GB2364934A

GB2364934A - Long-term sampling and sample size selecting device

Info

Publication number: GB2364934A
Application number: GB0015022A
Authority: GB
Inventors: Robin Middlemass Howie
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-06-20
Filing date: 2000-06-20
Publication date: 2002-02-13
Also published as: GB0015022D0

Abstract

This invention describes a long-term Sampling Device 20 which can be used for measuring airborne or other fluid-borne contaminants such as asbestos. Sampling Device 20 incorporates a size selecting device based on a helical elutriator. In practice a number of such long-term sampling devices may be used in parallel to ensure that at least one sampling device generates a sample suitable for analysis.

Description

<Desc/Clms Page number 1> LONG-TERM SAMPLING AND SAMPLE SIZE SELECTING DEVICE DESCRIPTION This invention relates to sampling airborne or fluid-bome contaminants. Fluid-borne contaminant concentrations are generally measured using real-time analysers or by drawing the contaminated fluid through a suitable collection medium and analysing the sample on the collection medium to determine the quantity of contaminant of interest. For example, in the case of airborne asbestos, potentially contaminated air is drawn through a membrane sampling filter and the filter is subsequently analysed to determine the number of respirable fibres present on the filter. In certain situations it is necessary to draw the fluid being sampled through a size selecting device, e.g. for sampling airborne crystalline silica concentrations, a size selector is used so that only respirable crystalline silica particles are collected.

The size selection devices used for airborne sampling are generally miniature cyclones, elutriators or fibre or foam filters. Such devices are generally used for sampling flow rates of the order of 1 litre/minute and are generally unsuitable for collecting small particles at very low sampling flow rates, e.g. for 2 - 3 micrometer diameter particles at flows of the order of 5 - 20 millilitres/minute.

Current techniques for sampling airborne asbestos concentrations using are unsuitable for sampling high concentrations over an extended period as the sample collected is too dense for the specified analysis technique. In addition, in many situations airborne asbestos is accompanied by airborne particulate matter. Such particulate matter can prevent recognition of asbestos fibres.

Experience has demonstrated that conventional size selecting devices can be used for asbestos sampling and that the use of such size selecting devices effectively increases the number of asbestos fibres observed on the sample collection filter.

It is therefore considered that there is a requirement for a means of sampling long- term airborne asbestos fibre concentrations and that such sampling should include a means of size selecting the material collected on the sampling filter to minimise the collection of non-fibrous particulate matter.

This invention describes a long-term sampling device which incorporates a size selection means and which is suitable for sampling over a full working shift while ensuring that the collected sample can be analysed using currently specified techniques

This invention is illustrated, by way of example only, with reference to Figure 1 which shows a conventional Horizontal Elutr iator 10 with sample air flow Inlet 11 and Outlet 12. The height of Inlet 11 is so selected that a particle of a selected size which enters Inlet 11 at half the entry height sediments at the bottom the Outlet 12. In practice, Horizontal Elutriators can be bulky if the selected size is set to small diameters and/or high sample flow rates. This invention is based on forming a conventional horizontal elutriator into a helical coil Helical Elutriator 14 can be formed by cutting a rectangular thread onto a suitable material such as brass or a conducting plastic and by closing the threads so formed with a thin walled tube of similar material, so effectively forming Passages 15. Helical Elutriator 14 therefore has Passages 15 with Inlet 16 and Outlet 17, or vice versa.

Figure 2 illustrates a complete long-term sampling device, by way of example only. Long-term Sampling Device 20 consists of three components, Filter Holder 21, Helical Elutriator 22 and Closure Tube 23. Components 21, 22 and 23 are constructed from brass or a suitable conducting plastic. Filter Holder 21 has Connector 24 which is coupled to a suitable sampling pump, not shown. Filter Holder 21 is threaded to provide a secure connection to Helical Elutriator 22. On assembly, Membrane Sampling Filter 25 is sealed between Filter Holder 21, Sealing Ring 26 and Helical Elutriator 22. Helical Elutriator 22 is formed by cutting a rectangular thread to form Helical Passages 27 between Helical Elutriator 22 and Closure Tube 23. Closure Tube 23 is sealed to Helical Elutriator 23 by Sealing Ring 31. Components 21, 22 and 23 can disassembled to permit cleaning. In operation, potentially contaminated air is drawn in through Entry 28, through Helical Passages 27, Outlet 29 and into central Chamber 30. The sampled air is drawn out of Chamber 30 through Membrane Filter 25. Chamber 30 helps to ensure that any size selected sample is uniformly deposited across the active area of Membrane Filter 25 and also reduces the weight of the complete long-term sampling device.

For a sample flow rate of 5 millilitres/minute and an effective membrane filter diameter of 6 mm, the density of asbestos sample on the membrane filter would be approximately 1,000 fibres per square mm in an ambient concentration of about 9 fibres/millilitre. For the above criteria, the overall long-term sampling device would be about 10 mm diameter by about 20 mm overall length.

The overall size selecting long-term sampling device would therefore be small.

To ensure that a sample suitable for analysis is obtained, it is anticipated that at least two such long-term sampling devices would be used in parallel. All sampling devices would be set to ensure the same size selection characteristic. With two sampling devices one device would sample at, say, 2 millilitres/minute, the other device would sample at, say, 20 millilitres/minute. Note: assuming that the passages in the two Helical Elutiators have the same cross sectional areas, the total passage length in the higher flow rate sampling device would be ten times longer than in the lower flow rate device. After sample collection, the membrane filters would be removed and prepared for analysis. Only the filter with the more optimum sample density would be analysed.

Given the uncertainty regarding the likely airborne asbestos fibre concentration, a sufficient number of suitably configured long-term sampling and size selecting devices would be used. For example, if the uncertainty in airborne asbestos fibre concentration was a factor of 100, four sampling devices would be used covering a similar range of sampling flow rates.

Claims

CLAIMS 1) A size-selecting sampling device based on a helical elutriator in which fluid- borne contaminant(s) passes through the helical elutriator under the influence of a suitable means and where the required size selection characteristic for the contaminant(s) of interest can be modified as required by varying the height and/or width of the elutriator channel and/or by varying the rate at which the fluid bearing the contaminant(s) passes through the elutriator.
2) A size selecting sampling device as described in Claim 1) above in which a number of such devices but with different fluid flow rates may be used in parallel to ensure that at least one sampling device generates a sample suitable for analysis.
3) A size selecting sampling device as described in Claim 1) above in which a number of such devices but with different characteristics may be used in parallel to obtain a size distribution for the contaminant(s) of interest.