heat transfer fluid - circulating medium (often a petroleum oil) that absorbs heat in one part of a system (e.g., a solar heating system or a remote oil-fired system) and releases it to another part of the system. Heat transfer fluids require high resistance to cracking (molecular breakdown) when used in systems with fluid temperatures above 260°C (500°F). See bulk temperature. Systems can be either closed or open to the atmosphere. To prevent oxidation in a closed system an inert gas is sometimes used in the expansion tank (or reservoir) to exclude air (oxygen). See gas blanket. If the system is open and the fluid is exposed simultaneously to air and to temperatures above 66°C (150°F), the fluid must also have good oxidation stability, since a protective gas blanket cannot be contained.