Considering flow conditions in the DPI test system has led to understanding more about why experts have improved the pharmacopeial protocol over time. Still, there are aspects of the fundamentals that are not yet clarified but worth understanding and accommodating in the protocols. The authors believe these include specifications for the solenoid valve, the effect of the pre-separator, control of the flow rate and measurement of the flow rate.
In this article, the authors think critically about the motion of air from the moment the solenoid valve gets its signal to open until the solenoid valve closes and the air motion comes to a halt. They also consider the fact that there are particles in the system, not just air. They describe ongoing investigations of remaining uncertainties in the testing equipment and methods. Their hope is that each reader who has a question can better think through any issue after reading this article.