Computer Automation of a Dry-Dust Generating System

Abstract

An automated system to minimize variability in an aerosol concentration over time in an animal exposure chamber was developed. This system includes a microprocessor-controlled stepper motor to regulate the speed of a dust generator, a real-time aerosol monitor to instantaneously measure chamber aerosol concentrations, and a personal computer with an analog-to-digital (A/D) converter, serial port, and relay board for signal input and output. A computer program was written to use these devices to produce a chamber concentration near a level chosen by the operator. This program was also written to start and stop an exposure after a specified time period and to continuously display the data received from the aerosol monitor on the computer screen during the exposure. A combination of operating conditions, which include a moderately high chamber flow rate (0.32 m³/min) and high generator rotation speed while using a small scraper blade, optimized the performance of the automated system by minimizing the concentration variability (CV) seen in the aerosol monitor readings. The CV, which indicates the average difference between consecutive monitor recordings, decreased from 2.79 mg/m³, when using the large scraper to produce a chamber concentration of 25 mg/m³, to 0.47 mg/m³ when using the small scraper at 25 mg/m³. When operating with these optimum conditions, 5 consecutive trials of 180 min produced concentrations of 10.52, 10.52, 10.57, 10.37, and 10.47 mg/m³. The system proved to be capable of calculating the appropriate generator speed needed to reach and maintain a desired concentration level after trials were performed to determine the amount of aerosol losses in the chamber. A total of 14 trials performed at 10 mg/m³ had an average actual-to-expected concentration ratio of 1.023 (SD = 0.067). The response of the concentration level to changes in generator speed after start-up varied in relation to the magnitude of the speed change. The system proved to be more accurate when the generator speed was changed to provide a change in chamber concentration of more than 2 mg/m³. The accuracy and reliability of the aerosol monitor were investigated by comparing the time-weighted average of the monitor recording with the average chamber concentration measured by a sample filter (M/F ratio). The average difference of the M/F ratio between consecutive trials was 3.36% when using the small scraper near 10 mg/m³ and 9.71% when using the large scraper near 25 mg/m³.