Measuring and testing – Particle size
Reexamination Certificate
2000-08-14
2002-06-25
Raevis, Robert (Department: 2856)
Measuring and testing
Particle size
Reexamination Certificate
active
06408704
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and devices for measuring airborne particles. In particular, the present invention relates to a method and apparatus for determining the concentrations of airborne particles in an aerosol as a function of aerodynamic particle size.
BACKGROUND OF THE INVENTION
The control of industrial and other environments increasingly requires ongoing knowledge of the quantities of airborne particles in the atmosphere. Dust and other non-gaseous particles are found suspended in air or other gaseous media. Such particles suspensions are referred to herein as aerosols. The effects of airborne particles as well as the control of airborne particles are dependent on the ways that the different particles move.
The motion in air of airborne particles and the deposition of such particles onto surfaces depends primarily on physical particle size. However, density, shape, surface properties and other characteristics of the particles also influence their airborne behavior. Usually these characteristics as well as all characteristics that affect particle motion and deposition are combined with the physical particle size into a property referred to as “aerodynamic particle diameter” or “aerodynamic particle size”. A common method for determining the size distribution of particles in a volume of air, or the “aerosol size distribution” of the particles, is to collect the airborne or “aerosol” particles on a filter and to subsequently size them under an optical or electron microscope. However, this method only yields the physical size, not the aerodynamic particle size. To determine how these particles behave in air, in addition to determining the physical particle size by this method, it would be necessary to determine the other characteristics of the particles that will affect their airborne behavior. This technique is time consuming. Therefore, methods of dynamically measuring the sizes of particles dynamically, while suspended in air, have been preferred.
A common method for dynamically measuring the concentration and size distribution of particles in the airborne state uses techniques such as optical single particle size spectrometry. In this method, one particle at a time is passed through an illuminated view volume, and the magnitude of light scattered by each particle is recorded. An optical single particle counter when used with this method is usually calibrated with spherical, monodisperse test particles of known particle density and optical characteristics, such as polystyrene latex (PSL) spheres. However, most airborne particles have their own light scattering and absorption characteristics, so that the “optical particle diameter” measured by single particle size spectrometry generally does not correspond to the “aerodynamic particle diameter”.
When using the optical single particle counting method, the device embodying this method can be dynamically calibrated to measure aerodynamic particle size by placing an impaction stage at its inlet. An impaction stage eliminates, from the aerosol, particles having an aerodynamic size above an aerodynamic threshold or “aerodynamic cut”. The aerodynamic cut of an impaction stage can be determined by theory or experiment. Through this type of calibration, each optical particle size of the optical single particle counter can be related to its equivalent aerodynamic particle size. To operate an optical single particle counter in the field, the impaction stage is removed from the inlet. When an optical single particle counter is calibrated over a wide particle size range, several impaction stages with different aerodynamic cut sizes are successively attached to the optical single particle counter's inlet. This calibration is done for each aerosol of a different anticipated airborne particle composition. Such a calibration process is very time consuming.
A simpler dynamic optical particle sensing method is “aerosol photometry”. In an aerosol photometer, the optical view volume is larger than in an optical single particle counter, thus accommodating a cloud of particles. However, the output from such a device depends not only on the optical characteristics of the particles, but also on the size distribution of all of the particles in the view volume. The light scattered by each particle depends on its refractive index and size.
Another dynamic, but more complex and expensive method for determining the aerodynamic particle size distribution in situ is by accelerating the aerosol particles in a nozzle and then measuring the “time of flight” of each particle between two points. In a nozzle or other acceleration field, aerodynamic drag accelerates large particles to a lesser extent than small particles. Thus, in an acceleration field, the time of flight between two points is longer for large particles than for small particles. The difference is caused by the difference in aerodynamic drag. Thus, this method determines the particle size distribution through a number of different techniques and geometric arrangements, as illustrated, for example, in U.S. Pat. Nos. 4,633,714; 5,296,910; 5,561,515; 5,679,907; and 5,701,012.
Based on the above, there remains a need for a more efficient and more easily used method and apparatus for measuring the concentrations of airborne particles, and particularly for determining the aerodynamic particle size distribution in an aerosol.
SUMMARY OF THE INVENTION
An objective of the present invention is to determine the concentrations of airborne particles in an aerosol as a function of aerodynamic particle size. A further objective of the present invention is to provide a method and apparatus by which airborne particle concentration in an aerosol can be simply, economically and rapidly determined.
A particular objective of the present invention is to provide a method and apparatus for dynamically measuring concentrations of airborne particles in an aerosol as a function of their aerodynamic particle sizes or diameters.
According to the principles of the present invention, an aerosol, or air that contains airborne particles, is passed through an aerodynamic cut device which performs an aerodynamic cut of the particles entering an aerosol particle sensor. The airborne particles in the aerosol can be any inert or viable airborne particles, examples of which include dust, fume, smoke, fog, mist, bacteria, pollen, fungal spores, fragments of biological or non-biological material and other non-gaseous particles, or combinations thereof. Preferably, the aerodynamic cut is achieved by a rotating element in the aerodynamic cut device that centrifugally removes from the aerosol particles above or below a specific aerodynamic cut size.
In accordance with the preferred embodiment of the invention, the aerodynamic cut is achieved by a rotating element in the aerodynamic cut device which centrifugally removes particles above a predetermined or selected aerodynamic particle size from the aerosol, preferably to a collecting surface within the aerodynamic cut device. The rotating element may include one or more disc or propeller blades mounted on a rotating shaft, or an impeller with radial, forward-curved or backward-curved blades or other structure that accelerates particles outwardly from the axis of the rotating element. The collecting surface may be an inner wall of the housing containing the rotating element or a rotating enclosure or may be an enclosure that contains the rotating blade or other such structure and which rotates with such structure about its axis of rotation.
In certain preferred embodiments of the present invention, the aerodynamic cut is continuously or intermittently varied by variably controlling the configuration or motion of the rotating element of the cut device so as to change the distribution of particles in the aerosol entering an inlet of an aerosol particle sensor. Preferably, the variation of the rotating element is achieved by operating the element at a sequence of rotational speeds, preferably a sequence in which the rotational speeds are varied fro
Raevis Robert
Wood Herron & Evans L.L.P.
LandOfFree
Aerodynamic particle size analysis method and apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Aerodynamic particle size analysis method and apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Aerodynamic particle size analysis method and apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2958153