In flue gas desulfurization and in the production of sulfuric acid, the catalytic oxidation of SO2 to SO3 is followed by the latter's absorption in water or dilute acid. If the conditions are less than optimal, sulfuric acid can escape through the stack in the form of an aerosol mist of finely dispersed droplets. If this is not detected immediately, grave environmental pollution can occur.
The measuring system used for this purpose consists of a turbidimeter for detecting the light scattered by the droplets and a sampling setup with an automatic temperature control system. The latter's job is to cool the stack gas down to about 120°C. At this temperature the sulfuric acid condenses, while the water remains in vapor form. This eliminates the problem of falsification by water droplets, and the reading indicates the sulfuric acid concentration selectively.