Once the freezing segment of the lyophilization cycle has completed, the vacuum system is engaged to reduce the pressure in the chamber and condenser to facilitate the drying process. During Primary Drying, the high vapor pressure of ice in the product relative to the low pressure in the chamber causes ice to sublime and the water vapor to leave the vial, passing through the vents of the partially inserted stopper. This creates a pressure differential from the product to the chamber. However, if the pressure in the chamber exceeds the vapor pressure of the product while in the presence of ice, water may not be able to sublime.
This handy tool provides a reference to the corresponding pressures at varied temperatures, as well as commonly used vacuum unit conversions.
(click for printable view)