Most manuafacturers technical manuals indicate that a vacuum of approximately 26 inches of mercury should be obtained in the last-effect shell when the temperature of the seawater is 85 F, and that the vacuum should be higher when the seawater is colder. Failure to ob-tain a vacuum of 26 inches of mercury, or more, can generally be traced to one of the following factors: air leaks, improper operation of air ejec-tors, insufficient flow of seawater, and ineffec-tive use of heat transfer surface in the distilling condenser.