Monitor for monitoring particles flowing in a stack

An instrument for monitoring particles flowing in a stack, comprising:

• (a) a light source for providing a light beam, and a sensor for detecting light scattered in a scattering zone from the light beam by the flowing particles, the light source and the sensor being arranged on a first side of the scattering zone;
• (b) a probe housing comprising:
• (i) a mount for fixing the housing to a wall of the stack;
• (ii) a proximal portion, closer to the mount and arranged to project into the stack in use, including a first aperture through which in use the light beam exits from the proximal portion;
• (iii) a distal portion, further from the mount, comprising a second aperture through which in use light from the light beam enters the distal portion after having been scattered from particles flowing in the stack, and a focusing mirror arranged to reflect and focus the scattered light;
• (iv) a medial portion, connecting the distal portion to the proximal portion so that there is a space between the proximal and distal portions, in which the scattering occurs;
• (v) a waveguide, passing from the distal portion through the medial portion and the proximal portion to the sensor and arranged to guide to the sensor the light reflected and focused by the focusing mirror.