Method and apparatus for the measurement of airborne fibres

Apparatus for measuring the quantity of fibers present in a gaseous fluid includes a pair of sloping shield electrodes (21) to shield charged fibers from the influence of a precipitating field other than in the region adjacent an optical detector (26).

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.

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 electrical potentials on the corona 
and precipitating electrodes can be varied for each fibre type, but are 
typically 3,000 and 300 volts. Fibres are precipitated on to the cover 
slip retaining their axial alignment. 
The number of fibres is directly related to the light scattering along and 
across the cover slip, and may be determined accurately by using high 
stability flash lamps 7, collimated by wedged light pipes 8, along and at 
right angles to the flow. The difference between alternate pulses is 
recorded using a single detector 9. 
U.S. Pat. No. 4,916,325 describes a method of precipitating electrically 
charged fibres on to a microscope slide using a single precipitator (or 
repeller) electrode). 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 shaped flow channel and the revised electrode scheme 
illustrated in FIG. 2. 
Referring now to FIG. 2, the electric field due to the precipitator 
electrode is screened from the charged fibres by the two sloping shield 
electrodes 21, mounted on an insulating body 22, which are held at the 
same potential (normally ground) as a conducting base plate 23. The fibres 
therefore move through a field free region and are not deposited until 
they are exposed to the field from a precipitator electrode 24. The 
deposition is thus constrained to occur in the small area 25 adjacent a 
light source and detectors 26. Since the fibres have been forced to move 
close to the slide surface in this region by the sloping shield electrodes 
the precipitation efficiency is high and fibre orientation is not 
significantly disturbed by high precipitator fields. In addition the flow 
velocity gradient experienced by the fibres as they reach the 
precipitation region also serves further to align them. 
Preferably, overload of the fibre monitor slide by coarse, non-respirable 
dusts is minimised by some form of elutriation. A parallel plate 
elutriator, working by gravitational settling have been used for this s 
purpose. However, its sampling characteristic is not omnidirectional and, 
because its plates need to be sealed along the edges, it is difficult to 
clean. 
We have found that a circular elutriator (FIG. 3) has none of these 
disadvantages. Such an elutriator, comprising annular discs 31 held by 
clamping screws and spacers 33,35 with a circumferential inlet passage 37 
and central outlet passage 38. Since neither the top, bottom or plates 
have seals along their edges, they are easily removed for cleaning.