An optical particle detector is configured to simultaneously detect at least two particles within a useful detection volume. The detector includes a retina capable of receiving light rays scattered by the particles and a dark reticle interposed between the useful detection volume and the retina. The dark reticle includes at least one optical aperture allowing a passage towards the retina of a part of first scattered light rays and of a part of second scattered light rays, and an opaque surface on a periphery of the at least one aperture, preventing a passage towards the retina of another part of the first and second scattered light rays so as to project onto the retina first and second scattering diagrams separated from each other.

A real-time silica monitoring system can include a plurality of off-site sensors positioned at geographic locations off-site from a hydraulic fracturing well site that detect and measure quantities of airborne silica particles, a plurality of on-site sensors positioned at geographic locations on a hydraulic fracturing well site that detect and measure quantities of air-borne silica particles, and one or more mobile sensors suitable to be carried by individual persons that detect and measure quantities of airborne silica particles. A monitoring system can include a computer system that can aggregate and store airborne silica measurements taken by one or more sensors and communicate data to a user.

Method and control system are provided for controlling values of process parameters of a pretreatment process of wood particles. A sampler is used to obtain a sample of a product flow of said pretreatment process after said wood particles have undergone steam explosion in a hemihydrolysis reactor. A particle measurement device is used to measure one or more characteristics of particles in said sample and to produce one or more pieces of measurement information indicative of the measured characteristics. Said one or more pieces of measurement information are used to select one or more values of one or more of said process parameters.

Method and control system are provided for controlling values of process parameters of a pretreatment process of wood particles. A sampler is used to obtain a sample of a product flow of said pretreatment process after said wood particles have undergone steam explosion in a hemihydrolysis reactor. A particle measurement device is used to measure one or more characteristics of particles in said sample and to produce one or more pieces of measurement information indicative of the measured characteristics. Said one or more pieces of measurement information are used to select one or more values of one or more of said process parameters.