"This publication describes the methods used to reduce evaporation loss from a free liquid surface in a fixed""""roof tank by separating the liquid and vapor regions@ and summarizes the characteristics and performance of floating covers and steel pans. A recommended method for evaluating evaporation loss reduction for either type is also included. The majority of the tanks in use in the petroleum industry are the fixed-roof type. Considerable progress has been made over the years in controlling evaporation@ but breathing and filling losses from fixed-roof tanks still are of significant magnitude in tanks containing volatile stocks. Reduction of evaporation loss conserves a valuable natural resource and in many instances it is necessary to provide a storage system which complys with emission control requirements for stationary sources. The apparatus used to achieve the loss reduction may be: Anyone of several nonferrous covers of proprietary design made for installation in a fixed-roof tank. and defined as internal floating covers. This type of system will be designated herein as floating covers. A pan-type steel floating roof designed in accordance with Appendix H of API Standard 650@ Welded Steel Tanks for Oil Storage@ and installed in a fixedroof tank. This system is defined as a covered floating roof@ and will be designated herein as a steel pan."