzer (DMA) in scanning electrical mobility sizers (SEMS) radically reduced the time required to measure number size distributions. Although the method has become ubiquitous, there is still confusion regarding various important parameters, including: detector time response, upstream impactor cut size, charger, plumbing delay time, appropriate scan times, relative humidity, and optimizing operating parameters for maximum data quality.

For example, the minimum scan time (t_scan) is determined by the ratio of the mean gas residence time in the DMA and the characteristic time for the voltage ramp (=t_scan/ln[HV_max/HV_min]). Here, the DMA sheath flow rate, DMA dimensions, and the voltage scan range each play a role. The ratio should be smaller than about 0.2 to ensure the voltage ramp has sufficient time to act on particles within the DMA. Another practical limitation of very short scan times is poor counting statistics if insufficient particles are counted in each size bin.

The plumbing delay time is the travel time of a particle from the entrance slit of the DMA to the detection point of the detector. It depends on the DMA dimensions, sheath and sample flow rates, interconnect tubing volume between DMA and detector, and the transport time within the detector. Given how many factors determine the plumbing delay time, it’s no wonder confusion surrounds it. The best determination of the plumbing delay time is where the up and down-voltage scans of PSLs match.

To demonstrate the impact of the factors listed above, a miniature SEMS (Brechtel Model 9404) will purposely be operated with both correct and incorrect values to demonstrate the impact on the measured ambient size distribution.