AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Improved Coincidence Correction in Condensational Particle Counters
STEVEN SPIELMAN, Gregory Lewis, Susanne Hering, Aerosol Dynamics Inc.
Abstract Number: 705 Working Group: Instrumentation and Methods
Abstract As with any instrument that records discrete events, an airborne particle counter can benefit by correcting for the fact that multiple particles can arrive simultaneously. For this work, we assume that condensationally-enlarged particles scatter light, which is captured by an optical detector and converted into an electrical signal. Pulse heights in this application are fairly uniform, even though the average rate can vary by many orders of magnitude. At low concentration, particles can be accurately counted by simply triggering on every pulse that exceeds some threshold. However, while the signal is above the threshold, the detector cannot detect an additional particle. This "dead time" can be inferred by assuming fixed pulse width, or by direct measurement. Some instruments, for example the TSI 3787, further refine the correction by recognizing that nearby pulses tend to join, due to the overlap of their tails. This technique works well up to an average particle rate of about 1.0 particles per FWHM (the full-width at half-maximum of a pulse) with a measured dead time of 80%. It fails at higher concentrations, when pulses are likely to include three or more particles.
We have developed a coincidence correction algorithm that can handle average pulse rates as high as 3.0 particles per FWHM. Accurate particle concentrations can be determined even with a dead time of 99.5%. We present modeling and experimental results, with both AC- and DC-coupled systems.