METHODS AND APPARATUS RELATING TO WALL-FLOW FILTERS

1 . A method of assessing a wall-flow filter, the wall-flow filter having an inlet end and an outlet end with the inlet end and the outlet end being separated by a porous structure, the method comprising:

a) establishing a gas flow through the wall-flow filter from the inlet end to the outlet end via the porous structure;

b) entraining particles in the gas flow so that the particles pass into the wall-flow filter through the inlet end causing at least a portion of the particles to be deposited on and/or in the porous structure;

c) using a photometer located downstream of the outlet end of the wall-flow filter to detect at a plurality of points in time throughout a detection period a photometer signal related to scattering and/or attenuation of electromagnetic radiation by particles that have passed through the porous structure and out of the outlet end of the wall-flow filter;

d) obtaining a photometer signal profile based on the detected photometer signal throughout the detection period; and

e) obtaining a characteristic of the wall-flow filter and/or making a qualitative assessment of the wall-flow filter based on the photometer signal profile.

2 . The method of claim 1 , wherein making the qualitative assessment of the wall-flow filter comprises comparing the photometer signal profile against one or more predetermined photometer signal profiles.

3 . The method of claim 1 , wherein making the qualitative assessment of the wall-flow filter comprises comparing one or more characteristics of the photometer signal profile against one or more threshold values.

4 . The method of claim 3 , wherein the one or more characteristics of the photometer signal profile include one or more of a peak height, a number of peaks, a decay rate, a decay time, and a numerical integral of the photometer signal profile.

5 . The method of claim 1 , wherein obtaining the characteristic of the wall-flow filter is based on numerical integration of the photometer signal profile throughout the detection period.

6 . The method of claim 1 , wherein obtaining the characteristic of the wall-flow filter and/or making the qualitative assessment of the wall-flow filter is accomplished without counting the particles before they flow into the inlet end of the wall-flow filter.

7 . The method of claim 1 , wherein the photometer signal is detected at a sampling rate of 0.5 to 5 samples/second.

8 . The method of claim 1 , wherein the photometer is zeroed multiple times throughout the detection period.

9 . The method of claim 1 , wherein all of the gas flow exiting the outlet end of the wall-flow filter passes through a flow cell of the photometer.

10 . The method of claim 1 , wherein the photometer comprises a flow cell, a light source and at least one light detector.

11 . The method of claim 10 , wherein the flow cell has a path length between the light source and the at least one light detector of 75 to 100 mm.

12 . The method of claim 10 , wherein the photometer comprises an LED light source; optionally a blue LED light source; optionally a blue LED light source emitting light with a wavelength of 400 to 490 nm.

13 . The method of claim 10 , wherein the photometer comprises an amplifier, wherein a detected voltage of the light detector is amplified by a factor of greater than 5,000 to produce the photometer signal.

14 . Apparatus for applying particles to a wall-flow filter, the wall-flow filter having an inlet end and an outlet end with the inlet end and the outlet end being separated by a porous structure, the apparatus comprising:

a) a holder for holding the wall-flow filter;

b) an inlet chamber in communication with the inlet end;

c) a vacuum generator in communication with the outlet end for establishing a gas flow through the wall-flow filter from the inlet end to the outlet end;

d) a device for spraying the particles into or within the inlet chamber;

e) a photometer located downstream of the outlet end of the wall-flow filter; and

f) a controller configured to:

i) detect at a plurality of points in time throughout a detection period a photometer signal of the photometer based on scattering and/or attenuation of electromagnetic radiation by particles that have passed through the porous structure and out of the outlet end of the wall-flow filter, and

ii) to obtain a photometer signal profile based on the detected photometer signal throughout the detection period; and

iii) to obtain a characteristic of the wall-flow filter and/or make a qualitative assessment of the wall-flow filter based on the photometer signal profile.

15 . The apparatus of claim 14 , wherein the controller is configured to zero the photometer multiple times throughout the detection period; optionally to zero the photometer before each detection of the photometer signal throughout the detection period.

16 . The apparatus of claim 14 , wherein the apparatus is configured such that all of the gas flow exiting the outlet end of the wall-flow filter passes through a flow cell of the photometer.

17 . The apparatus of claim 14 , wherein the photometer comprises a flow cell, a light source and at least one light detector.

18 . The apparatus of claim 17 , wherein the flow cell has a path length between the light source and the at least one light detector of 75 to 100 mm.

19 . The apparatus of claim 17 , wherein the photometer comprises an LED light source; optionally a blue LED light source; optionally a blue LED light source emitting light with a wavelength of 400 to 490 nm.

20 . The apparatus of claim 17 , wherein the photometer comprises an amplifier, wherein a detected voltage of the light detector is amplified by a factor of greater than 5,000 to produce the photometer signal.