The evaporation loss of oil, i.e. the amount of oil lost through evaporation under certain conditions, is expressed as a percentage by mass. The evaporation loss is proportional to the volatility of the oil. The greater the evaporation loss, the greater the oil consumption in practical application, so the amount of evaporation loss of oil under certain conditions should be limited. The evaporation of lubricating oil in use causes the lubricating oil quantity in the lubrication system to gradually decrease and needs to be replenished, and the viscosity increases, which affects the oil supply. The evaporation of hydraulic fluid in use also produces cavitation phenomenon and loss of efficiency, which may cause damage to the hydraulic pump. The data obtained by distillation method is only rough result, and the evaporation loss of lubricating oil products needs to be determined by special method. At present, China's method of determining the evaporation loss of lubricating oil is GB/T 7325 lubricating oil and grease evaporation loss determination method. The method is put in the evaporator lubricating oil specimen, placed in the specified temperature of the constant temperature bath, hot air through the surface of the specimen 22h. Then according to the specimen of the mass loss of evaporation loss calculation. According to this method, the evaporation loss of lubricating oil products can be determined at any temperature within 99-150℃. At present, this method is mainly used for evaporation loss assessment of synthetic lubricants in China. The main measurement methods abroad are: ASTM D972 of the United States, DIN 51581 of Germany and JIS K2220 (5.6) of Japan, etc.