GB2259982A

GB2259982A - Sampling a fluid to determine its contaminant content

Info

Publication number: GB2259982A
Application number: GB9120679A
Authority: GB
Inventors: Robin Middlemass Howie
Original assignee: INST OF OCCUPATIONAL MEDICINE
Current assignee: INST OF OCCUPATIONAL MEDICINE
Priority date: 1991-09-28
Filing date: 1991-09-28
Publication date: 1993-03-31
Also published as: GB9120679D0

Abstract

A method of sampling contaminant concentrations in a fluid medium includes diluting a sample withdrawn from the fluid medium, e.g. air, into a housing 1 with an inlet 6 for a dilution fluid e.g. clean air. The diluted sample thence passes into a mixing chamber 8 for deposit on or in a collection medium 14 for analysis, the fluid being drawn through the housing 1 by a pump 18. Process is especially of use at high contaminant concentrations. Suspended particulates in H2O samples can also be determined. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO SAMPLING This invention concerns improvements in or relating to sampling and more particularly has reference to an improved method of sampling contaminant concentrations in a fluid medium, and to an apparatus therefor.

Conventional sampling apparatus employed for monitoring airborne contaminant concentrations may be unsuitable for use where the contaminant concentrations are likely to be high, unless the sampling periods are relatively short since otherwise the sample taken could be too dense or too concentrated to permit accurate analysis. For example, when monitoring airborne respirable asbestos fibre concentrations using the standard cowled sampling head and with analysis by manual phase contrast optical microscopy, the maximum permissible sampling duration in fibre concentrations of 400 fibres per millilitre will be about 20 seconds to ensure that the density of fibres on the sampling filter does not exceed the 400 fibres per millimetre limit for accurate analysis.Such short sampling periods are unsuitable where long-term measurements of fibre concentrations are required, for example measurement of occupational exposure over a working shift of eight hours.

An object of the present invention is therefore to provide an improved method of and an apparatus for sampling contaminant concentrations in a fluid medium.

According to one aspect of the present invention a method of sampling contaminant concentrations in a fluid medium includes the steps of withdrawing a sample from the fluid medium, diluting the sample with an uncontaminated fluid, and collecting the diluted sample on a collection medium.

According to a second aspect of the present invention an apparatus for sampling contaminant concentrations in a fluid medium includes a housing having an entry portion provided with an inlet for a sample and an entry port for a dilution fluid, a mixing chamber within the housing leading from the entry portion, a collection medium located within the housing downstream of the mixing chamber, a fluid discharge port downstream of the collection medium, and means for producing fluid flow through the housing.

Conveniently, control means are provided for controlling the flow through the housing. The control means may include a mechanism for recycling the fluid as a dilution fluid through the inlet of the entry portion. The mechanism may incorporate a filter for the fluid.

In one embodiment suitable for use when large contaminant concentrations are likely to be experienced, one apparatus may be employed in series with another, but with the collection medium in the downstream housing and appropriate fluid entry and discharge ports provided.

By way of example only, a method of and apparatus for sampling contaminant concentrations in a fluid medium in accordance with the invention are described below with reference to the accompanying drawings in which: Figure 1 is a diagrammatic cross-section of a first embodiment of the apparatus; Figure 2 is a diagrammatic cross-section of a second embodiment of the apparatus.

Referring to Figure 1 there is shown an apparatus for sampling contaminant concentrations comprising a housing 1 having an entry portion 2 provided with an inlet 4 for a contaminated fluid, for example air, having an extension tube 5 and with an entry port 6 for a dilution fluid, the port 6 registering with the annulus 7 formed between the tube 5 and the wall of the entry portion. The entry portion 2 is suitably configured such as to match the required sampling characteristics, for example for inspirability, and may be made in component form to facilitate modification in accordance with the requisite standards.

The housing 1 further includes a mixing chamber 8 at the relatively lower end of which as viewed in the drawing is provided a discharge portion 10 having a mount or holder 12 for a collection medium 14 such as a filter. A discharge port 16 is provided in the end of the discharge portion 10 and is connected to a pump 18 which in use serves to induce fluid flow through the housing 1. Downstream of the pump 18 is a filter 20 which feeds into two control devices 22, 24, the device 22 having an outlet 26 connected to the inlet port 6 and device 24 having an outlet 28 discharging in this specific example to atmosphere.

The housing 1 is preferably produced from electrically conducting materials.

In operation, the apparatus in this particular example is utilised for sampling the environment and contaminated air flow is induced through the inlet 4 by the pump 18. The contaminated air passes through the tube 5 into the housing and the mixing chamber 8 and is simultaneously diluted therein by clean air introduced through the inlet port 6 and flowing via the annulus into the chamber. The contaminated air now diluted flows through the chamber 8 and to the collection medium 14 which may for example be a filter for capturing aerosols, the air passing through the medium having a flow rate V1. The air thence passes through the discharge port 16 and through the pump 18. The pump 18 delivers air via the filter 20 to the control devices 22, 24, air from the device 22 entering port 6 at a flow rate V2. The flow rate of the contaminated air passing through the inlet 4 being V1 - V2.In practice the relative areas of the inlet 4 and the annulus 7 are preferably scaled to secure that the air velocities from the entry 4 and the annulus 7 are equal, and the length of the mixing chamber 8 is selected to ensure adequate mixing of the sample and the dilution air prior to the collection medium.

The collection medium 14 is in practice removed after a suitable time and the captured contaminants are measured to provide the required information on the prevailing atmospheric conditions.

Referring now to Figure 2 like numerals of reference have been used as for Figure 1 where appropriate. In this embodiment, effectively two apparatuses of the kind depicted in Figure 1 are connected one to the other in series to provide a two-stage dilution for sampling where large dilution ratios are required.

Essentially the collection medium 14 is removed from the first stage 40 and repositioned in the second stage as at 42 in an appropriate holder 44 in the discharge portion of the second stage 46. The first and the second stages 40 and 46 are bridged by a transfer tube 48 defining an annulus 50 within stage 40 and an annulus 52 within stage 46. A discharge port 53 is associated with annulus 50 and a dilution entry port 54 is associated with annulus 52, a mixing chamber 56 being provided in the second stage 46 intermediate the annulus 52 and the collection medium 42 downstream of which is a discharge port 60 connected to a pump (not shown).

In operation, air is drawn through the collection medium 42 at a flow rate V3 and clean air is fed into the port 52 at a flow rate V4 which is less than V3. Thus a sample from the first stage mixing chamber 8 is drawn through the transfer tube 48 at a flow rate V3 - V4. The sizes of the transfer tube 48 and the annulus 52 are scaled such that the air velocities from both entering the mixing chamber 56 are equal. Air is withdrawn through port 53 at a flow rate V5. The total flow rate of air withdrawn from mixing chamber 8 is thus V5 + (V3 - V4). Clean air is fed into the port 6 at a flow rate V6 which is less than V5 + (V3 - V4). A sample from the environment is thus drawn through the inlet 4 of the first stage 40. The sizes of the the transfer tube 48 and annulus 50 are preferably scaled such that the air velocities entering both are equal. The mixing chamber 56 and the bridging arrangement between the first and second stages are preferably made of electrically conducting materials.

It is to be understood that whilst the second embodiment describes two stages in which dilution of the sample takes place, more than two stages may be provided for this purpose.

The invention is principally applicable to the collection of aerosol samples. However, the invention is applicable to the collection of relevant samples from any suitable fluid, for example water in which particles may be suspended.

Claims

1. A method of sampling contaminant concentrations in a fluid medium including the steps of withdrawing a sample from the fluid medium, diluting the sample with an uncontaminated fluid, and collecting the diluted sample on a collection medium.

2. A method according to Claim 1 including diluting the sample in two or more stages.

3. A method substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

4. An apparatus for sampling contaminant concentrations in a fluid medium including a housing having an entry portion provided with an inlet for a sample and an entry port for a dilution fluid, a mixing chamber within the housing leading from the entry portion, a collection medium located within the housing downstream of the mixing chamber, a fluid discharge port downstream of the collection medium, and means for producing fluid flow through the housing.

5. An apparatus according to Claim 4 in which control means are provided for controlling the flow through the housing.

6. An apparatus according to Claim 4 or 5 in which the control means includes a mechanism for recycling the fluid as a dilution fluid through the inlet of the entry portion.

7. An apparatus according to Claim 6 in which the mechanism includes a filter.

8. An apparatus according to any one of the preceding claims 4 to 7 in which the apparatus includes a further housing in series with the first, the collection medium being located in the said further housing, and a bridging section for connecting the housing and the further housing, the section incorporating adjacent discharge and entry portions respectively for the two housings.

9. An apparatus substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.