The Dust and Aerosol Measurement Feasibility Test
(DAFT) was designed to ensure that a modified P-Trak®—a key
component of the forthcoming NASA Smoke Aerosol Measurement Experiment
(SAME)—will perform properly in the unique environment of microgravity.
If the P-Trak® performs as expected, the device will be used in
SAME to provide data that will help scientists design better fire detectors
for future, longduration, manned missions.
The P-Trak® is a commercial device that
counts ultrafine particles (it can recognize particles as small as
0.02 micrometers in diameter). It works by passing particulate-laden
air through a heated chamber of vaporous isopropyl alcohol. The individual
particles serve as seeds around which the alcohol condenses when cooled,
forming droplets large enough to be detected and counted as they scatter
light from a laser beam (see the schematic on the other side). The
must be tested in microgravity prior to its use in SAME because
its alcohol condenser was redesigned to work properly in microgravity.
(The original smooth-walled P-Trak® condenser—meant for
Earth based use—depends on gravity.) NASA scientists modified
the unit’s condenser by forming microgrooves in its wall to
increase the alcohol flow back to the wick (see photo on right).
This modification was based on the knowledge gained from previous
microgravity fluids physics experiments conducted by researchers
at NASA Glenn Research Center
The main components of the
DAFT experiment, the P-Trak® and the DustTrak® aerosol monitoring
devices, were delivered to the International Space Station (ISS)
in February 2005. (The DustTrak® uses a sensor to determine
the intensity of light being scattered by particles and translates
that number into an aerosol mass measurement. It is being used to
test the accuracy of the P-Trak®
because it is insensitive to gravitational forces.) The rest set of
tests were conducted in February and March 2005 and the results were
encouraging. Importantly, the muddied condenser performed as expected.
The second set of tests will be conducted after the balance of DAFT
parts (including a calibrated aerosol source and an unmodified P-Trak®)
are delivered to the ISS.
the ISS tests conducted in 2005, astronauts performed several
steps: 1. Turned on the P-Trak®
and the DustTrak® 2. Attached high-efficiency particulate air
(HEPA) filters to the inlets to “zero” the particle counters
(since the HEPA filter removes all airborne particles, if the particle
counters do not read “zero” something is amiss) 3. Connected
the devices to a common sampling hose 4. Recorded the ISS environment
for several minutes 5. Reconnected the HEPA filters to “zero” the
particle counters 6. Turned off the P-Trak® and the DustTrak® The
tests were conducted in the U.S. Laboratory Module Destiny in front
of EXPRESS Rack 4; in the modules’s aft end and port side and
in Node 1. (The EXPRESS Rack is a standardized payload rack system
that transports, stores, and supports experiments aboard the ISS.
Node 1 is a hub to which various modules are attached.) Three of the
tests sampled undisturbed cabin air; the other test validated the
instruments at high particulate levels by having an astronaut create
airborne debris at the P-Trak® and DustTrak® inlets by separating
pieces of Velcro®.
Results So Far
The three tests that sampled the undisturbed
environment in the ISS showed very low levels of airborne particulates,
averaging fewer than 0.005 mg/m3 from the DustTrak® and fewer
than 15 particles/cm3 from the P-Trak® (these are typical readings
at the baseline). These numbers are dramatically lower than the values
recorded in the space shuttle in an earlier experiment (~0.050 mg/m3).
Lower levels are to be expected because the ISS has HEPA filtration
for the cabin air as compared to merely a fine screen on the shuttle
air handler. Furthermore, the typical shuttle crew of seven astronauts
generates more airborne particulate than an ISS crew of two astronauts.
Future DAFT Tests
The next series of tests will take place
once the remaining DAFT equipment is delivered to the ISS. These will
be performed with Arizona Road Dust (ARD)—a standard aerosol
test material of known particle size and distribution—and nitrogen
aerosol inside a 15-liter Mylar® bag. Testing a known particulate
will allow scientists to establish definitively how well the P-Trak® works