Gas separation: processes – With control responsive to sensed condition – Electric or electrostatic field
Patent
1995-01-19
1997-03-04
Chiesa, Richard L.
Gas separation: processes
With control responsive to sensed condition
Electric or electrostatic field
55270, 55DIG39, 95 78, 96 19, 96 64, B03C 336
Patent
active
056074972
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a passive dust sampler.
BACKGROUND OF THE INVENTION
Dust samplers are known in which the atmosphere containing the dust to be sampled is pumped through a collector (such as a filter medium), which traps the dust for subsequent analysis or measurement. Such a sampler requires a pump, which is bulky, and the pump in turn requires a power source, which adds to the mass and bulk. Because these samplers positively transport the dust to the collector, they may be called active dust samplers.
For personal use, it would be desirable to have a passive dust sampler, i.e. one without a pump or power source, which would sample dust at a rate proportional to its atmospheric concentration in the immediate vicinity, biassed (ideally) towards smaller particles, these generally being more harmful to the human lung. A personal sampler should also be lightweight.
A passive dust sampler must work by moving the sampled material relative to the air. Passive gas samplers make use of the diffusive motion of molecules to transport them to an adsorbing surface. Since the masses of all molecules of any chemical species are virtually identical, the average velocity of transport can be easily and accurately calculated. Dust particles also undergo diffusive motion but their average velocity is many orders of magnitude smaller than that of molecules, and so this is not an effective transport process. Capture must be brought about by external forces, and the only forces sufficiently large to be effective are gravity and electrostatic attraction.
J. Pich in Aerosol Science (1966) Ed. C. N. Davies, Academic Press, London 1966, discloses that a uniformly charged fibre captures charged particles at a rate dependent on the number of the charged particles passing per unit volume but independent of their velocity. In addition, the rate at which the charged body will collect charged particles of the opposite sign will be proportional to the electrical mobility of the particles. The electrical mobility of a particle is the velocity of the particle caused by an electric field, divided by the field strength.
If capture takes place by gravity, the same velocity-independence is observed, but the capture rate is proportional to the square of the aerodynamic diameter of the particles, and this would favour the capture of large particles and selectively exclude small ones, the opposite of what the invention seeks.
Vincent et al. in the Institute of Occupational Medicine's Report TM83/15 (1983) describe the variation of electrical mobility with particle size of industrial aerosols and find that although there is a tendency for larger particles to have a higher electrical mobility the distribution would lead to a much lower bias than that of gravitational capture.
SUMMARY OF THE INVENTION
The dust sampler according to the present invention comprises an insulator (for example an electret) in the form of a collector sheet having a known electrical charge when in use, mounted on a non-insulating base (e.g. certain glasses are sufficiently non-insulating), the sheet being surmounted by a conductor parallel thereto and preferably of at least the area of the sheet, the conductor being supported, and preferably electrically connected to the base, by (therefore preferably conductive) spacer means such as pillars or gridwork around the sheet, but preferably enclosing an area at most 110% of, or, in other cases, at least double or not more than quadruple, the sheet area. It will be understood that "non-insulator" and "conductor" are used interchangeably herein and can include not only metals etc. but also any material sufficiently conductive to be unable to retain a permanent charge, such as a semiconductor. The spacer means is for mechanical protection of the insulator and for support of the conductor, and is such that air can flow through the space between the insulator and the conductor. The spacer means may take the form of gridwork preferably not larger than 11/2 cm normal to the sheet, more preferably not larger than 1 cm. Its
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J. Pich. "Theory of aerosol filtration by fibrous and membrane filters" Institute of Physical chemistry, Czechoslovak Acad of Sciences, Prague, Czechoslovakia, pp. 223-285, 1966.
J. H. Vincent et al., "Measurement of the static electrification of airborne dusts in workplaces" Institute of Occupational Medicine. Report No. TM/83/15, Sep. 1983, pp. 1-112.
Pillai et al, "Applications of polymer electrets for pollution studies", Applied Physics Letters, vol. 52, No. 18, May 2, 1988, pp.1540-1541.
British Technology Group Limited
Chiesa Richard L.
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