As a freeze drying cycle begins the pressure is lowered below the pressure of the triple point of water, ~4.6mm of mercury to establish sublimation and enhance the free water molecules to travel from where they are in the product to the condensing surface where the pressure is lower.
This mass flow of vapor has its way with the entire system. One initially raises the shelf temperature to keep up sublimation, and then this vapor increasing the condenser temperature, causes a sort of constantly changing homeostasis which eventually raises the pressure in the system and then when the flow of vapor stops or slows, and the condenser temperature lowers due to lower load due in part to the ever increasing difficulty for these water molecules to exit the product. The ice gas interface recedes due to the serum bottle’s poor contact, the warping of product trays, and the heat transfer coefficient of the shelf to product , the radiation entering the sight glasses, the influence of the chamber wall, the position of the product on the shelf [nearer the edge or in the middle back?] all weighing into their incremental, random, influences. The powerful scientists and practitioners of freeze drying systems realized that control was necessary to assist in removing variability in the cycle, and that pressure control was the easiest way to accomplish this. So today there are many ways to control pressure, if you wish to incorporate pressure control into your system, or are at odds because this parameter is not optimized in your freeze drying protocols, Applewood Scientific can assist you in whatever way you envision, to set up studies which will enable you to make the difference in what matters: Product Consistency from batch to batch. After all, this is the important objective we seek.