Optics: measuring and testing – By particle light scattering – With photocell detection
Reissue Patent
1997-08-25
2001-09-04
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S246000, C250S576000
Reissue Patent
active
RE037353
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to air quality and, more particularly, to instruments for assaying airborne particulates.
BACKGROUND OF THE INVENTION
Particle counters and sensors are used to detect light scattered by particles entrained in a stream of fluid, e.g., in an air stream. Such counters and sensors draw air (with entrained particles) from a room, for example, and flow such air along a tube and through an illuminated sensor “view volume” to obtain information about the number and size of such particles. Such information results from an analysis of the very small amounts of light reflectively “scattered” by the particle as it moves through the view volume.
Some types of sensors flow such air along an enclosed transparent tube; others “project” the air and accompanying particles at a particular flow rate (often measured in cubic feet per minute) from one tube across an open space to another tube. In sensors of the latter type, there is no tube wall (however transparent such wall may be) to impair light scattering and collecting. In other words, the particle is briefly illuminated by a very-small-diameter light beam is it “flies” through an open space.
Among other uses, particle counters incorporating particle sensors are used to obtain a measure of air quality by providing information as to the number and size of particles present in some specified volume of air, e.g., a cubic meter of air. Even work environments which appear to human observation to be clean—business offices, manufacturing facilities and the like—are likely to have substantial numbers of microscopic airborne particles. While such particles are not usually troublesome to the human occupants, they can create substantial problems in certain types of manufacturing operations.
For example, semiconductors and integrated chips are made in what are known as “clean rooms,” the air in which is very well filtered. In fact, clean rooms are usually very slightly pressurized using extremely clean air so that particle-bearing air from the surrounding environs does not seep in. And the trend in the semiconductor and integrated chip manufacturing industry is toward progressively smaller products.
A small foreign particle which migrates into such a product during manufacture can cause premature failure or outright product rejection even before it is shipped to a customer. This continuing “miniaturization” requires corresponding improvements in clean-room environments (and in the related measuring instruments) to help assure that the number and size of airborne particles are reduced below previously-acceptable levels. While known particle counters and sensors have been generally acceptable for their intended purpose, certain disadvantages exist.
A disadvantage of known particle sensors involves the air passage, usually circular, along which air and entrained particles flow. In particular, such passage has a very small cross-sectional area. As a result, the pressure differential between the ends of the passage (sometimes referred to as the “pressure drop” across the passage) is quite high. It is not unusual to encounter a pressure drop in the range of 25-70 inches of water at a flow rate of about one cubic foot per minute (CFM). In the field of particle sensors, a pressure drop of 25-70 inches of water at that air flow rate is typical.
(Parenthetically, measuring pressure in inches of water is common. An analogy is found in older style blood pressure measuring devices which include a column of mercury contained in and visible through a graduated glass tube. Blood pressure is measured in “millimeters of mercury” and in such older style devices, blood pressure was equal to the column height. Blood pressure is still measured in millimeters of mercury but a different type of gauge is used to make the measurement.)
Because of the typical pressure drop along the very-small-area air flow passage, known sensors require a motor-driven positive displacement vacuum pump, usually of the diaphragm or vane type, to create enough vacuum to overcome such pressure drop. The necessary electric drive motor and vacuum pump are likely to be relatively heavy. And the motor requires outlet-sourced power; battery power is not practical because of the relatively large amount of power consumed. And because such a sensor requires an electrical cord and plug, it is not so readily moved from site to site, especially remote sites.
While the pressure drop along the air flow passage can be reduced by increasing the passage cross-sectional area, there is another design constraint which militates against that approach. To help assure accuracy in particle sensing and counting, all (or substantially all) of the air-entrained particles flowing along the passage must pass through the beam of light. Usually, the “flight path” of particles is perpendicular to such beam. However, the light beam is preferably sharply focused and its diameter is very small, e.g., less than about 0.1 inch. Therefore, the diameter of the air flow passage cannot be appreciably larger than that of the light beam and still assure that most or all of the particles will pass through the light beam and be detected.
The invention addresses these seemingly intractable difficulties and inconsistent design parameters in a unique and imaginative way.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an improved particle sensor overcoming some of the problems and shortcomings of the prior art.
Another object of the invention is to provide an improved particle sensor in which the air flow passage exhibits exceptionally low pressure drop at a flow rate of about one CFM.
Another object of the invention is to provide an improved particle sensor in which substantially all of the particles are directed through the light beam.
Yet another object of the invention is to provide an improved particle sensor which is lighter in weight than comparable conventional sensors.
Still another object of the invention is to provide an improved particle sensor which is battery powered and highly portable, even to remote sites. How these and other objects are accomplished will become more apparent from the following descriptions and from the drawing.
The invention is an improvement in a particle sensor of the type having a light beam with a beam long axis and also having an air flow tube with (a) an inlet end, and (b) a particle exit mouth. In the improvement, the cross-sectional area of the flow passage at the inlet end is quite large and is greater than the cross-sectional area of the exit mouth. And the exit mouth is elongate in a direction substantially parallel to the beam long axis and, preferably, is “race-track” shaped and has first and second side edges which are generally parallel to one another.
The flow passage (of relatively large area) dramatically reduces the pressure drop along the tube. And the long, relatively narrow exit mouth (about as wide as the width of the light beam) helps assure that particles flowing through the mouth pass through the beam.
More specifically, the air flow tube has an inlet portion and a nozzle portion. The latter has a first inlet section which has a minimum cross-sectional area, i.e., an area less than that of any section along the length of the inlet portion. Further, the nozzle portion has a first nozzle section which has a maximum cross-sectional area, i.e., an area greater than that of any section along the nozzle portion.
In a highly preferred embodiment, the cross-sectional area of the first inlet section is no less than the cross-sectional area of the first nozzle section. Additionally, the inlet portion has an enlarged second inlet section having a cross-sectional area greater than that of the first inlet section. The first inlet section and the first nozzle section have substantially the same shape, e.g., circular.
In another aspect of the invention, the sensor air flow tube extends along a flow axis and the sensor has an air blower (preferably a centrifugal blower) rather than the conventional positive-displacement vacuu
Chandler David L.
Kreikebaum Gerhard
Jansson, Shupe Bridge & Munger, Ltd.
Pham Hoa Q.
Venturedyne Ltd.
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