In-line smoke attenuator

1. An apparatus in an airflow path before a particle detector, wherein the apparatus removes a substantially constant proportion of all sizes of airborne particles from the airflow over time, the apparatus further including a flow splitting arrangement for dividing the airflow into at least a first sub-flow and a second sub-flow, and a filtering arrangement for filtering the first sub-flow; and

wherein the flow splitting device has an impedance to the passage of the sub-flows that is substantially greater than the flow impedance caused by the filtering arrangement.

2. The apparatus as claimed in claim 1 , wherein the filtering arrangement removes substantially all particulate matter from the first sub-flow.

3. The apparatus as claimed in claim 2 , wherein the filtering arrangement includes a HEPA filter.

4. The apparatus as claimed in claim 1 wherein the flow splitting arrangement includes a plurality of apertures through which the airflow passes to divide it into sub-flows.

5. The apparatus as claimed in claim 4 wherein the plurality of apertures formed in the flow splitting arrangement are substantially identical to each other.

6. The apparatus as claimed in claim 4 wherein the relative proportion of the airflow split into each sub-flow corresponds to the proportion of the apertures formed in the flow splitting arrangement configured to direct the airflow into each sub-flow.

7. The apparatus as claimed in claim 1 that further includes, at least one flow meter for determining a flow rate in any of the following:

an inlet to the apparatus;

an outlet from the apparatus;

a flow path through which one or more of the sub-flows passes.

8. The apparatus as claimed in claim 7 , including a plurality of flow meters.

9. An arrangement for conditioning a fluid flow, the arrangement including:

a first flow path;

a second flow path,

a filter arrangement to filter the fluid flow in the first flow path; and

a flow splitting arrangement for splitting the fluid flow into either the first flow path or the second flow path,

wherein the impedance to the fluid flow caused by the flow splitting arrangement is greater than the impedance caused to the fluid flow by the filter.

10. An arrangement for conditioning a fluid flow including:

a chamber including a fluid inlet and a fluid outlet;

an unfiltered fluid flow path extending between the fluid inlet and the fluid outlet;

a filtered fluid flow path extending between the fluid inlet and the fluid outlet;

a filtering means for filtering the fluid flowing through the filtered fluid flow path;

a flow splitting arrangement for splitting the fluid flow into the filtered fluid flow path or the unfiltered fluid flow path,

wherein the impedance to the fluid flow caused by the flow splitting arrangement is greater than that caused by the filtering means.

11. The arrangement for conditioning a fluid flow as claimed in claim 10 in which the flow splitting arrangement includes at least one first aperture leading to the filtered fluid flow path and at least one second aperture leading to the unfiltered fluid flow path, and wherein the flow impedance caused by each first and second aperture is the substantially the same.

12. The arrangement for conditioning a fluid flow as claimed in claim 11 wherein the proportion of the fluid flow split into the filtered fluid flow path and unfiltered fluid flow path respectively is determined by the relative number of first and second apertures.

13. A method of filtering an air sample prior to introduction to a particle detection means, the method including:

dividing the air sample into at least two sample flows;

filtering one or more of the sample flows;

restricting the flow of the sample flows by an amount greater than a flow restriction caused by the filtering of the one or more sample flows; and

re-combining at least some of the sample flows prior to introduction to a particle detection means.

14. The method as claimed in claim 13 wherein the at least one sample flow is not filtered before being re-combined with another sample flow.

15. The method as claimed in claim 13 wherein step of restricting the one or more filtered sample flows is performed either before or after the filtering of the sample flow.

16. The method as claimed in claim 13 , further including the step of measuring any one or more of;

a flow of filtered air;

a flow of unfiltered air;

a sample flow;

the flow of air prior to dividing it into sample flows; and

the flow of air after combining sample flows compared to a total flow.