A servomechanism for vapor concentration control in experimental exposure chambers

Abstract

The servomechanism established consisted of 5 components: 4 vapor generators, 4 stainless steel chambers built in parallel, a GLC with FIDs as a vapor analyzer, a computer with a printer and a flow controller. Methane was employed as a calibration gas. Vapor concentration analyses were repeated automatically, of which the time frequency was dependent on the retention time(s) of the test material(s). Concentration at command levels could be produced under the control of the computer by regulating the rate of flow through each vapor generator. In continuous 34 hour vapor generation experiments in which trichloroethylene (TRI) was used, 4 graded series of concentrations using 4 chambers in parallel were produced. The frequency of unacceptable values (i.e., values out of the ±15% range of the command concentration) was zero in the total 512 (128×4) measurements. The connection of multiple generators to each chamber made possible the exposure to mixed vapors at the desired concentration for each vapor constituent, even when volatilities of the constituents varied. In the 4 day - 9 hour test, TRI, tetrachloroethylene (TETRA) and the mixture of the two were used. The frequency of unacceptable values was once in 448 (112×4) determinations, indicating the high reliability of the system irrespective of single or mixed vapor exposure. It was further possible to simulate a real workroom environment, in which the concentration frequency followed a log-normal distribution; the generation of the vapor was so controlled that concentrations followed the directed sequence of computer-produced random variables with a given geometric mean (GM) and standard deviation (GSD). Performance with 206 ppm as GM and exp (0.68) as GSD was established when 200 ppm as GM and exp (0.7) as GSD were commanded. Vapor concentrations with wave forms following a cosine function or with repeated short term peaks were also generated.