Filter cleaning system for opacity monitor

A control system for coordinating the calibration and filter cleaning functions in an opacity monitor having a light source and a light sensor on opposite sides of a first optical path for measuring the opacity of a gas in the first optical path, comprising:

• calibration means operatively connected to the light source and sensor for calibrating the opacity monitor during a calibration period;
• a pair of protective pipes having facing open first ends and lying on the first optical path between the light source and sensor with the gas whose opacity is to be measured adapted to pass between said open first ends;
• a blower having an input for receiving air and an output for supplying air;
• a primary filter for supplying filtered air for purging said protective pipes;
• a secondary filter for supplying filtered blowdown air for blowdown cleaning of said primary filter;
• valve means connected to said blower input and output, to said primary filter and to said pair of protective pipes, having a first position for supplying purging air from said primary filter to said blower input and from said blower output to said protective pipes for purging said protective pipes, and a second position for supplying blowdown air from said secondary filter to said blower input and from said blower output to said primary filter for cleaning said primary filter;
• and
• control means connected to said calibration means and to said valve means for moving said valve means into said second position thereof when said calibration means is activated to calibrate the opacity monitor during said calbration period.