Calibration of hot-wire detector for particle concentration and size distribution in closed spacecraft cabin atmosphere

Burgess, W. A.

Reist, P. C.

English

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Study of space cabin atmospheres  Status report, Jul. 1 - Dec. 31, 1967

Aerosol generation for instrument calibration, calibration of aerosol particle analyzers, data analysis computer program, and particle monitoring in space cabin atmosphere stud

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Study of Space Cabin Atmospheres
NASA Research Grant NGR-22 -007-053
Status Report for the Period
July 1, 1966 to December 31, 1.966
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	ICATEGO
Submitted by
William A. Burgess
and
Parker C. Reist
Harvard School of Public Health
V
March 31, 1967
I. Summary of Acti-Tities
This report covers the work period from July 1 through
December 31, 1966. During the period, the principal activities
were on the generation of aerosols for instrument calibration,
the calibration of several Aerosol Particle Analyzers, the design
of a computer program for analyzing collected data, and the
evaluation of various sensing methods for monitoring large particles
in spacecraft.
A. Aerosol Generation for Instrument Calibration
The need for a monodisperse aerosol for the calibration
of light scattering devices and our use of polystyrene latices
to satisfy this need has been described in previous status
reports. Briefly, the generation technique involves the
atomization of a suspension of polystyrene latex particles in
water with a conventional nebulizer and subsequent drying of the
droplets using a dilution air stream. Theoretically, on
evaporation of the water, a monodisperse aerosol should be
produced. In practice one notes some dispersity in size since
some multiple particles may exist. In addition, contamination
of the latex solution can produce foreign particles of varying
sizes. Usually, however, the number of multiple particles is
small compared to the number of single particles, and, by using
ultra-pure dilution water, the number of particles arising
from contamination can be minimized.
As cited in previous reports our initial work with this
aerosol indicated that, in addition to the primary polystyrene
.V
latex, the light scattering device was detecting high
concentrations of particulate material less than O.3µ in
diameter. For a given percent solid concentration of the
latex :_n water, the relative air concentration of these
secondary particles increased with increasing particle size.
Other investigators have noticed this material and it has been
suggested tt:at :,hese secondary particles are dried emulsifier
which is present in droplets not cDnt^, ining latex spheres.
It is thought that the water soluble emulsifier contained in
droplets having a latex sphere plates out on the primary particle
surface during droplet evaporation and a single particle results.
However, when a droplet is generated which does not c::ntain
a latex sphere, it leaves a residue of emulsifier as a secondary
particle after drying. These secondary particles seriously
hampered the effective use of polysa-yrene latex as a calibrating
aerosol especially in the latex sizes below one micron.
During this report period we investigated the feasibility
of reducing the concentration of this secondary aerosol by
washj_ng the lat::x particles with :rater prior to use. Washing
is carried out with water filtered through a 0.1µ pore diameter
membrane fil".:er provided with a wire mesh backing plate. A
porous glass backing plate was used in early work but it was
found to introduce additional particulate material into the
wash water. In the washing procedure the original solution,
containing the latex particles, is filtered through an
-3-
appropriately sized membrane filter which passes the secondary
aerosol but filters out the latex spheres. Several volumes of
ultra-filtered water are then used to wash the latex spheres
collected on the filter. Finally, the spheres are resuspended
in another portion of ultra-filtered water. With this technique
we have been able to reduce the air concentration of the
secondary particles by a factor of at least ten. This reduction
has been helpful in our calibrating work, however, further
reduction is needed to provide a reasonable calibrating aerosol
in the size range below one micron.
A second phase of our activity during this report period
has been the study of the problems noted in the prediction
of the primary aerosol number concentration directly from
generator operating conditions. Obviously, the ability to
predict concentration would enhance the value of the aerosol
generator. During this study we noted that the air concentration
of latex particles increases with time as the nebulizer is
operated. There are several possiblA reasons fnr thifi hahat*inr.
The increase in concentration could be due to increased
evaporation of the liquid within the nebulizer, physical
blocking of the particles by the nebulizer orifice, or the
refluxing action of the solution. It is our opinion that the
refluxing action of the nebulizer causes this behavior. In any
event, we have found that if the nebulizer is operated in a
mode which wastes the liquid not aerosolized, that is, if there
is no refluxing, the generator output can be kept constant.
-I4-
If this operational mode cannot be tolerated, the concentration
increase can be eliminated by frequent changes of solution.
B. Instrument Calibration
A major part of our effort during this report period
was directed to a ,joint effort with Electronic Research
Center (ERC) personnel in the calibration of a number of
Aerosol Particle Analyzers (APA).
In these studies three different sizes of polystyrene
latex were used. The actual particle concentrations in the
calibrating airstream delivered to the instrument were deter-
mined by sampling with a membrane filter at a sampling rate
of about 7 liters per minute and counting the collected
particles using dark field microscopy.
Three sets of data were collected with each APA unit for
each particle size, and the results averaged. A set of back-
ground data was also collected and the average background was
subtracted from the particle size data. Instrument concentration
data were estimated by multiplying the APA count by 60.
Preliminary results show that the unit does not have sharp
channel responses. An ideal response would be a vertical line,
i.e. all particles of a given size would appear in a certain
channel and not be seen in any other channel. However, due to
the finite channel width,coupled with the occurrence of multiple
particles and fractional light scattering, these curves take a
characteristic shape with a distribution of counts in all
channels.
Table 1 gives measured APA efficiencies for the three
particle sizes used in these studies. For the 0.796µ and
1.305µ particles, the total 5 channel count was used. For
the 2.688 particles only, the counts in channels 1, 2, and
3 were used, since the counts in 4 and 5 were considered to
arise from instrument or aerosol background.
Table 1
Measured Counter Efficiency
APA Unit #3
Particle Size µ	 Actual Particle Cone.	 Measured Cone.
	
Counting
particles/ cu.ft.	 particles/cu.ft. Efficiency
0. 796' ---- 8.12 x 104	----
1.305 9.5 x 10= 5.38 x 105	 56.6
2.68 4.9 x 105 3.48 x lo5	 71.0
C.	 Computer Analyses of Collected Data
Characterization of a group of heterogeneously distributed
particles by a mean value and an associated standard deviation
has been a standard procedure for many years. Assumption of
normally distributed particle sizes has permitted the use of
probability paper for rapid determination of the mean and
standard deviation. For some skewed size distributions, log-
probability paper has been suggested to transform the distribu-
tion to a normal distribution.
Because many particle size distributions approxomate a
straight line when plotted on log-probability paper, it is
V-6-
general practice to determine the mean and standard deviation
graphically without resorting to slower, but more precise,
hand calculation procedures. Unfortunately, use of log-
probabtlity paper forces one to assume a log-normal size
distribution which in some instances, such as for particles
found in a non-gravitational environment, may not be the case.
Therefore, it was considered desirable to provide a rapid
method wing a digital computer for calculating deviation
parameters of a heterogeneously distributed cloud, which would
include arithmetic and geometric values of number, surface area,
and mass diameter, associated standard deviations and particle
number and mass concentration.
The staff has prepared a report describing a Fortran II
computer program for the IBM 1620 unit which makes these
computations. In this program, particular attention has been
paid to output format so that the data generated can be used
directly by individuals having little specific knowledge of the
program. This report, which was to have been issued during
this quarter on a local basis, will be submitted during the
next report period for consideration as an official NASA release.
D. _valuatin of Particle Sensors
An initial survey has been conducted of the three particle
sensing methods chosen for a feasib}lity study for monitoring
large particles in the spacecraft, namely the piezo-electric
crystal acoustical, and hot wire sensors.
-7-
This p iezo-electric cyratal sensor has been used for the
detection and size measurement of micro-meteoroids. Such
particles colliding with the crystal element present an
electr.cal output whose pulse height is proportional to
their diameter. A critical review of the piezo-electric
crystal sensor suggests that this sensor does not have the
sensitivity required for our application.
In our evaluation of the piezo-electric crystal for the
measurement of particle size distribution in the less than 30
micron range, it was necessary to consider the minimum momentum
sensitivity of existing crystal elements to establish the
particle detection sensitivity. The minimum momentum sensitivity
of a microphone detector for the Explorer I satellite is in
the order of 10-3 dyne-sec with 10 -5 dyne-sec as the feasible
minimum attainable in the future. Utlizing the level of 10-5
dyne-sic and considering impaction velocities of 1000 cm/sec,
one could detect particles down to approximately ^-5 microns.
A velocity approximately one order of magnitude larger would
be required to detect a 10 micron particle. This analyses
suggested that the piezo-electric detector would not be useful
for particle measurement in the size range below 30 microns,
and therefore further work on this sensor does not seem warranted
at the present time.
The hot wire and acoustical sensors are being evaluated and
will be discussed in the next status report.
-8--
II. Future Activities
The current work effort will be extended during the next
report period in the following areas:
1. Aerosol generation techniques will continue to be studied
to provide more suitable calibrating aerosols for light
scattering instruments.
2. The performance of hot wire and acoustical sensors will
be evaluated and the suitability of such devices for
speoecraft , meaeurements will be evaluated.


Study of space cabin atmospheres Status report, Jul. 1 - Dec. 31, 1967

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