Apparatus and method for monitoring particle flow in a stack

1. A method of monitoring particles flowing in a stack, the method comprising:

emitting light from a light source along an optical path for scattering from the particles;

rotating a rotatable monitoring assembly mounted in the optical path; and

detecting the scattered light using a detector;

wherein the rotatable monitoring assembly contains at least two apertures and the method further comprises rotating the rotatable monitoring assembly into a plurality of different configurations, including:

an operation configuration, wherein the light passes through the rotatable monitoring assembly and into the stack unimpeded;

a zero-check configuration, wherein the rotatable monitoring assembly blocks the light from reaching the stack;

a span-check configuration, wherein light of varying intensity passes from the light source through the rotatable monitoring assembly into the stack;

a contamination-check configuration, wherein the light is reflected through the rotatable monitoring assembly onto the detector, without entering the stack; and

a safety-shutter configuration, wherein the rotatable monitoring assembly protects the detector from particles in the stack,

wherein the rotatable monitoring assembly is positioned along an optical axis of the light source, and is rotatable about an axis parallel to the optical axis of the light source.

2. The method of claim wherein the monitoring of the particles flowing in the stack is of a density of the particles within a flow, a rate of flow of the particles, and/or a velocity of the particles.

3. Apparatus for monitoring particles flowing in a stack, the apparatus comprising:

a light source for emitting light along an optical path for scattering from the particles;

a rotatable monitoring assembly mounted in the optical path;

a detector for detecting the scattered light;

wherein the rotatable monitoring assembly contains at least two apertures and is configurable into a plurality of different configurations, including:

an operation configuration, wherein the light passes through the rotatable monitoring assembly and into the stack unimpeded;

a zero-check configuration, wherein the rotatable monitoring assembly blocks the light from reaching the stack;

a span-check configuration, wherein light of varying intensity passes from the light source through the rotatable monitoring assembly into the stack;

a contamination-check configuration, wherein the light is reflected through the rotatable monitoring assembly onto the detector, without entering the stack; and

a safety-shutter configuration, wherein the rotatable monitoring assembly protects the detector from the particles in the stack,

wherein the rotatable monitoring assembly is positioned along an optical axis of the light source, and is rotatable about an axis parallel to the optical axis of the light source.

4. The apparatus of claim 3 , wherein the at least two apertures include a first aperture and a second aperture, wherein the second aperture in the rotatable monitoring assembly is larger than the first aperture, and wherein the second aperture is formed from two superimposed circular apertures.

5. The apparatus of claim 3 , wherein a stopper is mounted on the rotatable monitoring assembly, such that when the rotatable monitoring assembly is in the zero-check configuration, the stopper is positioned to block light from the light source from passing through the rotatable monitoring assembly.

6. The apparatus of claim 3 , wherein at least one attenuator is used to attenuate light when the rotatable monitoring assembly is in the contamination-check configuration.

7. The apparatus of claim 6 , wherein the at least one attenuator is interchangeable in order to measure different characteristics of the light source.

8. The apparatus of claim 3 , wherein an air purge is positioned proximate to the rotatable monitoring assembly and is so arranged to cause an airflow to pass over the rotatable monitoring assembly that separates stack gas from the rotatable monitoring assembly.

9. The apparatus of claim 8 , wherein an air flow meter is positioned proximate to the rotatable monitoring assembly and is arranged to monitor a flow rate of purge air.

10. The apparatus of claim 3 , wherein the apparatus is configured to be installed in a wall of the stack.

11. The apparatus of claim 3 , wherein the light source is a laser.

12. The apparatus of claim 3 , wherein the detector is an imager.

13. The apparatus of claim 3 , wherein the rotatable monitoring assembly includes a third aperture for allowing light to pass from the light source to the stack.

14. The apparatus of claim 3 , further comprising a second light source.

15. An apparatus for monitoring particles flowing in a stack, the apparatus comprising:

a light source for emitting light along an optical path for scattering from the particles;

a rotatable monitoring assembly mounted in the optical path;

a detector for detecting the scattered light;

wherein the rotatable monitoring assembly contains at least two apertures and is configurable into a plurality of different configurations, including:

an operation configuration, wherein the light passes through the rotatable monitoring assembly and into the stack unimpeded;

a zero-check configuration, wherein the rotatable monitoring assembly blocks the light from reaching the stack;

a span-check configuration, wherein light of varying intensity passes from the light source through the rotatable monitoring assembly into the stack;

a contamination-check configuration, wherein the light is reflected through the rotatable monitoring assembly onto the detector, without entering the stack; and

a safety-shutter configuration, wherein the rotatable monitoring assembly protects the detector from the particles in the stack, wherein the rotatable monitoring assembly includes a third aperture for allowing light to pass from the light source to the stack.