STABLE GASEOUS ATMOSPHERES FOR NOSE-ONLY INHALATION USING MASS-FLOW CONTROLLERS

The maintenance of stable test atmospheres in small-volume inhalation systems such as a nose-only exposure manifold requires rapid and appro priate responses to fluctuations in a number of parameters. For exampl e, changes in flows or voltages can affect test compound generation ef ficiency and gas dilution ratios, resulting in excursions of the test concentration outside an acceptable range. A feedback control loop was designed that makes use of mass flow controllers in maintaining the t arget concentration of a test compound. The system was tested by gener ating ozone in a nose-only exposure manifold. An ozone analyzer provid ed updated concentration values every 30 s, which were compared to the user-defined target concentration recorded in the interface readout b ox. Values of gain, integral, and lead determining an appropriate resp onse of the controller to concentration fluctuations were implemented based on proportional-integral-derivative control algorithms. Using th is approach, the flow of ozone mixed with dilution air into the system was adjusted as required without user intervention, minimizing concen tration overshoots upon system startup, maintaining an ozone concentra tion within a range +/-5% of the target concentration, and responding rapidly and efficiently to line disturbances.