Particulate emissions eg from an incinerator 12 are filtered by a baghouse filter 16 (or electrostatic filter, not shown) and monitored by an instrument 22 to ensure they do not exceed specified levels before being released to atmosphere. To calibrate the instrument, a variable fraction of the particle-laden gas stream from the incinerator 12 is arranged to bypass the filter and merge with the filtered gas in outlet duct 18 to provide various particle concentration levels for the instrument to measure. During shutdown of the incinerator 12, two bag filters 32 are removed and replaced with adjustable orifice units (40, Figs 2 and 3), which provide a variable size flowpath bypassing the filter. By adjustment of the orifice units with the incinerator operational, it is possible to achieve readings at the instrument over the whole range of expected particle concentration levels. Each reading is then compared with the particle concentration as measured by a standard reference method, to calibrate the instrument.
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