COMPUTER AUTOMATION OF A DRY-DUST GENERATING-SYSTEM

An automated system to minimize variability in an aerosol concentratio n over rime in an animal exposure chamber was developed. This system i ncludes a microprocessor-controlled stepper motor to regulate the spee d of a dust generator, a real-time aerosol monitor to instantaneously measure chamber aerosol concentrations, and a personal computer with a n analog-to-digital (AID) converter, serial port and relay board for s ignal input and output. A computer program was written to use these de vices to produce a chamber concentration near a level chosen by the op erator. This program was also written to start and stop an exposure af ter a specified time period and to continuously display the data recei ved from the,e aerosol monitor on the computer screen during the expos ure. A combination of operating conditions, which include a moderately high chamber Now rate (0.32 m(3)/min) and high generator rotation spe ed while using a small scraper blade, optimized the performance of the automated system by minimizing the concentration variability (CV) see n in the aerosol monitor readings. The CV, which indicates the average difference between consecutive monitor recordings, decreased from 2.7 9 mg/m(3), when using the large scraper to produce a chamber concentra tion of 25 mg/m(3), to 0.47 mg/m(3) when using the small scraper at 25 mg/m(3). When operating with these optimum conditions, 5 consecutive trials of 180 min produced concentrations of 10.52, 10.52, 10.57, 10.3 7, and 10.47 mg/m(3). The system proved to be capable of calculating t he appropriate generator speed needed to reach and maintain a desired concentration level after trials were performed to determine the amoun t oi aerosol losses in the chamber. A total of 14 trials performed al 10 mg/m(3) had an average actual-to-expected concentration ratio of 1. 023 (SD = 0.067). The response of the concentration level to changes i n generator speed after start-up varied in relation to the magnitude o f the speed change. The system proved to be more accurate when the gen erator speed was changed to provide a change in chamber concentration of more than 2 mg/m(3). The accuracy and reliability of the aerosol mo nitor were investigated by comparing the rime-weighted average of the monitor recording with the average chamber concentration measured by a sample filter (MIF ratio). The average difference of the MIF ratio be tween consecutive trials was 3.36% when using the small scraper near 1 0 mg/m(3) and 9.71% when using the large scraper near 25 mg/m(3).