Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1994-03-14
1995-08-15
Allen, Stephone B.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
356338, G01N 1506
Patent
active
054421900
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the measurement of airborne fibres and, in particular to apparatus suitable for the measurement of the level of asbestos fibres in the air.
Some fibrous dusts present a hazard to man when inhaled as they have been shown to be human carcinogens. Asbestos fibres are the best Known example, and are controlled in the workplace by filtering a Known volume of air, and examining the sample under phase contrast optical microscopy. By counting fibres, a concentration in the original air can be determined.
There is a need for a cheap portable instrument that directly indicates the levels of fibres without the need to wait for a filter sample and microscope examination. Leak testing during the stripping of fibrous insulation is an example. To that end an approach to fibrous aerosol monitoring was made by combining the electrostatic precipitation of aligned fibres with differential light scattering along and at right angles to the fibre axis.
A prototype instrument has been constructed based on a parallel-plate precipitator with inertial fibre alignment. Fibre detection is based on a difference circuit which measures the scattered light from two low powered flash-lamps. The instrument is calibrated to read in fibres per ml of air sampled.
This instrument provides apparatus for measuring the quantity of fibres present in a gaseous fluid comprising means for creating of flow of said gaseous fluid through a chamber, aligning means to align the orientation of fibrous particles flowing into said chamber, electrical charging means to charge said particles within said chamber, electrical precipitating means to precipitate said fibrous particles on to carrier means, illuminating means to illuminate said carrier means with radiation and measuring means to measure radiation transmitted from said carrier means, thereby to derive an indication of the number of said particles precipitated thereon.
The disadvantage of this is that the precipitated deposit occurs over the whole length of the slide. Since the detection of fibres relies on analysing the light scatter, it would be advantageous if the precipitated deposit could be concentrated into this area. This can be achieved by using a modified flow channel and electrode.
According to the present invention there is provided apparatus for measuring the quantity of fibres present in a gaseous fluid comprising means for creating of flow of said gaseous fluid through a chamber, aligning means to align the orientation of fibrous particles flowing into said chamber, electrical charging means to charge said particles within said chamber, electrical precipitating means to precipitate said fibrous particles on to carrier means, illuminating means to illuminate said carrier means with radiation, measuring means to measure radiation transmitted from said carrier means and electrical screening means to shield said particles from the influence of said electrical precipitating means in regions not adjacent said measuring means, thereby to derive an indication of the number of said particles precipitated thereon.
An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings in which:
FIGS. 1A and 1B respectively show in elevation and plan a diagrammatic representation of our earlier portable airborne fibre monitor,
FIG. 1C shows a plan view of the microscope cover slip of FIG. 1A
FIG. 2 shows in elevation a modified fibre monitor in accordance with the present invention, and
FIG. 3 is a schematic diagram of an elutriator suitable for use with the monitor of FIG. 2.
Referring now to the drawings, a pump draws air containing fibres into a slotted inlet nozzle 1 of the instrument where they are aligned by inertia. These aligned fibres are then charged by a corona discharge existing between a wire 2 and a first plate electrode 3. The charged fibres are then precipitated on to a microscope cover slip 4 beneath a conducting glass window 5 connected to a second plate electrode 6 which serves to precipitate the fibres. The el
REFERENCES:
patent: 2909960 (1959-10-01), Orr, Jr. et al.
patent: 4166703 (1979-09-01), Kirsch et al.
patent: 4916325 (1990-04-01), Rood et al.
Allen Stephone B.
British Technology Group Ltd.
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