Design and Development of a Sub-micron Electrical
Particle Counter
Eric McKeever1, Suresh Dhaniyala2
Abstract:
Aerosol detection and size distribution measurement is important to assess its impact on human
health and global climate. Particle characteristics are also important for many industrial applications.
The currently available aerosol detection instruments provide high-resolution measurement, but at
significant cost and large sizes. In our research, we intend to develop a compact and inexpensive particle
detector for applications in miniature instrumentation. This detector is based on corona charging of
particles and electrical detection of highly charged particles using an image charge technique. The
detectors performance will be evaluated with laboratory measurements.
Introduction\Background:
Aerosol is a suspension of solid or liquid particles in gas with sizes ranging from 1 nm to
100  m. They can adversely affect the global climate, the urban environment, and human health. They
are also increasingly used in industrial applications such as microelectronics, pharmaceuticals, and
nanotechnology. The behavior and role of particles is a function of their size and number and thus
accurate measurements of both are required. While aerosol electrical classifiers (that can select particles
by size) are well developed (Knutson and Whitby 1975, Zhang et al. 1995) and widely used, new
detectors that are compact and accurate are required.
Currently particle counting is usually done using a condensation nuclei counter (CNC) (Agarwal
and Sem, 1978) or an electrical aerosol detector (EAD) (TSI Inc.). CNCs are widely used as they can
count individual particles but they are expensive and bulky. CNCs count particles by condensing butanol
vapor on them and then these grown particles are detected as they pass through a laser diode. EADs
charge particles and then collect them on a conducting filter and the current from the collected charges is
correlated to aerosol concentration. The low resolution of EAD makes it unsuitable for single particle
counting.
P.H.W. Vercoulen (1991) and Fuerstenau and Wilson (2003) have both developed electrical
particle detectors that can detect single particles. P.H.W. Vercoulen (1991) designed an aerosol detector
that can detect charge on large (>300 nm) particles. His device works by charging the aerosol particles
and then sending them through a coil of wire. An induced voltage in the wire is then monitored to
1
Honors Class 2004, Department of Mechanical and Aeronautical Engineering, Clarkson
University
2
Mentor, Professor in Department of Mechanical and Aeronautical Engineering, Clarkson
University
Oral Presentation
measure particle charge. Fuerstenau and Wilson developed a detector that may be used to measure the
charge of airborne particles on Mars with sizes of 1 to 100 m. They detect particles by sending them
through a tube electrode. As particles pass through the electrode an image charge is produced. That
image charge is then amplified by a charge sensitive preamplifier and the signal is monitored.
Objectives
We intend to develop a non-contact electrical particle counter that will detect sub-micron
particles individually. The detector will be used with a DMA (differential mobility analyzer) to obtain
aerosol size distributions. The detector will be able to detect sub-micron particles individually, and hence
be applicable in measurements where aerosol concentrations are low. The proposed detector is designed
such that eventual miniaturization of the instrument is possible.
The detector will consist of a tube electrode and a charge sensitive preamplifier similar to that of
Fuerstenau and Wilson (2003). To use that technique, particles will have to be highly charged (> 1000
charges), but sub-micron particles cannot be charged to those levels (Hinds, 1999). Therefore, particles to
be detected will be grown and then charged prior to detection. Particles will be grown using a
condensation growth chamber similar to a CNC. A corona charging unit will be developed to charge
particles. The sub-micron electrical aerosol detector will have several uses. For example it could have a
great impact in personal monitoring. It has been shown that particles smaller than 2.5m are inhaled and
deposit deep in the lungs (Dockery et al., 1993; Pope et al. 1995; EPA 1997).
Component Development:
The proposed detector has three major components; the condensation chamber, the corona
charger, and the tube electrode detector. Currently the corona chargers and the tube electrode detector are
being built. The condensation chamber will be the last component to be designed and built because large
particles can be used to test the functionality of the charger and detector.
The single particle detector will be similar to the one being developed by Fuerstenau and Wilson
(2003). The detector will consist of a tapered glass tube with tube electrode on the end (Figure 1). As
highly charged particles pass through the tube electrode they will induce an image charge on the
electrode.
Figure 1. Single Particle Detector
The magnitude of the induced image charge is dependent on the ratio of half the axial length of the
electrode (C) and the radius of the electrode (a) (Weinheimer 1987). As this ratio increases the ratio of
charge induced (q’) by charge on a single particle (q) approaches 1 (Figure 2). The electrode shown in
Figure 1 has a C/a ratio of 3.4 which corresponds to a q’/q ratio of approximately 0.95.
Figure 2. The Fractional Induced Charge on a Finite Electrode (Weinheimer 1987)
The charge induced in the cylinder will then be amplified by a charge sensitive preamplifier
(Cremat CR-110) and shaped by a shaping amplifer (Cremat CR-200). The amplified and shaped signal
will then be sent to a 16 bit data acquisition card (National Instruments 6036e). The resolution of the
DAQ card, the gain on the preamplifier, and the geometry of the tube electrode determine the minimum
detectable amount of charge per particle.
A corona charger will be used to charge the particles to the minimum detectable charge per
particle. The charger consists of a sharp needle held at -2.5 kV and a T pipe fitting that is grounded
(Figure 3). At this voltage the air around the tip of the needle breaks down. Positive ions are attracted to
the needle and negative ions are repelled. This creates high concentrations of negative ions ( 10 6
# / cm 3 ) in the aerosol flow. These ions attach to the large aerosol particles until the charge limit of each
particle is reached.
Corona Charger
Figure 3. Corona Charger
After the flow leaves the charger it reaches an ion trap that removes all ions that are not attached
to particles. An ion trap is an exposed wire in the flow kept at a low positive voltage (~ 20 V).
The excess negative ions in the flow are attracted and become attached to the ion trap while the
particles being larger in size flow through because they have greater inertia. The flow contains
only large highly charged aerosols after the ion trap.
Results:
The corona charger has been built and tested. Ion concentrations of 106 ions per cm3 have been
measured. The ion concentration downstream of the needle has been observed to increase exponentially
as the flowrate past the needle increases. This ion concentration peaks when the flow is removing ions
from the needle at the same rate that the needle is producing them. The detector is currently being
assembled and it will be tested shortly.
Conclusions and Future Work:
A sub-micron electrical particle counter is being developed. This detector will be useful in
experiments where individual particle detection is desired. The detector is electrical, so eventually it may
be miniaturized and produced inexpensively.
Acknowledgements:
I would like to thank Stephen Fuerstenau and Gregory Wilson for the information they provided on single
particle detection.
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