10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Enabling Long-Term Operation of a Motion-Tolerant, Water-Based Condensation Particle Counter
STEVEN SPIELMAN, Gregory Lewis, Nathan Kreisberg, Susanne Hering, Aerosol Dynamics Inc.
Abstract Number: 1586 Working Group: Instrumentation
Abstract The International Space Station presents a unique challenge for monitoring the particulate air quality to which astronauts are exposed. Sensors must be small, operate at near zero-gravity, and yet be capable of long-term operation. Other applications, such as personal monitoring, require tolerance to tipping and jarring, which imposes similar limitations on the instrument design. While optical and electrometer-based sensors generally meet the requirements of motion-tolerance, zero-gravity, long-term operation, this is not the case for condensation particle counters.
Previously we introduced motion-tolerant, a self-sustaining, water-based condensation particle counter. Called MAGIC, for moderated aerosol growth with internal water cycling, this instrument has no internal water reservoirs, and thus tolerates tipping and jarring, yet operates for days to weeks on a single charge of the wick. Using the “moderated” water condensation growth tube approach, water vapor from the heated, wet-walled condensation section is captured by the wick in the cooled downstream section. Sustained operation is accomplished through a combination of extraction of additional water from the sampled air stream, and internal capture of water vapor by the wick. Because there are no exposed liquid reservoirs, the instrument can be operated in any orientation, or under zero-gravity.
Presented here is the adaption of MAGIC to long-term operation, with the aim of providing many months of continuous operation. Key developments are a (1) wick moisture sensor to enable direct control of the wick water, and (2) a new dead time correction method which allows use of a wider, and more robust focusing nozzle without loss of high particle concentration measurements.
The wick moisture sensor uses an optical sensor to assess the wick transparency. Because the wick material is a porous structure with a fine (sub-10 µm) pore size, it scatters light when dry. When wet, the pores fill with water and it becomes translucent. We inserted a dark material under the outer layer of the wick to creates a structure that darkens when wet. The sensor measures the reflected light to generate a signal that varies with moisture content. Tests under conditions in which the wick is slowly dried demonstrate that the sensor shows deviation from a fully wetted state well in advance of the loss of instrument performance. Coupled with an algorithm to control the temperature of the water-recovery stage, this provides a direct, verifiable means to ensure long-term maintenance of a properly wetted wick.
The second change focused on development of a droplet detector that is less sensitive to optical alignment. This in turn leads to longer light scattering pulses for individual droplet, and hence a higher rate of coincidence for the same particle concentration. As in many condensation particle counters, we directly measure fraction of time during which the scattered light signal is above the threshold, and therefore unable to distinguish an incoming, coincident pulse. Yet, because of the finite threshold and overlapping tails of neighboring pulses, this measured time is longer than the effective dead time. For years condensation particle counters have estimated the effective dead time by multiplying the measured dead time by an empirical dead time correction factor. However, at very large dead times this approach over-corrects. Presented here is an alternative approach that provides accurate counting when measured dead times are in excess of 90%.