Characteristics of an aerosol photometer while automatically controlling chamber dilution-air flow rate

Trials were conducted to determine those factors that affect the accuracy o f a direct-reading aerosol photometer when automatically controlling airflo w rate within an exposure chamber to regulate airborne dust concentrations. Photometer response was affected by a shift in the aerosol size distributi on caused by changes in chamber flow rate. In addition to a dilution effect , flow rate also determined the relative amount of aerosol lost to sediment ation within the chamber. Additional calculations were added to a computer control algorithm to compensate for these effects when attempting to automa tically regulate flow based on a proportional-integral-derivative ( PID) fe edback control algorithm. A comparison between PID-controlled trials and th ose performed with a constant generator output rate and dilution-air flow r ate demonstrated that there was no significant decrease in photometer accur acy despite the many changes in flow rate produced when using PID control. Likewise, the PID-controlled trials produced chamber aerosol concentrations within 1% of a desired level.