A particle sensing apparatus for determining particle concentration in a fluid comprising: a fluid-flow passageway having an upstream inlet to receive the fluid and a downstream outlet to exhaust the fluid; a recirculation passageway in fluid communication with the fluid-flow passageway, where a portion of the fluid-flow passageway and the recirculation passageway define a recirculation loop; a flow control arrangement for generating a flow of said fluid between the upstream inlet and the downstream outlet and around the recirculation loop; a first particle sensor (16, 16a) located in the recirculation loop; a force applicator (30, 30a) to apply a force (34, 34a) to the particles entrained in the fluid such that the particles move relative to said flow, the force applicator being arranged to control the passage of the particles either into the recirculation loop or to the downstream outlet; and a processor adapted to determine the concentration of particles in the fluid based on the concentration of particles detected at the first particle sensor (16, 16a), characterised in that the apparatus also comprises an inlet particle sensor for detecting an inlet concentration of particles in the fluid, said inlet particle sensor being located in the fluid-flow passageway between the upstream inlet and said portion of the fluid flow passageway; and wherein the processor is adapted to compare said inlet concentration to a predetermined upper concentration threshold and a predetermined lower concentration threshold and control the force applicator to direct the particles towards the recirculation passageway if said inlet concentration is lower than or equal to the lower concentration threshold, or towards the outlet if said inlet concentration is higher than or equal to the upper concentration threshold. a fluid-flow passageway having an upstream inlet to receive the fluid and a downstream outlet to exhaust the fluid; a recirculation passageway in fluid communication with the fluid-flow passageway, where a portion of the fluid-flow passageway and the recirculation passageway define a recirculation loop; a flow control arrangement for generating a flow of said fluid between the upstream inlet and the downstream outlet and around the recirculation loop; a first particle sensor (16, 16a) located in the recirculation loop; a force applicator (30, 30a) to apply a force (34, 34a) to the particles entrained in the fluid such that the particles move relative to said flow, the force applicator being arranged to control the passage of the particles either into the recirculation loop or to the downstream outlet; and a processor adapted to determine the concentration of particles in the fluid based on the concentration of particles detected at the first particle sensor (16, 16a), characterised in that the apparatus also comprises an inlet particle sensor for detecting an inlet concentration of particles in the fluid, said inlet particle sensor being located in the fluid-flow passageway between the upstream inlet and said portion of the fluid flow passageway; and wherein the processor is adapted to compare said inlet concentration to a predetermined upper concentration threshold and a predetermined lower concentration threshold and control the force applicator to direct the particles towards the recirculation passageway if said inlet concentration is lower than or equal to the lower concentration threshold, or towards the outlet if said inlet concentration is higher than or equal to the upper concentration threshold.