Abstract: A semi-automatic critical point drying apparatus for the preparation of biological or organic specimens for examination thereof by scanning electron microscopy. In the preparation of such specimens, the specimen is placed in a chamber into which is introduced a transitional fluid, such as liquid CO.sub.2, which replaces the dehydrating liquid in the specimen as a part of the drying process. In critical point drying, the transitional fluid in the chamber is heated to elevate the chamber temperature and pressure above the critical point of the fluid. A five-way indicator valve replaces the manually adjustable valve of prior art apparatus for enabling the operator easily to control the cooling, filling, purging and bleeding operations of such a critical point drying apparatus.

Abstract: A critical point drying apparatus for sample preparation in electron microscopy is automated to proceed in order through multiple operational modes in drying the specimen. These modes are: precooling, in which a specimen chamber is cooled; starting, in which the specimen chamber is filled with liquid CO.sub.2 : purging, in which a limited amount of CO.sub.2 passes through the specimen chamber; heating, in which the specimen chamber is heated to elevate pressure to the critical pressure and heat the specimen to the critical temperature; bleeding, in which the specimen chamber is depressurized to atmospheric pressure at a very slow rate; and finishing, in which the chamber is vented and an indicator signals the end of the drying operation. These steps are automatically activated in sequence when desired mode conditions are met or can be selectively controlled in a manual mode.